The structural organization and adrenergic innervation of the carotid arterial system of the giraffe (Giraffa camelopardalis).код для вставкиСкачать
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 LITERATURE CITED appears to be greater in the cerebral circulation than in other vascular beds (Faraci and Heistad, 1990). This Appleton, A.B., and G.M.H. Waites 1957 A surgical approach to the study has demonstrated that the cephalic segment of superior cervical ganglion and related structures in the sheep. J. Physiol., (Lond.), 135:52-57. the carotid artery, the internal maxillary artery, raH.S. 1986 Does gravitational pressure of blood hinder flow to mus anastomoticus, and the arteria anastomotica are Badeer, the brain of the giraffe? Comp. Biochem. Physiol., 83A:207-211. richly supplied by adrenergic nerves which penetrate Baldwin, B.A. 1964 The anatomy of the arterial supply to the cranial deeply into the tunica media. White et al. (1973) demregions of the sheep and ox. Am. J. Anat., 115:lOl-118. onstrated in the seal, for example, that the proximal Baldwin, B.A., and F.R. Bell 1963a Blood-flow in the carotid and vertebral arteries of the sheep and calf. J. Physiol., (Lond.), 167: segments of the arteries that vasoconstrict during the 448-462. so-called diving reflex, such as the renal, mesenteric, Baldwin, B.A., and F.R. Bell 1963b The effect on blood pressure in the and femoral arteries, show a high degree of penetration sheep and calf of clamping some of the arteries contributing to the cephalic circulation. J. Physiol., (Lond.), 167:463-479. of adrenergic nerves into the tunica media. In contrast, G.A., and F.R. Bell 1963c The anatomy of the cerebral cirthose arteries that remain patent, such as the coronary Baldwin, culation of the sheep and ox. The dynamic distribution of the and cerebral arteries, showed adrenergic nerves blood supplied by the carotid and vertebral arteries to the cranial largely a t the media-adventitia junction (White et al., regions. J. Anat., 97:203-215. Daniel, P.M., J.D.K. Dawes, and M.M.L. Prichard 1953 Studies on the 1973). carotid rete and its associated arteries. Philos. Trans. R. SOC. The rich innervation of the internal maxillary arLond., (Biol.), 237Bt173-208. tery, ramus anastomoticus, and arteria anastomotica de la Torre, J.C., and J.W. Surgeon 1976 A methodological approach in the giraffe resembles that of the vessels that unto rapid and sensitive monoamine histofluorescence using a modified, glyoxylic acid technique: The SPG method. Histochemistry, dergo reflex vasoconstriction in the diving animals. It 49%-89. is suggested that these vessels undergo reflex vasocon- Du Boulay, G.H., and P.M. Verity 1973 The Cranial Arteries of Mamstriction a s the animal lowers the head in a similar mals. Williams Heinemann (William) Medical Books, London manner a s the renal, femoral, and mesenteric arteries and Tonbridge. do in the seal. This would give rise to a n increased Faraci, F.M.. and D.D. Heistad 1990 Regulation of laree cerebral arteries and cerebral microvascular pressure. Circ. &s., 66t8-17. resistance to blood flow which, in turn, opposes the Franklin, K.J., and F. Haynes 1927 The histology of the giraffe’s tendency toward increased blood pressure to cause incarotid, functionally considered. J. Anat., 62:115-117. creased blood flow. Acting in this manner, these ves- Goetz, R.H. 1962 A study in the adaptations of the circulation to gravitational stresses. Proc. Rudolf Virchow Med. SOC. New York, sels may provide a “physiological sphincter” or “gate21:llO-127. valve control” mechanism similar to that described in Goetz, R.H., and E.N. Keen 1957 Some aspects of the cardiovascular the limb arteries of this animal where a n anatomical system in the giraffe. Angiology, 8:542-564. constriction, composed of a hypertrophied tunica media Goetz, R.H., J.V. Warren, O.H. Gauer, J.L. Patterson, Jr., J.T. Doyle, E.N. Keen, and M. McGregor 1960 Circulation ofthe giraffe. Circ. and dense sympathetic innervation, has been found Res., 8:1049-1058. (Kimani et al., 1991). Goto, M., and Y. Kimoto 1966 Hysteresis in stress-relaxation of blood It is concluded that the arteries in the cephalic cirvessels studied by a universal tensile testing instrument. Jpn. J. culation are endowed with a muscular structure and Physiol., 16t179-184. sympathetic nerves which in combination may provide Hernandez, M.J., M.E. Raichle, and H.L. Stone 197111972 The role of the sympathetic nervous system in cerebral blood flow autoregthe animal with a n array of mechanisms to control its ulation. Eur. Neurol., 6t175-179. cranial circulation. This control will depend on the pre- Jaeger, M. 1962 Study of the elasticity and tension of the carotid vailing haemodynamic demands and may not require artery of the cow in comparison with the aorta and coronary vessels. Helv. Physiol. Pharmacol. Acta, 20:7-24. any extraneous mechanisms contrary to the views expressed by Warren (1974). In this connection, other Keatinge, W.R., and C. Torrie 1976 Action of sympathetic nerves on inner and outer muscle of sheep carotid artery and effect of presstudies have demonstrated that baroreflexes modulate sure on nerve distribution. J. Physiol., (Lond.),257:699-712. heart rate and other sympathetic regulation in this Kimani, J.K. 1979 Some aspects of the structural organization of the 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; lent. Ph.D. Thesis, University of Nairobi, 417 pp. Millard et al., 1986). Morphological evidence has been Kimani, J.K. 1983 The structural organization of the carotid arterial provided for the existence of a carotid sinus in this system of the giraffe (Giruffu cumelopurdulis). Afr. J. Ecol., 21: animal (Kimani, 1979; Kimani and Mungai, 1983). 317-324. This implies that the giraffe, like other animals, is ca- Kimani, J.K. 1987 Structural organization of the vertebral artery in the giraffe (Grruffu cumelopurdulis). Anat. Rec., 21 7:256-262. pable of regulating its cerebral blood flow despite the 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 ruffu cumelopurdulis). Anat. Rec. 229t103-108. Kimani, J.K., and J.M. Mungai 1983 Observations on the structure and innervation of the presumptive carotid sinus area in the giraffe (Grruffu cumelopurdulis). Acta Anat., 115t117-133. Lawrence, W.E., and R.E. Rewell 1948 The cerebral blood supply in the giraffdae. Proc. Zool. SOC. Lond., 118:202-212. McCalden, T., D. Borsook, C.J. Shimeli, D. De Vos, P.C. Pieterse, and B. De Klerk 1977 Autoregulation and haemodynamics of giraffe carotid blood flow. South Afr. J. Sci., 73:278-279. Mekata, F. 1984 Different electrical responses of outer and inner muscle of rabbit carotid artery to noradrenaline and nerves. J. Physiol., (Lond.),346589-598. Millard, R.W., A.R. Hargens, K. Johansen, K.S. Pettersson, R. Bur- CAROTID ARTERIAL SYSTEM OF G. CAMELOPARDALZS roughs, D.G.A. Meltzer, D.H. Gershuni, and W. van Hoven 1986 Baroreflex modulates heart rate in the giraffe. Fed. Proc., 45: 758. Nilsson, O . , S. Booj, A. Dahlstrom, A.R. Hargens, R.W. Millard, and K.S. Pettersson 1988 Sympathetic innervation of the cardiovascular system in the giraffe. Blood Vessels, 25:299-307. Somlyo, A.P., and A.V. Somlyo 1968 Vascular smooth muscle. I. Normal structure, pathology, biochemistry and biophysics. Pharmacol. Rev., 20t197-272. Sparks, R.V., J r . 1964 Effects of quick stretch on isolated vascular smooth muscle. Circ. Res., 15t254-260 (suppl., 1). Speden, R.N. 1960 The effects of the initial strip length on the noradrenaline-induced isometric contraction of arterial strips. J. Physiol., (Lond.), 154t15-25. Van Citters, R.L., D.L. Franklin, D.F. Vatner, T. Patrick, and J.V. 377 Warren 1969 Cerebral haernodynamics in the giraffe. Trans. AsAm. Phys., 82:293-303. Van Citters, R.L., W.S. Kemper, and D.L. Franklin 1966 Blood pressure responses of wild giraffes studied by radio telementry. Science, 152t384-386. Van Citters, R.L., W.S. Kemper, and D.L. Franklin 1968 Blood flow and pressure in the giraffe carotid artery. Comp. Biochem. Physiol., 24t1035-1042. Waites, G.M.H. 1960 The influence of the occipito-vertebral anastomoses on the carotid sinus reflex of the sheep. J. Exp. Physiol., 45:243-251. Warren, J.V. 1974 The physiology of the giraffe. Scientific Am., 231: 96-105. White, F.N., M. Ikeda, and R.W. Elsner 1973 Adrenergic innervation of large arteries in the seal. Comp. Gen. Pharmacol., 4t271-276. SOC.