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The structural organization and adrenergic innervation of the carotid arterial system of the giraffe (Giraffa camelopardalis).

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THE ANATOMICAL RECORD 230:369-377 (1991)
The Structural Organization and Adrenergic
Innervation of the Carotid Arterial System of the
Giraffe (Giraffa camelopardalis)
JAMES KIRUMBI KIMANI AND ISAAC 0. OPOLE
Department of Human Anatomy, University of Nairobi, Nairobi, Kenya
ABSTRACT
The sympathetic innervation of the giraffe (Giraffacamelopardalis) carotid arterial system is described in this study using the sucrose-potassium
phosphate-glyoxylic acid (SPG) method. The brachiocephalic and bicarotid trunks
showed a paucity of sympathetic innervation. Smooth muscle nests observed in the
outer layers of the tunica media in these arteries revealed a rich network of sympathetic nerve fibres. The common carotid artery showed numerous sympathetic
nerve fibres particularly in the outer muscular zone of the tunica media. The
internal maxillary, ramus anastomoticus, and arteria anastomotica also revealed
a rich sympathetic innervation and a deep penetration of the nerve fibres into the
tunica media. It is suggested that the rich sympathetic innervation of the giraffe
carotid arteries maintains a basal tonic state in the smooth muscle in the tunica
media. This, in turn, may enable the animal to maintain a relatively high rate of
blood flow in the carotid arteries in diastole despite the pressure run-off. It is
further suggested that the muscular structure and dense sympathetic innervation
of the internal maxillary and its branches to the carotid rete mirabile provide the
animal with a n array of mechanisms to modulate its cranial circulation particularly when i t bends its head to drink.
The long neck of the giraffe poses a number of haemodynamic problems to this animal, as the heart lies
approximately midway between the ground and the
head, a total distance of about 4-5 m in the adult (Fig.
la). The heart, therefore, placed 2-2.5 m below the
head, has to generate enough pressure to overcome the
hydrostatic pressure of approximately 200 mm Hg to
permit adequate perfusion of the brain (Goetz and
Keen, 1957; Goetz et al., 1960; Goetz, 1962). The brain,
on the other hand, has to be protected from excessive
gravitational pressure when the animal brings its head
down to drink, a difference in height, and concomitantly, therefore, a hydrostatic pressure, of up to 2.5 m
of water in addition to the pressure generated by the
heart (Goetz and Keen, 1957; Goetz et al., 1960; Goetz,
1962; Van Citters et al., 1966, 1968, 1969; Warren,
1974; McCalden et al., 1977). However, as noted by
Kimani (1987) neither the presence of a n extracranial
carotid-vertebral anastomosis (Lawrence and Rewell,
1948), nor the presence of a n intracranial rete mirabile
caroticum in the cavernous venous sinus (Goetz and
Keen, 1957; Goetz, 19621, nor the antigravity effect of
the cerebrospinal fluid suggested by Warren (19741,
seems to explain the mechanisms involved completely.
Faced with the above dilemma, Kimani (1979) suggested t h a t some of the mechanisms by which the giraffe ensures a n adequate cerebral perfusion pressure,
and protection of the brain when i t lowers its head to
drink, may be a function of the hypertrophied wall of
the left ventricle, the morphology of the blood vessels,
and appropriate barostatic reflexes. A corollary to this
suggestion is the demonstration in a recent study (Ki0 1991 WILEY-LISS, INC
mani, 1987) that the carotid and vertebral arteries,
although found in the neck, have a different structural
organization which, in turn, may imply that the two
blood vessels are subjected to different haemodynamic
demands. Pertinent to this is also the fact that, in the
giraffe, as in most other artiodactyls the vertebral
blood does not participate in the supply of cephalic
structures because it is confined to the cervical region
by the pressure barrier in the carotid-vertebral anastomosis (Daniel et al., 1953; Appleton and Waites,
1957; Waites, 1960; Baldwin and Bell, 1963a-c; Baldwin, 1964; Du Boulay and Verity, 1973). It is conceivable, therefore, that the structural organization of the
carotid arterial system of the giraffe, characterised, in
the main, by a largely muscular structure, constitutes
part of the remarkable ability of this animal to autoregulate its cranial circulation.
Little is known, however, about the autonomic innervation of either the common carotid arteries andlor
their intra- and extracranial branches in this animal.
This study therefore looks at the distribution of sympathetic nerves in the carotid arterial system of the
giraffe. An attempt is made to elucidate further the
role of the sympathetic innervation in the regulation of
cerebral blood flow in this animal.
Received March 19, 1990; accepted November 26, 1990.
Address reprint requests to Professor James K. Kimani, Department of Human Anatomy, University of Nairobi, P.O. Box 30197,
Nairobi, Kenya.
370
J.K. KIMANI AND 1.0.OPOLE
Fig. 1, a: A photograph of the giraffe to demonstrate its horn-hoof
height of approximately 4-5 m. Note the relatively long neck and the
approximate position of the heart (arrow). b A photograph showing
the terminal portion of the giraffe left common carotid artery at the
level of the angle of the mandible and the carotid rete after dissection
and removal from a gross specimen. 1, common carotid artery; 2,
vertebral artery; 3, carotid-vertebral anastomotic branch; 4, remnants of the internal carotid artery (reflected downward); 5, occipital
artery; 6, glossopharyngeal nerve; 7, caudal auricular artery; 8, su-
perficial temporal artery; 9, linguofacial trunk; 10, external maxillary artery; 11,inferior alveolar artery; 12, ramus anastomoticus; 13,
internal maxillary artery; 14, sphenopalatine artery; 15,infraorbital
artery; 16, external ophthalmic artery; 17, arteria anastomotica; 18,
carotid rete mirabile. c: A photograph showing the branching of the
aorta and brachiocephalic trunk in the giraffe. 1,pulmonary trunk; 2,
arch of the aorta; Za, left vagus nerve; 3, brachiocephalic trunk; 4, left
subclavian artery; 5, right subclavian artery; 6, left vertebral artery;
7, right vagus nerve; 8, bicarotid trunk; 9, common carotid arteries.
MATERIALS AND METHODS
Materials used in this study were obtained from a
total of 15 adult Maasai giraffes (Giraffa camelopardalis). The animals were collected over a period of 15
years under a licence issued by the Department of
Wildlife Conservation and Management of the Ministry of Tourism and Wildlife of the Government of
Kenya. Due to stringent culling regulations only two
females were used in this study. One of them had died
a t the Nairobi National Park, while the other had been
accidentally wounded by a stray bullet and had to be
sacrificed. The animals were otherwise healthy.
Materials for light microscopic study were obtained
from 10 animals. The neck was separated from the
trunk between the seventh cervical and first thoracic
vertebrae and later fixed by intracarotid and intravertebral perfusion with 10% formal-saline solution. The
heart and the major blood vessels arising from i t were
removed in the field and immediately fixed by immersion in a solution of 10% formal-saline. The specimens
were then dissected for gross study of the cranial vascular system subsequent to which histological specimens were obtained serially a t regular intervals of 5
cm caudocranially. These specimens were processed for
paraffin-embedding, sectioning, and staining with
Weigert’s resorcin fuchsin counterstained with Van
Gieson’s stain or Masson’s trichrome stain.
Materials for fluorescent histochemistry were removed from five animals immediately after the animals were shot. The specimens were excised, quickly
cut into small blocks, wrapped in aluminium foil, and
immediately frozen in dry ice in a thermos flask. The
specimens were then transported to the laboratory for
cryostat sectioning. The tissue specimens were embedded using OCT compound (Tissue-Tek 11) on a precooled chuck in a cryostat chamber at -30°C. Sections
were cut at 16 pm and prepared for fluorescent microscopy using the sucrose-potassium phosphate-glyoxylic
acid (SPG) method for tissue monoamines according to
de la Torre and Surgeon (1976) as follows: cryostat sec-
CAROTID ARTERIAL SYSTEM OF G. CAMELOE’ARDALZS
tions were picked up on glass slides and immediately
dipped three times in SPG solution containing 6.8%
sucrose, 3.2% potassium phosphate, and 1.0% glyoxylic
acid a t pH 7.4. The slides were then blow-dried for 5
minutes using a hair drier set a t “medium” point to
give a drying temperature of 4WC, subsequent to which
the slides were placed on a flat metal plate in a n oven
maintained a t 100°C for 5 minutes with the sections
being covered with liquid paraffin. The slides were removed from the oven, mounted under coverslips using
fresh liquid paraffin, and examined with a Leitz Ortholux fluorescent microscope, using a 25014 ultra high
pressure mercury lamp with a Leitz BP. 546120 filter
block. Photographs were taken using Kodak Tri-X 400
film.
After cryostat sectioning for fluorescence histochemistry, tissue blocks were removed from the chucks and
immersed in 4% formal-saline solution for 7 days and
processed for light microscopic study a s outlined above.
RESULTS
Gross Anatomical Findings
The ascending aorta of the giraffe gives rise to a
single brachiocephalic trunk which sends off a subclavian artery on either side (4 and 5 in Fig. lc) and gives
rise to the vertebral arteries (6 in Fig. lc) before continuing as the bicarotid trunk (8 in Fig. lc). The bicarotid trunk, a short vessel measuring about 10 cm in
length in the adult, divides at the root of the neck into
right and left common carotid arteries (9, 9 in Fig. lc).
Each of these ascends deep to the jugular vein, to the
base of the skull where they branch dorsally and ventrally, dividing into the two terminal branches, the internal maxillary and superficial temporal arteries (13
and 8 in Fig. lb).
The first dorsal branch is the anastomotic branch
between the carotid and vertebral arteries, whose origin is variable (3 in Fig. lb). The next dorsal branch is
the vestigial extracranial portion of the internal carotid artery (4 in Fig. lb). This cord-like structure proceeds medially to the other branches of the carotid artery to enter the base of the skull (Fig. lb). The carotid
artery then gives the occipital and caudal auricular
arteries (5 and 7 in Fig. lb). Ventrally, the carotid artery gives the lingual and external maxillary (or facial)
arteries (9 and 10 in Fig. lb). The internal maxillary
artery gives the ramus anastomoticus and arteria
anastomotica, supplying the carotid rete (12 and 17 in
Fib. lb). Other branches arising from the internal
maxillary are the inferior alveolar (mandibular alveolar), sphenopalatine, infraorbital, and malar arteries
(11-16 in Fig. lb). Through the carotid rete (17 and 18
in Fig. lb), the internal maxillary artery constitutes
the principal source of blood supply to the brain.
Histological Findings
The brachiocephalic trunk, as well a s the initial portion of the bicarotid trunk, are thick, elastic vessels
consisting mainly of parallel elastic lamellae between
which are smooth muscle cells, collagenous tissue, and
a few elastic fibers, as described by Kimani (1983).
Patches or nests of smooth muscle cells are found in the
outer zones of the tunica media of the brachiocephalic
trunk (Fig. 2a).
The terminal portion of the bicarotid trunk and the
371
initial part of the common carotid artery show a transmural zonation characterised by a luminal elastic zone
and a n adventitial muscular zone (Fig. 2b). The transformation from the proximal carotid segment as the
carotid artery is followed cranially in the neck is characterised by a marked attenuation of the elastic fibre
content on the luminal side of the tunica media and a
corresponding increase in smooth muscle content (Fig.
2c).
The internal maxillary and the superficial temporal
arteries both have a largely muscular tunica media
surrounded by a prominent tunica adventitia (Fig. 2d).
A number of muscle bundles ensheathed by fibroelastic
trabeculae were seen at the media-adventitial junction.
The rest of the tunica media showed a fine network of
elastic fibres between the smooth muscle cells.
The ramus anastomoticus and arteria anastomotica
(12 and 17 in Fig. l b ) showed a fairly thick and preponderantly muscular tunica media, surrounded by a
prominent tunica adventitia (Fig. 2e, f). The smooth
muscle content revealed a compact organization with
minimal connective tissue trabeculae as seen in the
common carotid artery (Fig. 2e, f). Arterioles and endothelium-lined spaces, conceivably venules, were observed in the tunica adventitia (Fig. 2f).
Fluorescent Histochemical Findings
In the brachiocephalic and bicarotid trunks, fluorescent histochemistry revealed the presence of adrenergic nerve terminals within the muscular nests described in the outer zone of the tunica media (Fig. 3).In
contrast, relatively few adrenergic nerve terminals
were demonstrated within the contiguous elastic
lamellae. The carotid artery, on the other hand, showed
a rich adrenergic innervation, with adrenergic nerves
being found well into the tunica media, specifically in
the outer muscular layers (Fig. 4a, b). These adrenergic
nerve varicosities were mainly located in the fibroelastic septa, previously noted between the smooth muscle
bundles (Fig. 2c). The inner half to one third of the
tunica media showed a paucity of adrenergic nerves,
while no nerve fibres were demonstrated in the tunica
intima (Fig. 4a). Regional differences between the
proximal, middle, and distal segments were not apparent, except for a craniad deeper penetration of nerves
into the tunica media (Fig. 5a, b).
The superficial temporal and internal maxillary arteries revealed a rich sympathetic innervation, with
fluorophores occurring throughout the outer two thirds
(Fig. 6a, b). Sympathetic nerve fibres were demonstrated deep in the tunica media, although no fibres
were seen either in the tunica intima or the juxtaintima1 zone of the tunica media (Fig. 6b).
The ramus anastomoticus and arteria anastomotica
were also densely supplied with adrenergic nerves. The
outer halfltwo thirds of the tunica media in both arteries showed intense fluorescence of sympathetic nerve
fibres (Fig. 7a, b). A few fluorophores were found in the
inner half of the tunica media, but the juxtaintimal
zone was free of sympathetic nerve fibres (Fig. 7b).
DISCUSSION
Observations made in this study have revealed that
the giraffe carotid arteries have a dense sympathetic
innervation in which the nerves are characteristically
372
J.K. KIMANI AND 1.0. OPOLE
Fig. 2. Light photomicrographs of the carotid arterial system of the
giraffe. a: Outer layer of the tunica media of the brachiocephalic artery showing a muscular nest (m) within the tunica media. The luminal side shows concentric layers of elastic lamellae (E). Weigert’si
Van Giesson’s stain. ~ 1 2 5 b. Proximal segment of the common
carotid artery close to its origin from the bicarotid trunk. Note the
transmural zonation of the tunica media into a luminal elastic (a) and
an outer muscular (b) zone. L, lumen; B, tunica adventitia. Weigert’si
. Common carotid artery about 50 cm
Van Giesson’s stain. ~ 3 0 c:
cranial to its origin from the bicarotid trunk. Note the presence of
bundles of smooth muscle (m) in the tunica media (A) separated by
fibroelastic septa (s). L, lumen, B, tunica adventitia. Masson’s tri-
chrome stain. x 30. d Proximal segment of the internal maxillary
artery. The tunica media (A)shows a largely muscular structure with
a few isolated bundles a t the media-adventitia border. L, lumen; arrow, internal elastic lamina; B, tunica adventitia. Masson’s trichrome
stain. x 125. e: Ramus anastomoticus showing compact smooth muscle in the tunica media (A). Note the presence of arterioles and
venules (arrows) in the tunica adventitia (B).Masson’s trichrome
stain. x 125. f: One of the branches of the arteria anastomotica. Note
the compact arrangement of smooth cells in the tunica media (A). B,
tunica adventitia; L, lumen; arrow, internal elastic lamina. Masson’s
trichrome stain. x 125.
located in the outer muscular zone of the tunica media.
This is in conformity with recent findings based on
immunofluorescence against specific antisera to dopamine-p-hydroxylase, neuropeptide Y, neurofilament, and synapsin I (Nilsson e t al., 1988). However,
the material used by Nilsson e t al. (1988) was limited
and no attempts were made to demonstrate the mode of
innervation of the cephalic branches of the carotid artery, particularly those that supply the brain. In the
present study sympathetic nerves were found to penetrate very deeply into the tunica media of the superficial temporal, internal maxillary, ramus anastomoticus, and the arteria anastomotica compared to the
proximal segments of the carotid artery near the root of
the neck.
As noted by Nilsson et al. (1988), the physiological
significance of the dense innervation of the giraffe carotid arteries is not known. Keatinge and Torrie (1976)
have shown that in the sheep carotid artery the luminal smooth muscle cells are more responsive to circulating catecholamines than the outer innervated medial smooth muscle cells. Mekata (1984) has also
demonstrated different electrical responses between
the outer and inner smooth muscle cells of the rabbit
carotid artery to adrenaline and sympathetic nerves.
Perhaps a similar functional heterogeneity exists between the smooth muscle cells in the non-innervated
adluminal zone and those in the outer innervated parts
of the tunica media in the giraffe carotid artery. In this
regard, the latter may respond largely to neurogenic
CAROTID ARTERIAL SYSTEM OF G. CAMELOPARDALIS
373
Fig. 3. Fluorescent histochemical photomicrograph of a muscular nest in the brachiocephalic trunk to
illustrate the presence of sympathetic varicosities (arrows) between the smooth muscle bundles (m). fs,
fibroelastic septa. x 250.
control, and the former to circulating and/or diffusing
catecholamines. The question remains, however, as to
whether the carotid arteries in the giraffe neck maintain a basal tonic state contrary to the views expressed
by Nilsson et al. (1988).
Kimani (1987) has shown that the vertebral artery of
the giraffe has a different structural organization from
the carotid artery (at least in its transition zone). This
indicates that the two blood vessels, though found in
the neck, are made to bear completely different haemodynamic stresses. Furthermore, in the full-term giraffe
fetus this segment of the carotid artery has a preponderantly elastic structure without evidence of transmural zonation (Franklin and Haynes, 1927; Kimani,
1979,1983). This suggests that the presence of smooth
muscle on the adventitial side and the functional maturation of sympathetic innervation occur postnatally.
Thus the structural organization of the carotid arterial
system of the giraffe, characterised in the main by a
largely muscular structure and a rich sympathetic innervation constitutes part of the anatomical basis for
the remarkable ability of this animal to autoregulate
its cranial circulation. A corollary to this suggestion is
the fact that a characteristic feature of the behaviour of
the bovine carotid arteries is a contraction which occurs when the internal pressure is raised above 130 cm
of water (Jaeger, 1962). Other workers have shown
that a rapid stretch of a vascular wall preparation, a s
may occur in systole, causes a sudden increase in tension followed by a n exponential rather than a precipitous decay (Speden, 1960; Sparks, 1964; Goto and Kimoto, 1966; Somlyo and Somlyo, 1968). However, a s
noted in the cerebral arteries (Hernandez et al., 19711
1972), the rich sympathetic innervation of the giraffe
carotid artery may maintain a basal tonic state. This,
in turn, may enable the artery to respond in a more
myogenic fashion when subjected to stretch unlike a
dennervated vessel which acts more like a flaccid tube.
According to Van Citters et al. (1968,1969),the giraffe
can maintain a relatively high rate of blood flow in the
carotid arteries in diastole despite the pressure run-off.
Other studies have demonstrated the presence of a
standing wave in the carotid arterial system of this
animal whose node is somewhere in the proximal part
(Goetz et al., 1960; Van Citters et al., 1968).
Previous studies have, however, pointed out that the
systolic pressure generated by the giraffe heart is in
favour of a n unassisted cerebral circulation such a s a
peristaltic wave along the carotid artery andior a siphon-like effect of the venous blood rushing down the
jugular vein (Goetz and Keen, 1957; Goetz et al., 1960;
Goetz, 1960). Badeer (1986) has argued strongly that
gravitational forces acting in a “siphon” mechanism
could ensure blood flow through the cerebral vascular
bed in an arrangement where the negative pressure in
the jugular venous system offsets that in the carotid
arterial system necessary for cerebral perfusion. These
suggestions, although attractive, fail to explain why
structural differences exist between the carotid and
vertebral arterial systems (Kimani, 1983, 1987).
The head-down position in the giraffe introduces a
totally new, and directly opposite, plethora of problems
that have intrigued scientists for many years. Physiological studies have demonstrated that the systolic
blood pressure a t the base of the skull rises from 120350 mm Hg when a standing animal lowers its head
(Goetz and Keen, 1957; Goetz e t al., 1960; Van Citters
et al., 1966, 1968, 1969; Warren, 1974).Attempts have
been made in previous studies to explain the mechanisms that underlie the ability of this animal to lower
or raise its head without damaging the cerebral blood
vessels, but neither the presence of the rete mirabile
in the cavernous venous sinus (Goetz and Keen,
1957; Goetz et al., 1960, Goetz, 1962), nor the occurrence of a n extracranial carotid-vertebral anastomosis
(Lawrence and Rewell, 1948), nor the antigravity effect
374
J.K. KIMANI AND 1.0. OPOLE
Fig. 4. Fluorescent histochemical photomicrographs of the common
carotid artery a few centimetres cranial to its origin from the bicarotid trunk. a: Luminal portion of the tunica media (A)showing elastic layers (arrow head) and a few bright fluorescent sympathetic
nerve fibres (arrow). Note the absence of nerve fibres in the luminal
zone. L, lumen. x 250. b Outer layer of the tunica media (A)to illustrate the presence of sympathetic nerves (arrows) running along the
fibroelastic septa between the smooth muscle bundles (m) as demonstrated in Figure 2c. Note the network of sympathetic nerve fibres a t
the media-adventitia border (arrowheads). x 250.
Fig. 5. Fluorescent histochemical photomicrographs of the distal
segment of the common carotid artery close to the carotid-vertebral
anastomotic branch as shown in Figure Ib. a: Luminal portion of the
tunica media (A) showing a few elastic lamellae (arrowhead). Note
the presence of a few sympathetic nerves (arrow) deep in the tunica
media and their absence in the luminal zone. E, internal elastic lamina; L, lumen. x 400. b Outer layer of the tunica media (A) to illustrate the presence of numerous sympathetic nerves (arrows) located
within the fibroelastic septa between bundles of smooth muscle cells
(m). B, tunica adventitia. x 400.
of the cerebrospinal fluid within which the intracranial extracerebral vessels are bathed (Warren, 1974)
seems to explain the mechanisms involved completely.
Little is, however, known about the role of the blood
vessels that supply the brain; namely, the internal
maxillary artery, arteria anastomotica, and ramus
CAROTID ARTERIAL SYSTEM OF G. CAMELOPARDALZS
375
Fig. 6. Fluorescent histochemical photomicrographs of the internal
maxillary artery as shown in Figure lb. a: Proximal part of the internal maxillary artery to show the distribution of sympathetic
nerves (arrows1 in the outer two thirds of the tunica media (A).The
luminal zone is characteristically devoid of nerve fibres. Arrow heads,
media adventitia border; E, internal elastic lamina. x 250. b: Luminal zone ofthe tunica media (A)as shown in Figure 6a above. Note the
presence of sympathetic nerves in the outer zone of the picture (arrows) and their absence in the luminal zone. E, internal elastic lamina. x400.
Fig. 7. Fluorescent histochemical photomicrographs of the arteria
anastomotica as shown in Figure Ib. a: Luminal zone of the tunica
media to demonstrate the deep penetration of sympathetic nerves
(arrow) into the tunica media (A). Note the absence of sympathetic
nerve fibres in the juxtaintimal zone of the tunica media. E, internal
elastic lamina. x 400. b: Outer layers of the tunica media (A1 to show
the rich sympathetic innervation (arrows) in which varicose nerve
fibres run along the fibroelastic layers between smooth muscle cells.
Arrowheads, media adventitia border; B, tunica adventitia. x 400.
anastomoticus. These distributing arteries would be
expected to play a major part in modulating blood flow
to the brain due to their preponderantly muscular
structure a s demonstrated in this study. Pertinent to
this suggestion is the finding that carotid blood flow in
the head-up position is not significantly different from
376
J.K. KIMANI AND 1.0. OPOLE
the values obtained if the animal lowers its head to the grant from the University of Nairobi, Deans Commitground (McCalden et al., 1977). Furthermore, recent tee.
studies have revealed that resistance of large arteries
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animal although their responses to such challenges
carotid arterial system of the giraffe (Giruffu cumelopurdulis)
with special reference to the carotid sinus baroreceptor equivahave not been determined (Van Citters e t al., 1969;
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J.K. 1983 The structural organization of the carotid arterial
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This implies that the giraffe, like other animals, is ca- Kimani, J.K. 1987 Structural organization of the vertebral artery in
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Kimani, J.K., R.N. Mbuva, and R.M. Kinyamu 1991 Sympathetic
attendant haemodynamic challenges.
innervation of the hind-limb arterial system in the giraffe (GiACKNOWLEDGMENTS
The authors are grateful to the Department of Wildlife Conservation and Management in the Ministry of
Tourism and Wildlife of the Government of Kenya for
granting permission to cull the animals used in this
study. Our thanks also go to the technical staff in the
Department of Human Anatomy, the University of
Nairobi, for their technical assistance, and lastly to
Mrs. Mary Kiarie and Ms. Hydah Were for typing the
manuscript. This work was supported by a research
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SOC.
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