322 THE ANATOMY OP THE HORSE. 



3. The Middle Sacral Artery is an extremely slender vessel, and not 

 always present. Search for it in the angle between the internal iliacs. 

 Arising from the summit of that angle, it passes mesially backwards on 

 the sacrum. It is of interest as representing the large vessel which in 

 some animals continues the aorta to the coccygeal region. 



4. The Cceliac Axis is detached as soon as the aorta passes through 

 the hiatus aorticus. 



5. The Anterior Mesenteric — the largest of the branches — is detached 

 at the 1st lumbar vertebra. 



6. The Renal Arteries, right and left, arise from the sides of the 

 aorta at the articulation between the 1st and 2nd lumbar vertebrse. 



7. The Spermatic Arteries, right and left, come off a few inches 

 behind the renals, viz., between the 3rd and 4th lumbar vertebrse, and 

 one generally a little in advance of the other. As seen in the 

 dissection, each passes obliquely backwards and outwards over the 

 ureter and circumflex iliac artery to gain the internal abdominal ring, 

 where it joins the other constituents of the spermatic cord. In the 

 cord it has a remarkably tortuous disposition; and, although a long 

 vesssel, it detaches no branches of any size until it reaches the testicle. 

 In the mare it is represented by the ovarian ai'tery, which passes in a 

 tortuous manner between the layers of the uterine broad ligament to 

 reach the ovary. It gives off a uterine branch to the uterine horn. 



8. The Posterior Mesenteric Artery. This vessel is usually detached 

 at the 4th lumbar vertebra, a little behind the origin of the spermatics, 

 but this relationship may be reversed. 



The External Iliac Artery is, speaking generally, the vessel of supply 

 to the hind limb. It is regarded as a terminal branch of the aorta, and 

 it has its root at the body of the 5th lumbar vertebra. It descends with 

 a curved course at the pelvic inlet, and at the anterior border of the pubis 

 it is directly continued as the femoral artery. It is placed immediately 

 beneath the peritoneum, and each is related on its outer side to the 

 psoas parvus, sartorius, and iliacus muscles, the tendon of the first of these 

 separating it from the great crural nerve. On the inner side it is related 

 successively to the common iliac and external iliac veins, the former 

 separating it from the internal iliac artery. Its branches are: — 



1. The Circumflex Iliac Artery. This is a large artery detached 

 from the outer side of the external iliac close to its origin. It passes 

 outwards across the psoas muscles ; and at the outer edge of the psoas 

 magnus it divides into an anterior and a posterior branch. The former 

 is distributed in the flank, beneath the internal oblique muscle ; and the 

 latter perforates the oblique muscles near the bony angle of the haunch, 

 and descends to the thigh. 



2. The Artery of the Cord (Plate 44). This is a slender vessel 

 arising close to the preceding, or it may come from the aorta itself. 



