200 CIRCULATORY SYSTEM. 



aorta. Arising in pairs from its sides, they wind upward, covered 

 by the psoas parvus, and, pursuing the inter-vertebral spaces, 

 pierce the fibres of the inter-lransversales lumborum, and ramify 

 and expend their branches among the muscles covering the loins. 

 In the course of their ramifications they enter into communications 

 with the last of the intercostals and the circhraflex artery of tlie 

 ileum. Near their origins, each detaches a branch which enters 

 the vertebral canal by the correspondent spinal foramen, also 

 twigs to the psoas parvus and inter-transversales. 



Below the last lumbar vertebra the aorta bifurcates into two 

 pairs of large and important arterial trunks, — the external and 

 internal iliacs. 



The Internal Iliac Arteries 



Are the product of a kind of second bifurcation ; for there is still 

 a continuation of the trunk of the aorta after it has given off the 

 external iliacs : and this forms a remarkable difference between it 

 and the division of the corresponding vessel in the human subject, 

 the horse having no common iliac arteries. The internal iliacs are 

 larger than the external, but extremely short. They diverge from 

 the parent trunk, backward and outward, and hardly quit the 

 body of the vertebra before each gives off a large branch, the 

 artery of the bulb : the vessel continues a little further, and by the 

 side of the sacral articulation splits into three nearly equal 

 divisions; — the obturator, gluteal, and lateral sacral arteries. 



The artery of the culb springs from the under and outer 

 part of the internal iliac, and proceeds in its course backward 

 between the trunk of the artery and the external iliac vein, which 

 lattei' at the same time is a little above it. It next makes a sweep 

 backward, around the sides of the pelvis (to which it is closely 

 bound by peritoneuui and cellular adhesions); then insinuates 

 itself between the laminse of the sacro sciatic ligament, and con- 

 tinues along the upper border of the ischium to the ischial arch, 

 the side of which it turns round to reach the bulb of the penis, 

 wherein it terminates. In the female, this artery sends its termi- 

 nating branches to the bulb of the vagina and lining membrane of 

 that canal. Its branches are — 1. The umbilical artery, or rather 

 the remains of that vessel which in the foetus nearly equalled in 

 size the iliac arteries themselves, and was surpassed by none in 

 regard of importance. In the young subject these vessels com- 

 monly remain pervious as far as the bladder ; but in process of 

 time they degenerate into the round ligaments of the bladder ; in 

 the description of which organ further notice will be taken of 

 them. 2. Sundry small vesical branches, in passing, to the 

 bladder. 3. In leaving the pelvis, the prostatic artery, which 



