Abdomen and Pelvis 
positioned about 1 to 2 mm more cranial than the 
left adrenal gland. The right lateral lobe of the liver 
overlaps the right adrenal ventrally. Both adrenal 
glands are almost completely embedded in areolar 
tissue containing much fat (Fig. 5-48). 
The red brow^n to dark brow^n color of the smooth 
surfaces of the adrenal glands resembles that of the 
kidneys (Fig. 5-49). The glands are ovoid in shape, 
with a length of about 4 mm and width about 2 mm 
(Tables 3, 5, 12-12e). 
Histologically, the adrenals show the typical structure of a darker 
cortical substance and a higher medullary substance (Fig. 5-57). The 
cortex of the adrenal glands exhibits an indistinct segmentation because 
the transition of the zona fasciculata, composed of small columnar cells, 
into the zona glomerulosa and the zona reticularis is indistinctly de- 
marcated. The substance of the medulla is constructed from single cords 
that are separated bv vessels and vascular capillaries. 
The suprarenal arteries (aa. suprarenales), 
which arise from the renal arteries, supply the 
adrenal glands; the venous return is via the supra- 
renal veins {vu. suprarenales) , which join the renal 
veins. 
5.6.5 Caudomesocolic and 
Retroperitoneal Vascularization 
5.6.5.1 Arteries 
The cranial mesenteric artery (Fig. 5-33) gives 
off a series of intestinal branches to the jejunum (aa. 
jejunales) and ileum {aa. dei); an ileocolic branch 
{a. ileocolica) to the ileum, caecum and the proxi- 
mal colon; a right colic branch {a. colica dextra) to 
the ascending colon; and a middle colic branch {a. 
colica media) to the transverse colon. The middle 
colic artery often anastomoses with branches from 
the caudal mesenteric artery {a. mesentenca cau- 
dalis), which arises from the ventral aorta near the 
level of its origin. The left colic branch of the caudal 
mesenteric artery supplies the descending colon, 
while the cranial rectal artery is the caudal continu- 
ation of the caudal mesenteric artery, and supplies 
the rectum (Fig. 5-58). 
Close to the origin of the cranial mesenteric ar- 
tery, the paired renal arteries arise to supply the 
kidneys, adrenals and other retroperitoneal tissue 
(Fig. 5-34). From the ventral side of the aorta at the 
level of the caudal pole of the left kidney, two rela- 
tively small vessels, the testicular arteries (aa. testi- 
culares), originate in males, passing eventually 
through the inguinal canal {canalis inguinalis); the 
ovarian arteries (aa. ovaricae) are the female 
counterpart. 
At the level of the cranial pole of the left kidney, 
the abdominal aorta approaches the left wall of the 
caudal vena cava and, after crossing the left renal 
vein dorsally, is applied ventrally to the vena cava 
until it bifurcates at the level of the sixth lumbar 
vertebra into its largest branches, the paired com- 
mon iliac arteries (aa. iliacae communes), (Fig. 5- 
58) and the smaller median sacral artery (a. sacralis 
mediana) and median caudal artery (a. caudalis 
mediana), which continue the aorta into the tail. 
The common iliac arteries run initially along the 
medial borders of the psoas major muscles and then 
turn obliquely laterally. In males, they are covered 
by the vesicular glands (§■//. vesiculares). In the 
female, they are crossed ventrally by the uterine 
horns, caudal to which they bifurcate into the in- 
ternal and external iliac arteries (aa. iliacae in- 
ternae et externae). 
5.6.5.2 Veins 
The caudal vena cava (vena cava caudalis) is 
formed from the junction of the right and left com- 
mon iliac veins (vv. iliacae communes) at an acute 
angle craniodorsal to the bifurcation of the abdom- 
inal aorta at the level of the sixth lumbar vertebra. 
Each common iliac vein is formed by the junction of 
the external and internal iliac veins (vv. iliacae 
externae et internae) a short distance from the ingu- 
inal ligament (lig. inguinale) just medial to the 
origin of the respective arteries. Shortly after its 
point of origin, the vena cava crosses dorsal to the 
right common iliac artery and continues cranially 
along the right side of the abdominal aorta. At 
approximately the same level as the origin of the 
testicular arteries, the caudal vena cava takes up the 
right testicular vein (v. testiculans), while the left 
testicular vein empties into the caudal side of the 
left renal vein. In females, both ovarian veins (vv. 
ovaricae) empty symmetrically into the caudal vena 
cava. Cranial to the terminus of the right testicular 
vein, the caudal vena cava takes up the renal veins. 
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