Download Downloaded - Royal Society Open Science

Downloaded from http://rsos.royalsocietypublishing.org/ on May 7, 2017
rsos.royalsocietypublishing.org
Research
Cite this article: O’Brien HD. 2017 Cranial
arterial patterns of the alpaca (Camelidae:
Vicugna pacos). R. Soc. open sci. 4: 160967.
http://dx.doi.org/10.1098/rsos.160967
Received: 28 November 2016
Accepted: 22 February 2017
Subject Category:
Biology (whole organism)
Subject Areas:
developmental biology/evolution
Keywords:
artiodactyla, carotid rete, rete mirabile
caroticum, vasculature
Author for correspondence:
Haley D. O’Brien
e-mail: [email protected]
Cranial arterial patterns
of the alpaca (Camelidae:
Vicugna pacos)
Haley D. O’Brien
Department of Anatomy and Cell Biology, Oklahoma State University Center for Health
Sciences, 1111 West 17th Street, Tulsa, OK 74107, USA
HDO, 0000-0002-8758-6571
Artiodactyl cranial arterial patterns deviate significantly
from the standard mammalian pattern, most notably in
the possession of a structure called the carotid rete (CR)—
a subdural arterial meshwork that is housed within the
cavernous venous sinus, replacing the internal carotid artery
(ICA). This relationship between the CR and the cavernous
sinus facilitates a suite of unique physiologies, including
selective brain cooling. The CR has been studied in a number
of artiodactyls; however, to my knowledge, only a single study
to date documents a subset of the cranial arteries of New
World camelids (llamas, alpacas, vicugñas and guanacoes). This
study is the first complete description of the cranial arteries
of a New World camelid species, the alpaca (Vicugna pacos),
and the first description of near-parturition cranial arterial
morphology within New World camelids. This study finds that
the carotid arterial system is conserved between developmental
stages in the alpaca, and differs significantly from the pattern
emphasized in other long-necked ruminant artiodactyls in that
a patent, homologous ICA persists through the animal’s life.
1. Background
Living camelids are divided into two subfamilies within the
Camelidae: the Camelini, or Old World camels (e.g. bactrian
and dromedary camels), and the Lamini, or New World camels
(llamas, alpacas, guanacoes and vicuñas). Although it is known
that camelids possess a carotid rete (CR) [1–9], there are few
complete cranial arterial descriptions for this clade, outside of
dromedaries. In particular, there are no current, widely available
descriptions of the entire cranial arterial system of New World
camelids, in spite of behaviours, habitats and growing economic
uses that lend import to thorough and accurate documentation
of camelid cranial arteries. Formerly distributed throughout a
variety of environments, including temperate regions of North
Electronic supplementary material is available
online at https://dx.doi.org/10.6084/m9.
figshare.c.3713026.
2017 The Authors. Published by the Royal Society under the terms of the Creative Commons
Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted
use, provided the original author and source are credited.
Downloaded from http://rsos.royalsocietypublishing.org/ on May 7, 2017
Four cadaveric alpaca heads were obtained from the collection of S. Williams at Ohio University: two
mature males, one mature female and one near-term stillborn of unknown sex. All specimens died
of natural causes during the course of unrelated research studies. No animals were sacrificed for the
purpose of this study. Shortly after death, the adult alpacas were stored frozen and the stillborn alpaca
was preserved in formaldehyde. Data collection follows the methods of Holliday et al. [49] and O’Brien
& Williams [50], wherein specimens are injected with a radiopaque injection medium, computerized
tomography (CT) scanned and then digitally rendered in three dimensions. These more recently derived
methods allow soft and hard tissue interactions to be examined in tandem, without the need to destroy
either material. For all specimens, either the right or left common carotid artery (CCA) was cannulated
(18-gauge angiographic cannula for the stillborn specimen or 1/8-inch PVC tubing (Nalgene) in the
adult specimens). Cannulae were fixed in place with surgical ligature and adhesive. The arterial system
was then manually flushed with warm water for 10 min, followed by perfusion with 90–300 ml of 10%
................................................
2. Material and methods
2
rsos.royalsocietypublishing.org R. Soc. open sci. 4: 160967
America and Europe, living members of the Camelidae are now found in specialized habitats. The
geographical ranges of Lamini camelids are restricted to the Andes Mountains of South America,
where they are high altitude specialists and routinely encounter extremes in temperature and lengthy
periods without meteoric water [10–13]. This oxygen and water depravation, as well as the extremely
high and low environmental temperatures these animals routinely encounter can result in negative
consequences for unspecialized mammals; as such, New World camelids have a number of physiological
specializations for coping with the hypoxia and low atmospheric pressures endemic to high altitudes,
particularly within the cardiopulmonary system [14–17]. The CR, a cranial arterial meshwork well
developed in artiodactyls, may facilitate some of these extreme aspects of Lamini ecology.
The CR is a subdural arterial meshwork that supplies the majority of oxygenated blood bound for
the brain of nearly all artiodactyls. The rete is housed within the cavernous venous sinus, which receives
venous blood that has been warmed or evaporatively cooled by the maxilloturbinates [18–21]. The high
surface area of the arterial meshwork enables rapid heat exchange between the arterial and pooled
venous blood. The result of this exchange is thermal conditioning of the blood bound for the brain, which
in turn leads to a suite of physiological consequences that may benefit high-altitude artiodactyl species.
First, the CR may protect the brain from altitude hypoxia, as suggested by Caputa [21]. Second, in the
arid habitats of Camelini, the CR plays a role in selective brain cooling [22,23]. Continuity between the
carotid and ophthalmic retia [4] may further play a role in moderating the temperature of the globe
of the eye to protect the animal’s vision [24,25]. Overall, living camelids probably benefit from the
increased water economy furnished by CR-mediated hypothalamic cooling [26–35]. In spite of the range
of plausible physiological benefits imparted by possession of a CR, there is currently, to my knowledge,
no comprehensive documentation of the cranial arteries of the Lamini outside of a detailed description
of the intracranial cerebral arterial circle by Kiełtyka-Kurc et al. [9].
Additionally, camelids are notable for their elongated necks––morphology that may result in atypical
haemodynamic and developmental patterns. Although doubt has been cast on a haemodynamic role
for the CR in long-necked artiodactyls [36], ontogenetic changes have been documented in the cranial
arterial patterns of other long-necked artiodactyls, such as giraffes [37]. Fully mature adult artiodactyls
frequently lack an internal carotid artery (ICA) even though arterial development begins from the
same embryonic scaffolding as other mammals [38–40]. With the exception of tragulids and perhaps
dromedaries [4,9,41,42], the artiodactyl ICA diminishes in diameter or obliterates at a variable point
during ontogeny. Some documented artiodactyls lose a patent internal carotid in prenatal ontogenetic
stages (Sus scrofa scrofa and Giraffa camelopardalis [37,39,43–46]), others maintain a patent vessel until
shortly after parturition (Capra hircus hircus, Ovis aries [46]) and still others possess a patent internal
carotid until sexual maturity (Bos taurus, Bubalis bubalis [46–48]). Evidence within Old World camelids
(Camelus dromedarius) suggests that the ICA may persist through the animal’s entire life [1,2,4,6,9,39].
Although the developmental mechanism driving the elimination of the artiodactyl ICA is currently
unknown, emergent patterns from the few documented taxa suggest that patency is maintained further
into life in artiodactyls with larger body sizes or longer gestation periods, with the exception of giraffes
[37]. This study aims to fully describe adult and neonatal cranial arterial patterns of the alpaca, Vicugna
pacos, in order to document structures important for oxygen metabolism and thermoregulation, as well
as to establish a baseline for potential ontogenetic shifts in arterial morphology within a long-necked
and large-bodied artiodactyl.
Downloaded from http://rsos.royalsocietypublishing.org/ on May 7, 2017
3.1. Cranial arteries of the adult alpaca
3.1.1. Branches of the external carotid artery
Overall, the branching patterns of cranial arteries were conserved among adult specimens. The branches
of the external carotid artery (ECA) of the alpaca are summarized in the electronic supplementary
material, table S1 and visualized in figures 1–4. The ECA (arteria [a.] carotis externa) begins to branch
extensively deep to the condylar process of the mandible. The first major branch of the ECA is the
occipital artery (a. occipitalis; figure 1). From the superior surface of the ECA, the occipital artery shares
a short trunk with the ICA (a. carotis interna) and the condylar artery (figures 1–3). The trunk uniting
these arteries is variable—the vessels may arise in close proximity to each other or from a common
trunk, as described (figures 1–3). As the occipital artery ascends, it scours the deep surface of the jugular
process and the posterior surface of the temporal crest (mastoid contribution; crista supramastoidea). The
artery terminates by splitting into smaller branches that permeate the occipital region (nuchal ligament
(ligamentum nuchae) and muscles) and a larger caudal meningeal artery (figures 1 and 2). The latter enters
the cranium via a large mastoid foramen (foramen mastoideum) before radiating across the caudal half of
the meninges. The ICA itself is reduced in calibre, and the vessel does not leave a medial bullar groove
as it ascends to the basicranium as in other artiodactyls with a homologous ICA (figures 2 and 3; see e.g.
Moschiola [42]; Tragulus [51]). As the ICA enters the basicranium, it traverses the promontorial foramen,
coursing within a carotid canal in close association with the promontorium of the petrosal. The ventral
surface of the petrosal is scoured by a slight transpromontorial sulcus. This sulcus corresponds to direct
contact by the ICA of non-artiodactylan mammals [52–57]; however, in the adult alpaca, the ICA does
not make direct contact with the petrosal. The presence of a transpromontorial sulcus in adult alpacas
may be an effect of arterial reduction during ontogeny (see below). Upon entering the cranium, the ICA
leaves a notch at the rostral-most extent of the epitympanic wing (sensu [55,57]; this structure is also
referred to as the ‘pole of the promontorium’ sensu [58] and the ‘anteromedial flange’ sensu [59]). Once
inside the brain case, the ICA communicates with the CR (figures 2–4; also illustrated for other Lamini
by Kiełtyka-Kurc et al. [9]).
Following the occipital and ICA branches, a large artery arises from the superolateral surface of the
ECA, immediately posterior to the tympanohyal (os tympanohyoideum). This artery is highly dendritic
and supplies a large region equivalent to the distribution of the caudal auricular artery of other
artiodactyls (a. auricularis caudalis), as well as some of the typical distribution of the superficial temporal
(a. temporalis superficialis) and caudal masseteric arteries (a. masseterica profunda; figure 2). The majority
................................................
3. Description: results and discussion
3
rsos.royalsocietypublishing.org R. Soc. open sci. 4: 160967
One-Point anticoagulant solution (depending on specimen size). A higher concentration of anticoagulant
was used to flush the formalin-preserved specimen to assist removal and breakdown of blood clots (33%
One-Point). Following initial specimen preparation, radiopaque latex vascular injection was conducted,
following the criteria for complete perfusion outlined in O’Brien & Williams [50]. All specimens were
manually injected with a solution of 40% Liquid Polibar Plus barium sulfate suspension (BaSO4, E-Z-Em,
Westbury, NY) in 60% red liquid latex injection medium (Ward’s, Rochester, NY). Perfusion continued
until latex emerged from the contralateral CCA. The volume of injected medium ranged from 5 ml in the
stillborn specimen to 30 ml in the adult specimens. Acetic acid (10% glacial acetic acid solution) was used
to set any extravasated latex. For clarity in digital rendering, the venous system was not perfused.
Following radiopaque latex injection, specimens were CT scanned at the Holzer Clinic in Athens,
Ohio, on a Philips Brilliance 64-slice CT scanner. Scan resolution for all specimens was 0.67 mm slice
thickness, 150 kV and 80 mA, yielding an initial voxel size of 0.693359 × 0.693359 × 0.5. The resultant data
were up-sampled to improve resolution in Avizo (version 7.0; VSG). Grey-scale values were averaged
across voxels and smaller specimens were up-sampled to a size of 0.1 × 0.1 × 0.1 mm, whereas larger
specimens were up-sampled to 0.3 × 0.3 × 0.3 mm. Up-sampling decreases the average voxel size without
affecting the inherent quality of the data. This technique generates a visually smoother surface upon
reconstruction. Because a 40% barium solution yields stark contrast between hard tissues (cartilage
and bone), the skull and arteries were segmented based on distinctive grey-scale values. Manual
segmentation was then employed to verify the accuracy of the model. Segmented morphology was
then rendered in three dimensions with minimal use of smoothing algorithms (setting of 2 on a scale of
10). Note that vascular nomenclature largely follows that codified in the Nomina Anatomica Veterinaria
(2012), with accepted terminology in Latin following the first reference to a vessel.
Downloaded from http://rsos.royalsocietypublishing.org/ on May 7, 2017
STA
TFA
cDT
rDT
EO
OPR
MAL
4
................................................
rsos.royalsocietypublishing.org R. Soc. open sci. 4: 160967
OC
CMA
FA
MXL
BUC MDL
LA
IOB
Figure 1. Superficial and orbital arteries of the head of the alpaca, Vicugna pacos, lateral perspective. BUC, buccal artery; cDT, caudal
deep temporal artery; CMA, ‘common’ auricular artery; EO, external ophthalmic artery; FA, facial artery; IOB, infraorbital artery; LA,
lingual artery; MAL, malar artery; MDL, mandibular labial artery; MXL, maxillary labial artery; OC, occipital artery; OPR, ophthalmic rete;
rDT, rostral deep temporal artery; STA, superficial temporal artery; TFA, transverse facial artery.
STA
CMA
ICA
TFA
FA
cDT
LA
rDT
MA
EO
MXL
OPR
BUC
MDL
MAL
DLA
gPAL IOB
iAL
Figure 2. Superficial and orbital arteries of the head of the alpaca, Vicugna pacos, lateral perspective. Skull transparent to expose
deeper arteries. Note the ICA extending from the carotid artery to the braincase and the enlarged ophthalmic rete (OPR). BUC, buccal
artery; cDT, caudal deep temporal artery; CMA, ‘common’ auricular artery; DLA, deep lingual artery; EO, external ophthalmic artery; FA,
facial artery; gPAL, greater palatine artery; iAL, inferior alveolar artery; ICA, internal carotid artery; IOB, infraorbital artery; LA, lingual
artery; MA, maxillary artery, MAL, malar artery; MDL, mandibular labial artery; MXL, maxillary labial artery; OPR, ophthalmic rete;
rDT, rostral deep temporal artery; STA, superficial temporal artery.
of branches from this parent artery supply the auricular region, without ramification by additional,
individual auricular arteries (figure 2). As such, a more accurate term for this vessel may be the ‘common
auricular artery’. After branching from the ECA, the common auricular artery then ascends between
the jugular process and the posterior surface of the auditory bulla, scoring the superior surface of the
Downloaded from http://rsos.royalsocietypublishing.org/ on May 7, 2017
OPR
CR
CAC
5
ICA
ECA
IOB gPAL sPAL DLA SLA iAL MA LA
Figure 3. Sagittal section of the skull and arteries of the alpaca, Vicugna pacos. CAC, cerebral arterial circle; CMA, ‘common’ auricular
artery; CON, condylar artery; CR, carotid rete; DLA, deep lingual artery; ECA, external carotid artery; gPAL, greater palatine artery; iAL,
inferior alveolar artery; ICA, internal carotid artery; IOB, infraorbital artery; LA, lingual artery; MA, maxillary artery; OC, occipital artery;
OPR, ophthalmic rete; SLA, sublingual artery; sPAL, sphenopalatine artery.
mastoid, caudal to the external acoustic meatus. Ventral to the external acoustic meatus, the common
auricular artery distributes a posterolateral zygomatic branch that ramifies the temporomandibular
joint (TMJ) and contributes extensive, smaller arteries to the posterosuperior border of the masseter, a
region typically supplied by caudal branches from the superficial temporal artery [60]. A stylomastoid
artery (a. stylomastoidea) departs from the medial surface of the common auricular artery at the level
of the stylomastoid foramen. Finally, at the level of the crista supramastoidea, the common auricular
artery splits into rostral and caudal terminal branches. This rostral branch is not homologous with the
a. auricularis rostralis proper, which derives from the superficial temporal artery in non-camelid
artiodactyls [46,60]. The caudal termination of the common auricular artery supplies the posterior
scalp/superior occipital region as well as the cartilaginous pinna of the ear. The rostral division of the
common auricular artery supplies the remainder of the ear and gives off a proper branch to the posterior
portion of the temporalis muscle.
In the alpaca, the lingual and facial arteries do not form a common linguofacial trunk (truncus
linguofacialis), as the facial artery (a. facialis) has a separate origin on the lateral wall of the ECA (figure 2).
The facial artery has a somewhat deviant course relative to other artiodactyls and many other mammals,
in that it approaches the superficial facial region from the caudal border of the mandibular ramus (ramus
mandibulae) instead of the ventral border of the mandible (corpus mandibulae margo ventralis). In this
respect, its course is somewhat reminiscent of the transverse facial artery (a. transversa facei). The facial
artery transmits a number of small vessels to the digastric, masseter and buccinator muscles, providing
a substantial volume of the blood supply to the muscles supporting the oral cavity. It terminates by
splitting into superior and inferior labial arteries (a. labialis superior and a. labialis inferior).
The lingual artery (a. lingualis) is the third and anterior-most major branch of the ECA, departing
from the ventral surface of the parent vessel caudal to the greater horn of the hyoid (figures 2–4). It
distributes through the parenchyma of the tongue as expected, but has an aberrant branching pattern to
ramify regions of the ventral mandibular border typically supplied by branches of the facial artery. Near
the emergence of the lingual artery from the ECA, the sublingual artery (a. sublingualis) parts from the
lingual artery (figures 2 and 3). The sublingual artery of the alpaca is relatively large and compensates
for the absence of the submental artery. In addition to supplying the sublingual gland, the sublingual
artery courses along the ventral border of the mandible, supplying the floor of the mouth and the
lingual vestibule. The final branch of the ECA is a greatly reduced and partially perfused superficial
temporal artery (figures 1 and 2). The typical distribution of the superficial temporal artery is ramified
by a sizeable branch of the ‘common auricular artery’ (described above; figures 1 and 2). The transverse
facial artery originates on the anterior aspect of the superficial temporal artery and courses obliquely to
................................................
OC
CON
rsos.royalsocietypublishing.org R. Soc. open sci. 4: 160967
CMA
Downloaded from http://rsos.royalsocietypublishing.org/ on May 7, 2017
(a) CMA cDT
rDT
sPLP
OPR
EO
aETH
MAL
6
................................................
rsos.royalsocietypublishing.org R. Soc. open sci. 4: 160967
OC
aETH
(b)
IOB
ECA
iAL
BUC MA
MAL
OPR
EO
sPAL gPAL
lPAL
PTG
gPAL
CR
RA
IOB
CAC
ICA
VS
OC
Figure 4. Orbital and palatine arteries of the alpaca, Vicugna pacos, from lateral (a) and (b) medial views. aETH, anterior ethmoidal
artery; BUC, buccal artery; CAC cerebral arterial circle; cDT, caudal deep temporal artery; CMA, ‘common’ auricular artery; CR, carotid rete;
ECA, external carotid artery; EO, external ophthalmic artery; gPAL, greater palatine artery; iAL inferior alveolar artery; ICA, internal carotid
artery; IOB, infraorbital artery; lPAL, lesser palatine artery; MA, maxillary artery; MAL, malar artery; OC, occipital artery; OPR, ophthalmic
rete; PTG, pterygoid branches; RA, ramus anastomoticus; rDT, rostral deep temporal artery; sPAL, sphenopalatine artery; sPLP, superior
palpebral artery; VS, ventral spinal artery.
curve around the caudal border of the mandibular ramus near the TMJ (articulation temporomandibularis;
figures 1 and 2). It perfuses a small quadrant of the masseter and sends a terminal ramus to the capsule
of the TMJ.
3.1.2. Branches and distribution of the maxillary artery
The branches and distribution of the alpaca maxillary artery (MA) are summarized in the electronic
supplementary material, table S2 and visualized in figures 1–4. Common to contemporary artiodactyls,
Downloaded from http://rsos.royalsocietypublishing.org/ on May 7, 2017
The arterial blood supply to the eye and orbit of the alpaca are summarized in the electronic
supplementary material, table S3 and imaged predominantly in figure 4. The CR extends rostrally,
outside of the braincase, to participate in the formation of an OPR through which the eye and periorbita
are supplied (figure 4). The eyeball is directly supplied by the central artery of the retina (a. centralis
retinae), which originates as a derivative of the cerebral arterial circle and exits the optic foramen within
the optic nerve. The remainder of the globe of the eye is supplied by the internal ophthalmic artery (a.
ophthalmica interna), which originates from an anastomosis between the cerebral arterial circle and the
CR. The EO artery (a. ophthalmica externa) condenses from an extensive network of rami among the MA
and ophthalmic and carotid retia (figures 1–4). From this meshwork, the EO artery consolidates prior
................................................
3.1.3. Arterial blood supply to the eye and orbit
7
rsos.royalsocietypublishing.org R. Soc. open sci. 4: 160967
the MA (a. maxillaris) is the major source of oxygenated blood to the brain (figures 3 and 4; [47]). The only
source of collateral flow to the cerebral arterial circle (circulus arteriosus cerebri) is through the ventral
spinal artery (figure 4). The MA distributes blood to the brain (via the CR), pterygoid muscles, palate,
nasal cavity, oral cavity, ethmoidal region, frontal region, cranial sinuses, superficial facial structures
above the maxillary tuberosity (including the upper lip and fleshy portion of the rostrum), the maxillary
dentition, the dentary (including mandibular alveoli), and the lower lip and chin (figures 1–4). The
caudal deep temporal (cDT) artery (a. temporalis profunda caudalis) is the first major branch to arise as
a dorsal branch from the MA (figures 1, 2 and 4). This arterial offshoot courses deep to the posterior
border of the coronoid process of the mandible, ultimately perfusing the temporalis muscle. The cDT
artery shares a common origin with the inferior alveolar and masseteric arteries (a. alveolaris inferior; a.
masseterica), which proceed inferior and lateral to the MA, respectively (figures 1, 2 and 4). The inferior
alveolar artery enters the mandibular canal via the mandibular foramen. As it courses through the
inferior alveolar canal, the alveoli of the mandibular dentition are supplied before the artery ultimately
exits the mental foramen. The superficial distribution of the artery is to the lower lip and chin. The
masseteric artery separates from the MA/cDT complex dorsal to the inferior alveolar artery. After
separating from the MA, the masseteric artery hooks around the neck of the condylar process, after
which it supplies the lateral facial/zygomatic region and its eponymous muscle (figures 1 and 2). The
dorsal termination supplies the TMJ (figure 2). Deep to the coronoid process, the buccal artery (a. buccalis)
emerges from the lateral surface of the MA (figures 1, 2 and 4). This small artery traces the ventral
surface of the zygomatic process before bifurcating into the inferior palpebral artery (a. palpebralis inferior)
and a branch to the buccinator muscle between the coronoid process and the posterior border of the
maxilla.
Distal to the cDT artery on the dorsal surface of the MA, a variable number of rami connect the MA
to the CR through the foramen orbitorotundum. It is through these arteria anastomotica that the brain
receives almost all of its oxygenated blood (figures 2–4). The alpaca CR is further ramified via the MA
by a ramus anastomoticus that, as in true ruminant artiodactyls (Pecora), enters the brain case through the
foramen ovale, as well as by a reduced but patent ICA as described above (ramus anastomoticus: figure 4;
ICA: figures 2–4). Additional collateral contributions to the CR of the alpaca include the vertebral arteries
(figure 4; a. vertebralis; via the ventral spinal artery (a. spinalis ventralis)— the vertebral arteries do not
make a direct contribution to intracranial circulation). The cerebral arterial circle and blood supply to the
brain of camelids is discussed in detail by Kiełtyka-Kurc et al. [9], and will therefore not be duplicated
here. These authors identify variation within cerebral arterial circle vessels for both New and Old World
camelids [9]. The MA then anastomoses freely with the ophthalmic rete (OPR; figures 2–4; rete mirabile
ophthalmicum). Near the pterygoid crest, the rostral deep temporal artery (a. temporalis profunda rostralis)
departs the MA from within this periorbital anastomotic network. Immediately caudal to the OPR, the
rostral deep temporal artery proceeds superiorly along the anterior temporal line, sending perforating
branches into the anterior border of the temporalis muscle (figures 1, 2 and 4). Also within this region,
several pterygoid branches (rami pterygoidei) depart the internal surface of the MA (figure 4). These
branches have a short course before perfusing the pterygoid muscles. The rostral termination of the
MA is tripartite: the external ophthalmic (EO) artery departs medially (a. opthalmica interna) and the
malar artery superiorly (a. malaris), with the infraorbital artery (a. infraorbitalis) continuing anteriorly
(figures 2–4). The infraorbital artery has two divisions: a lateral division, which continues through the
infraorbital canal, and a medial division (the descending palatine artery; a. palatina descendens), which
serves as the parent vessel for the greater, lesser and sphenopalatine (a. palatina major, a. palatina minor
and a. sphenopalatina, respectively) arteries, which supply the soft and hard palates, the nasal septum,
internal nasal vestibule and turbinates (figure 4).
Downloaded from http://rsos.royalsocietypublishing.org/ on May 7, 2017
A single stillborn alpaca was available as a representative of early developmental stages. The stillborn
alpaca (age estimated as proximate to full gestation) had been preserved in formalin for several years
prior to injection. As such, the arterial walls were hardened and several distributing arteries (infraorbital,
labial and dorsal nasal) did not perfuse into the rostral-most soft tissues (figure 5). The stillborn alpaca
specimen included both head and neck, providing an opportunity to describe the branches of the CCA
(a. carotis communis) low in the neck (figure 5).
3.2.1. Arteries of the neck
The arteries of the neck of the stillborn alpaca are summarized in the electronic supplementary material,
table S4, and all arteries are presented in figure 5. Deviations in CCA patterning across ontogeny may
indicate an arterial mode of postural hypertension mitigation endemic to long-necked artiodactyls, such
as giraffes [37]. Of particular interest is an anastomosis between enlarged occipital and vertebral vessels
that courses through the alar foramen of the atlas—the ‘alar artery’, first noted in giraffes [61] (a. alaris,
sensu [37]). In a description of the cerebral arteries of the closely related Old world camel, Camelus
dromedarius, Kanan [4] notes the presence of such an anastomosis, and, indeed, an alar artery is present
branching from the CCA of the stillborn Vicugna. This vessel therefore follows the same course in all
three long-necked artiodactyl species (Camelus [4]; Giraffa [37]; Vicugna, figure 5a).
Unlike the condition described for developing giraffes (stillborn and 6 month old; [37]), the stillborn
alpaca retains a large, fully patent ICA (figure 5a,b). The ICA arises near the occipital artery from a
common point on the superior surface of the CCA, thus providing a precise demarcation between the
CCA and the ECA. The ICA of the stillborn alpaca makes direct contact with the petrosal, unlike the adult
in which the carotid canal is incompletely filled by the ICA. Below this transition, the cranial thyroid
(a. thyroidea cranialis) and descending pharyngeal (a. pharyngea descendens) arteries arise from the anterior
surface of the CCA (in order from proximal to distal). The descending pharyngeal artery originates close
to the hyoid, superior to the larynx.
3.2.2. Branches of the external carotid artery and course of the internal carotid artery
The branches of the ECA of the stillborn alpaca are imaged in figure 5. The ECA is the rostral continuation
of the CCA after the occipital, condylar and internal carotid arteries depart the dorsal surface of the
parent artery near the jugular process. In the stillborn alpaca, the ECA primarily supplies the superficial
face and scalp, and the deeper regions of the cranium below the level of the zygomatic bone (e.g. floor of
the oral cavity, portions of the cranial base). The ICA is a substantial and easily identifiable branch of the
CCA that arises as the caudal-most branch from a common origin with the occipital and condylar arteries.
The ICA briefly courses anteriorly, medial to the occipital and condylar vessels, before ascending towards
the tympanic floor on the deep surface of the large tympanic bulla. This association leaves a groove on the
medial wall of the bulla—a groove that is not present in fully developed adult skulls. As the ICA courses
in close proximity to the petrosal, the artery makes direct contact with the bone, leaving a groove that
remains in the adult even as the artery becomes relatively reduced in calibre. Within the open, developing
................................................
3.2. Cranial arterial patterns of the stillborn alpaca
8
rsos.royalsocietypublishing.org R. Soc. open sci. 4: 160967
to transecting the orbital region (figure 4). It supplies the periorbita, extraocular muscles and ethmoidal
region (figure 4). The ciliary trunk and ciliary vessels (aa. ciliares) are not perfused. Direct branches of
the MA supply the superficial periorbita. The lacrimal artery (a. lacrimalis) is a small division of the
MA, which arises slightly proximal to the carotid/OPR complex. It courses laterally towards the closed
post-orbital bar, where it divides into lacrimal and superior palpebral branches (a. palpebralis superior).
Collateral circulation to the superficial orbit is by the malar artery, which originates from the infraorbital
artery (figures 1, 2 and 4). Prior to entering the orbital portion of the infraorbital canal, the infraorbital
artery gives off the tortuous malar artery. The malar follows the anterior margin of the orbit, contacting
the lacrimal bone, and exiting the orbit via a notch near the lacrimal fossa. In addition to the superficial
orbit, the malar artery perfuses the caudal portion of the face.
The infraorbital artery is the rostral continuation of the MA. The MA bifurcates into palatine and
infraorbital arteries near the common orbital tendinous ring and courses ventral to the periorbita. The
orbital portion of the infraorbital artery has few branches, but gives rise to the malar artery, as described
above. The infraorbital artery then proceeds to the rostrolateral facial region through its eponymous
canal. Upon exiting the infraorbital foramen, the infraorbital artery branches extensively across the lateral
nasal and superior labial regions.
Downloaded from http://rsos.royalsocietypublishing.org/ on May 7, 2017
(a)
9
ALA
ICA
OC
IOB
DNA
FA
TFA
BUC
(b)
CCA
CMA
OC
ICA
LA
CCA
s’orb
dent
EO
OPR
MA
zyg
Figure 5. Cranial arteries of the stillborn alpaca, Vicugna pacos, (a) superficial lateral perspective and (b) deep lateral perspective with
supraorbital (s’orb), dentary (dent) and zygomatic (zyg) bones cut. Sectioned arteries are indicated in orange. Perfusion did not extend
rostrally beyond the skull, into the soft tissue structures of the nasal and labial regions. ALA, alar artery; BUC, buccal artery; CCA, common
carotid artery; CMA, ‘common’ auricular artery; DNA, dorsal nasal artery; EO, external ophthalmic artery; FA, facial artery; ICA, internal
carotid artery; IOB, infraorbital artery; LA, lingual artery; MA, maxillary artery; OC, occipital artery; OPR, ophthalmic rete; STA, superficial
temporal artery; TFA, transverse facial artery.
tympanic floor, the ICA courses antero-superiorly before anastomosing with the posterior extent of the
CR on the internal surface of the basisphenoid, definitively anterior to the petrosal.
The remaining arteries follow the same pattern between the stillborn and adult specimens, and
the branching patterns described in the electronic supplementary material, tables S1 through to S3
are therefore not replicated. Interestingly, the ophthalmic and carotid retia are already reasonably well
developed in the stillborn specimen. The full formation of the OPR in early ontogeny may be owing to
the fact that the ophthalmic vessels condense from large vascular plexuses consisting of the primordial
ophthalmic, nasal, ethmoidal and cerebral vessels [62]. The extent of the CR may result from a different
................................................
CMA
rsos.royalsocietypublishing.org R. Soc. open sci. 4: 160967
STA
Downloaded from http://rsos.royalsocietypublishing.org/ on May 7, 2017
Digital anatomical data collection and dissection techniques indicate the presence of a patent, functional
ICA in both stillborn and mature alpacas. This conclusion supports recent work performed by KiełtykaKurc et al. [9], who describe a well-formed ICA present across both New World and Old World camelids.
The functional presence of this vessel is unusual in mature artiodactyls, as suid and pecoran artiodactyls
are known to obliterate this vessel throughout ontogeny [1,2,4,37,39,43–48]. Overall, the major cranial
arterial branching patterns of the alpaca do not differ significantly from those studied in a comparative
context by Kiełtyka-Kurc et al. [9], nor do they differ significantly from the patterns detailed for the
domestic dromedary [4,6,65]. New and Old World camelids inhabit strikingly different habitats which
may promote their own unique demands on the animals’ cardiovascular systems. That there are few
major discrepancies between the arteries supplying the head and CR of these animals suggests that
the cranial vascular patterns of camelids reflect phylogeny rather than function. Because camelids are
frequently reconstructed as the earliest group of extant artiodactyls to originate [66–69], the maintenance
of a patent ICA in this group suggests a possible stepwise loss of this artery throughout Artiodactyla.
This phenomenon should be validated on a clade-wide scale.
The stillborn specimen described in this study provides additional data regarding the timing of
artiodactyl cranial artery patterning. In the alpaca, the adult cranial arterial pattern is established prior
to birth, contrary to the gradual, post-natal obliteration of the ICA in Bos and Bubalis [46,48]. A similar
phenomenon of an ontogenetically conserved pattern in the arteries supplying the brain has also been
noted recently for giraffes; however, for giraffes, the ICA is absent from an early age [37], rather than
persistent throughout life as in camelids. These developmental patterns are strikingly different, in spite
of the fact that giraffids and camelids have long gestational periods (greater than 325 days; [70,71]) and
face a similar cardiovascular burden owing to neck elongation. A paucity of data on anatomical variation,
developmental timing and genetic signalling necessitates further studies to assess whether these patterns
arise through macroevolutionary, physiological and/or environmental mechanisms. Untangling the
pattern and process underlying the evolution and development of the CR may help elucidate how
this group came to occupy highly variable habitats—including the extreme environments preferred
by alpacas and other camelids. Low specimen numbers should be interpreted conservatively, and
future studies should focus on the inclusion of additional specimens, particularly earlier ontogenetic
stages.
Data accessibility. 3D.stl files are available at Figshare. Files can be opened with Preview for Mac or with a
number of freeware programs and sites (e.g. http://www.viewstl.com/). Adult Alpaca Arteries: http://dx.doi.org/
10.6084/m9.figshare.3185095 Adult Alpaca Skull: http://dx.doi.org/10.6084/m9.figshare.3185086 Stillborn Alpaca
Arteries: http://dx.doi.org/10.6084/m9.figshare.3185074 Stillborn Alpaca Skull: http://dx.doi.org/10.6084/m9.
figshare.3185068.
Competing interests. I have no competing interests.
Funding. Funding for this research was provided by The American Society of Mammalogists Grants-In-Aid, the Society
for Integrative and Comparative Biology Grants-In-Aid, the Ohio University Student Enhancement Award, the Ohio
University Graduate Student Senate and Oklahoma State University’s Center for Health Sciences.
Acknowledgements. This study could not have been conducted without the generosity of Susan Williams, who loaned
all specimens described in this study. Kenneth Wheeler skillfully assisted in specimen preparation and injection;
................................................
4. Conclusion
10
rsos.royalsocietypublishing.org R. Soc. open sci. 4: 160967
mechanism, which requires further investigation on comparative anatomical and proximate, mechanistic
scales. Compared to other non-camelid artiodactyls, the CR of the stillborn alpaca, although well
developed, is somewhat smaller than expected at parturition. This may be owing to the presence of a
patent ICA. By birth, Sus, Ovis, Capra and Giraffa possess retia that occupy nearly the entire floor of the
basicranium, in the absence of an ICA [37,39,46,63]. Contrarily, in Bos and Bison, which maintain patent
ICA’s at birth [46,48], the CR does not yet fill the compartment lateral to the sella turcica––a similar extent
to that of the stillborn alpaca. Wible [39] notes that early artiodactyl embryos maintain a tubular ICA
associated with the intracranial surface of the basisphenoid. These studies suggest that the ICA is initially
formed in artiodactyls, but degenerates or becomes stenotic across development. This degeneration may
result in hypoxia to the lateral sellar compartment. Hypoxia is a known mechanism for the recruitment
of vascular endothelial growth factors [64]. The potential linkage between ICA obliteration, hypoxia and
rete formation should be explored in further detail and in a mechanistic context to resolve whether rete
ontogeny is influenced by intrinsic developmental processes, or extrinsic factors, such as altitude and
environment.
Downloaded from http://rsos.royalsocietypublishing.org/ on May 7, 2017
C. Pugh, B. Keener and J. Sands at the Holzer Clinic in Athens, Ohio, conducted all CT scans used in this study.
The insightful comments of two anonymous reviewers greatly improved the quality and content of this contribution.
19. Walker JEC, Wells RE, Merrill EW. 1961 Heat and
water exchange in the respiratory tract. Am. J. Med.
30, 259–267. (doi:10.1016/0002-9343(61)90097-3)
20. Romer AS, Parsons TS. 1986 The vertebrate body, 6th
edition. Philadelphia, PA: Saunders College Press.
21. Caputa M. 2004 Selective brain cooling: a multiple
regulatory mechanism. J. Therm. Biol. 29, 691–702.
(doi:10.1016/j.jtherbio.2004.08.079)
22. Schroter RC, Robertshaw D, Filali RZ. 1989 Brain
cooling and respiratory heat exchange in camels
during rest and exercise. Resp. Phys. 78, 95–105.
(doi:10.1016/0034-5687(89)90145-X)
23. Elkhawad AO. 1992 Selective brain cooling in desert
animals: the camel (Camelus dromedarius). Comp.
Biochem. Phys. A 101, 195–201. (doi:10.1016/03009629(92)90522-R)
24. Dewhirst MW, Viglianti BL, Lora-Michiels M, Hanson
M, Hoopes PJ. 2003 Basic principles of thermal
dosimetry and thermal thresholds for tissue
damage from hyperthermia. Int. J. Hyperthermia
19, 267–294. (doi:10.1080/0265673031000
119006)
25. Yarmolenko PS, Moon EJ, Landon C, Manzoor A,
Hochman DW, Viglianti BL, Dewhirst MW. 2011
Thresholds for thermal damage to normal tissues:
an update. Int. J. Hyperthermia 320, 343.
26. Schmidt-Nielsen K. 1979 Desert animals:
physiological problems of heat and water. New York,
NY: Dover.
27. Kuhnen G. 1997 Selective brain cooling reduces
respiratory water loss during heat stress. Comp.
Biochem. Phys. A. 118, 891–895. (doi:10.1016/S03009629(97)00235-1)
28. Jessen C, Dmi’el R, Choshniak I, Ezra D, Kuhnen G.
1998 Effects of dehydration and rehydration on body
temperatures in the black Bedouin goat. Pflüg. Arch.
Eur. J. Phy. 436, 659–666. (doi:10.1007/s0042400
50686)
29. Baker MA. 1989 Effects of dehydration and
rehydration on thermoregulatory sweating in goats.
J. Physiol. 417, 421–435. (doi:10.1113/jphysiol.1989.
sp017810)
30. Baker MA, Nijland JM. 1993 Selective brain cooling
in goats: effects of exercise and dehydration.
J. Physiol.-London 471, 670–692. (doi:10.1113/
jphysiol.1993.sp019922)
31. Caputa M, Feistkorn G, Jessen C. 1986 Effects of
brain and trunk temperatures on exercise
performance in goats. Pflüg. Arch. 406, 184–189.
(doi:10.1007/BF00586681)
32. Kuhnen G, Jessen C. 1994 Thermal signals in control
of selective brain cooling. Am. J. Physiol. 267,
R355–R359.
33. Hetem RS. 2009 Adapting to climate change: the
effect of desertification on the physiology of
free-living ungulates. PhD thesis, University of the
Witwatersrand, Johannesburg, South Africa.
34. Vesterdorf K, Blanche D, Maloney SK. 2011 The
cranial arterio-venous temperature difference is
related to respiratory evaporative heat loss in a
panting species, the sheep (Ovis aries). J. Comp.
35.
36.
37.
38.
39.
40.
41.
42.
43.
44.
45.
46.
47.
48.
Physiol. B 181, 277–288. (doi:10.1007/s00360010-0513-7)
Strauss WM, Hetem RS, Mitchell D, Maloney SK,
Meyer LCR, Fuller A. 2015 Selective brain cooling
reduces water turnover in dehydrated sheep. PLoS
ONE 10, e0115514. (doi:10.1371/journal.pone.
0115514)
O’Brien HD, Bourke J. 2015 Physical and
computational fluid dynamics models for the
hemodynamics of the artiodactyl carotid rete.
J. Theor. Biol. 386, 122–131. (doi:10.1016/j.jtbi.
2015.09.008)
O’Brien HD, Gignac PM, Hieronymus TL, Witmer LM.
2016 A comparison of postnatal arterial patterns in
a growth series of giraffe (Artiodactyla: Giraffa
camelopardalis). PeerJ 4, e1691. (doi:10.7717/peerj.
1691)
Schummer A, Wilkens H, Vollmerhaus B, Habermehl
K-H. 1981 The anatomy of the domestic animals.
Volume 2, the circulatory system, the skin, and the
cutaneous organs of the domestic mammals. Berlin,
Germany: Paul Parey Press.
Wible JR. 1984 The ontogeny and phylogeny of the
mammalian cranial arterial pattern. PhD thesis,
Duke University, Durham, NC, USA.
Wible JR. 1986 Transformations in the extracranial
course of the internal carotid artery in mammalian
phylogeny. J. Vert. Paleontol. 6, 313–325.
(doi:10.1080/02724634.1986.10011628)
Fukuta K, Kudo H, Sasaki M, Kimura J, Bin Ismail D,
Endo H. 2007 Absence of carotid rete mirabile in
small tropical ruminants: implications for the
evolution of the arterial system in artiodactyls.
J. Anat. 210, 112–116. (doi:10.1111/j.1469-7580.2006.
00667.x)
O’Brien HD. 2015 Cranial arterial pattern of the Sri
Lankan spotted chevrotain, Moschiola memmina,
and comparative basicranial osteology of the
Tragulidae. PeerJ 3, e1451. (doi:10.7717/peerj.
1451)
Tandler J. 1906 Zur Entwickelungsgeschichte der
arterillen Wundernetze. Anatom. Hefte. I Abteilung.
31, 236–267.
Von Hofmann L. 1914 Die Entwicklung der
Kopfarterien bei Sus scrofa domesticus. Morphol.
Jahrb. 48, 645–671.
Llorca FO. 1933 Über die Entwicklung der
Arterienbogen beim Schweine. Z. Anat.
Entwicklungs. 102, 335–347. (doi:10.1007/BF0213
4542)
Daniel PM, Dawes JDK, Prichard MML. 1953 Studies
of the carotid rete and its associated arteries. Phil.
Trans. R. Soc. Lond. B 237, 173–204. (doi:10.1098/rstb.
1953.0003)
Gillilan LA. 1974 Blood supply to the brains of
ungulates with and without a rete mirabile
caroticum. J. Comp. Neurol. 153, 275–290.
(doi:10.1002/cne.901530305)
Bamel SS, Dhingra LD, Sharma DN. 1975 Anatomical
studies on the arteries of the brain of the buffalo
(Bubalus bubalis). Anat. Anz. 137, 440–446.
................................................
1. Lesbre MF-X. 1903 Recherches anatomiques sur les
Camélidés. Arch. du Mus. d’Hist. Nat. de Lyon. 8,
1–195.
2. Tayeb MAF. 1951 A study on the blood supply of the
camel’s head. Brit. Vet. J. 107, 147–155.
3. Nickel R, Schwarz R. 1963 Verleichenden
Betrachtung der Kopfarterien des Haussäugetiere
(Katze, Hund, Schwein, Rind, Schaf, Ziege, Pferd).
Zbl. Vet. Med. A 10, 90–120.
4. Kanan CV. 1972 Observations on the distribution of
the external and internal ophthalmic arteries in the
camel (Camelus dromedarius). Acta Anat. 81, 74–82.
(doi:10.1159/000143747)
5. Badawi H, El-Shaieb M, Kenawy A. 1977 The arteria
maxillaris of the camel (Camelus dromedarius).
Zbl. Vet. Med. C: Anat. Histol. Embryol. 6, 21–28.
(doi:10.1111/j.1439-0264.1977.tb00417.x)
6. Smuts MMS, Bezuidenhout AJ. 1987 Anatomy of the
dromedary. Oxford, UK: Clarendon Press.
7. Frąckowiak H. 2006 The artery of the head in some
mammalian orders. In Animals, zoos and
conservation: zoological garden in Poznan (eds
E Zgrabczyńska, P Ćwiertnia, J Ziomek), pp. 171–180.
Poznań, Poland: Kontekst Publisher House.
8. Jerbi H, Fehri Y, Matoussi A. 2013 The arteries of the
head and neck of one-humped camel (Camelus
dromedarius). Hiron 1, 8–12.
9. Kiełtyka-Kurc A, Frąckowiak H, Zdun M, Nabzdyk M,
Kowalczyk K, Tołkacz M. 2014 The arteries on the
base of the brain in the camelids (Camelidae). Ital. J.
Zool. 81, 215–220. (doi:10.1080/11250003.2014.
901428)
10. Wilson DE, Reeder DM. 2005 Mammal species of the
world: a taxonomic and geographic reference, 3rd
edn, vol. 1. Baltimore, MD: Johns Hopkins University
Press.
11. Baldi B, Lichtenstein G, González B, Funes M, Cuéllar
E, Villalba L, Hoces D, Puig S. 2008 Lama guanicoe.
The IUCN Red List of Threatened Species
e.T11186A3260654.
12. Lichtenstein G, Baldi R, Villalba L, Hoces D, Baigún
R, Laker J. 2008 Vicugna vicugna. The IUCN Red List
of Threatened Species e.T22956A9402796.
13. Wheeler JC. 2012 South American camelids- past,
present and future. J. Camelid Sci. 5, 1–24.
14. Sillau AH, Cueva S, Valenzuela A, Candela E. 1976 O2
transport in the alpaca (Lama pacos) at sea level
and at 3,300 m. Resp. Physiol. 27, 147–155.
(doi:10.1016/0034-5687(76)90070-0)
15. Harris P, Heath D, Smith P, Williams DR, Ramirez A,
Krüger H, Jones DM. 1983 Pulmonary circulation of
the llama at high and low altitudes. Thorax 37,
38–45. (doi:10.1136/thx.37.1.38)
16. Heath D, Williams DR. 1959 Life at high altitude.
Baltimore, MD: University Park Press.
17. Monge C, León-Vellarde F. 1991 Physiological
adaptations to high altitude: oxygen transport in
mammals and birds. Physiol. Rev. 71, 1135–1172.
18. Negus VE. 1958 The comparative anatomy and
physiology of the nose and paranasal sinuses.
London, UK: Harcourt Brace/Churchill Livingstone.
rsos.royalsocietypublishing.org R. Soc. open sci. 4: 160967
References
11
Downloaded from http://rsos.royalsocietypublishing.org/ on May 7, 2017
66.
67.
68.
69.
70.
71.
Embryol. 28, 271–272. (doi:10.1046/j.1439-0264.
1999.00199.x)
Price SA, Bininda-Emonds ORP, Gittleman JL. 2005
A complete phylogeny of the whales, dolphins and
even-toed hoofed mammals (Cetartiodactyla).
Biol. Rev. 80, 445–473. (doi:10.1017/S1464793105
006743)
O’Leary MA, Gatesy J. 2008 Impact of increased
character sampling on the phylogeny of
Cetartiodactyla (Mammalia): combined analysis
including fossils. Cladistics 24, 397–442.
(doi:10.1111/j.1096-0031.2007.00187.x)
Spaulding M, O’Leary MA, Gatesy J. 2009
Relationships of Cetacea (Artiodactyla) among
mammals: increased taxon sampling alters
interpretations of key fossils and character
evolution. PLoS ONE 4, e7062. (doi:10.1371/journal.
pone.0007062)
Hassanin A et al. 2013 Pattern and timing of
diversification of Cetartiodactyla (Mammalia,
Laurasiatheria), as revealed by a comprehensive
analysis of mitochondrial genomes. C. R. Biol. 335,
32–50. (doi:10.1016/j.crvi.2011.11.002)
San-Martin M, Copaira M, Zuniga J, Rodreguez R,
Bustinza G, Acosta L. 1968 Aspects of reproduction
in the alpaca. J. Reprod. Fertil. 16, 395–399.
(doi:10.1530/jrf.0.0160395)
Skinner JD, Hall-Martin AJ. 1975 A note on foetal
growth and development of the giraffe Giraffa
camelopardalis giraffa. J. Zool. 1, 73–79.
12
................................................
57. O’Leary MA. 2010 An anatomical and phylogenetic
study of the osteology of the petrosal of extant and
extinct artiodactylans (Mammalia) and relatives.
Bull. Am. Mus. Nat. Hist. 335, 1–206. (doi:10.1206/
335.1)
58. Luo Z, Gingerich PD. 1999 Terrestrial Mesonychia to
aquatic Cetacea: transformation of the basicranium
and evolution of hearing in whales. Univ. Michigan
Papers Paleontol. 31, 1–98.
59. Wible JR. 2003 On the cranial osteology of the
short-tailed opossum Monodelphis brevicaudata
(Didelphidae, Marsupialia). Ann. Carnegie Mus. 72,
137–202.
60. Sisson S, Grossman JD. 1967 The anatomy of the
domestic animals, 4th edn. Philadelphia, PA: WB
Saunders.
61. Lawrence WE, Rewell RE. 1948 The cerebral blood
supply in the Giraffidae. Proc. Zool. Soc. Lond. 118,
202–212. (doi:10.1111/j.1096-3642.1948.tb00373.x)
62. Padget DH. 1948 The development of the cranial
arteries in the human embryo. Contrib. Embryol. 32,
205–262.
63. Tandler J. 1899 Zur verleichenden Anatomie der
Kopfarterien bei den Mammalia. Denkschr. oest.
Acad. Wiss. Mathem. Naturw. Klasse. 67, 677–784.
64. Kim K-R, Moon H-E, Kim K-W. 2012 Hypoxia-induced
angiogenesis in human hepatocellular carcinoma.
J. Mol. Med. 80, 703–714. (doi:10.1007/s00109-0020380-0)
65. Ocal MK, Erden H, Ogut I, Kara ME. 1999 A
quantitative study of the circulus arteriosus cerebri
of the camel (Camelus dromedarius). Anat. Histol.
rsos.royalsocietypublishing.org R. Soc. open sci. 4: 160967
49. Holliday CM, Ridgely RC, Balanoff AM, Witmer LM.
2006 Cephalic vascular anatomy in flamingos
(Phoenicopterus ruber) based on novel vascular
injection and computed tomographic imaging
analysis. Anat. Rec. 288A, 1031–1041. (doi:10.1002/
ar.a.20374)
50. O’Brien HD, Williams SH. 2014 Using biplanar
fluoroscopy to guide radiopaque vascular
injections: a new method for vascular imaging. PLoS
ONE 9, e97940. (doi:10.1371/journal.pone.
0097940)
51. O’Leary MA. 2016 Comparative basicranial anatomy
of extant terrestrial and semiaquatic Artiodactyla.
Bull. Am. Mus. Nat. Hist. 409, 1–55. (doi:10.1206/
0003-0090-409.1.1)
52. Van Kampen PN. 1905 Die Tympanalgegend des
Säugetierschädels. Gegenbaurs Morphol. Jahrb. 34,
321–722.
53. Conroy GC, Wible JR. 1978 Middle ear morphology of
Lemur variegatus: some implications for primate
paleontology. Folia Primatol. 29, 81–85.
(doi:10.1159/000155829)
54. Cartmill M, MacPhee RDE, Simons EL. 1981 Anatomy
of the temporal bone in early anthropoids, with
remarks on the problem of anthropoid origins. Am.
J. Phys. Anth. 56, 3–21. (doi:10.1002/ajpa.13305
60102)
55. MacPhee RDE. 1981 Auditory regions of primates
and eutherian insectivores: morphology, ontogeny,
and character analysis. Contrib. Primatol. 19,
1–282.
56. Wible JR. 1983 The internal carotid artery in early
eutherians. Acta Palaeontol. Polonica 28, 174–180.