416 THE HORS. 
THE OCCIPITAL ARTERY passes backwards, deeply hidden by the muscle, 
of the neck and the transverse process of the atlas, where it unites with the 
vertebral artery. 
THE INTERNAL CAROTID, a2 comparatively small artery, ascends towards 
the base of the skull, which it enters at the point of the petrous part of 
the temporal bone, and supplies the brain in ¢ommon with the vertebrai 
artery, with which it freely anastomoses. 
THE POSTERIOR AORTA must now be described. It is much longer and 
of larger diameter than the anterior, commencing opposite the fourth 
dorsal vertebra, where it lies at some little distance below the body of 
that bone. Passing upwards and backwards it becomes closely connected 
with the bodies of the vertebra, lying a little to the left, and having the 
cesophagus and vena azygos on the right, and the thoracic duct on the left. 
Here it is called the thoracic aorta; but passing through the crura of the 
diaphragm it enters the abdomen, and receives the name of abdominal 
aorta. The thoracic division supplies small branches to the bronchi and 
cesophacus, as well as the intercostal arteries to all but the four or five an- 
terior intercostal spaces. After passing through the diaphragm, the aorta 
gives off the phrenic arteries right and left to the diaphragm, and then 
supplies the important arteries of the viscera, namely : (a) the Celiac 
artery, dividing into the splenic, gastric, and hepatic arteries ; (6) the 
anterior mesenteric ; (c) the renal; (d) the spermatic ; (e) the posterior 
mesenteric; (f) the lumbar arteries; and finally, just below the last 
lumbar vertebra, it subdivides into (g) the two internal, and (h) the two 
external iliac arteries. In the horse there is no common iliac artery, 
as in man, the four being given off in one group, but the two internals 
generally forming a short continuation of the trunk. 
THE INTERNAL ILIAC ARTERY has a very short trunk, which passes 
backwards and outwards in close connexion with the sacrum. Ite first 
branch is (a) the umbilical artery. It then gives off (6) the artery of the 
bulb, after which and just opposite the sacro-iliac articulation it divides 
into a leash of branches, which are (c) the obturator, (d) the lateral sacral, 
and (e) the gluteal artery. The wmbdlical artery is almost entirely obli- 
terated in the adult, but a small branch still remains passing along the 
cord which exists as the only remnant of the large artery which in the 
foetus carries on the circulation peculiar to that condition. Zhe artery of 
the buib supplies the bladder and the internal organs of generation. The 
obturator artery give off branches to the muscles of the haunch, and finally 
ends in the internal pubic artery, which gives blood to the penis and 
adjacent organs. The lateral sacral artery proceeds backwards along the 
side of the sacrum to the bones of the tail, along which it ramifies. 
Lastly, the gluteal artery passes out of the pelvis through the hole in the 
sacro-sciatic ligament in company with the sciatic nerve, and supplies 
muscular branches to the glutei. 
THE EXTERNAL ILIAC ARTERY is smaller than the internal, and takes the 
same course as far as the articulation, beyond which it passes, lying just 
within the brim of the pelvis, in close contact with the psoas and iliacus 
muscles and covered by the peritoneum. About midway between the 
symphisis pubis and the anterior spinous process of the ilium it gives off 
the circumflex artery of the ilium, and then receives the name of the 
femoral artery. At this point the femoral vein lies posterior to it, and it 
is also accompanied by the internal saphena nerve. Proceeding in an 
oblique direction down the middle of the haunch, it reaches the hollow at 
the back of the stifle joint, where it is called the popliteal artery, and 
