350 THE ANATOMY OF THE HORSE. 



just considered communicates with the contiguous ganglion of the sym- 

 pathetic cord, by one or more branches detached at the intervertebral 

 foramen; and the sacral nerves send each a filament to the pelvic plexus. 



The Sympathetic Gangliatbd Cord in the sacral region. This is the 

 direct backward continuation of the lumbar cord. It is placed on the 

 inferior surface of the sacrum, internal to the inferior sacral foramina, 

 the lateral sacral artery intervening between it and the inferior primary 

 branches of the sacral nerves at their points of emergence. It possesses 

 a ganglion opposite each of the first three sacral foramina; and, as before 

 said, it communicates by filaments passing between these ganglia and 

 the corresponding spinal nerves. The emergent branches of these 

 ganglia are very slender, and pass to the cellular tissue beneath the 

 sacrum, or to the contiguous blood-vessels. The cord terminates at the 

 last ganglion, either abruptly, or by a filament passing on to the middle 

 coccygeal artery. 



The Internal Iliac Artery (Plate 48). This is one of the terminal 

 branches of the posterior aorta. Beginning at the intervertebral disc 

 between the 5th and 6th lumbar vertebrae, it passes downwards and 

 backwards across the articulation between the last lumbar transverse 

 process and the sacrum, and then across the sacro-iliac articulation; and 

 at the upper part of the ilio-pectineal line, a little above the eminence of 

 the same name, it divides into the iliaco-muscular and obturator arteries. 

 The vessel is covered by the peritoneum, and in the first inch or two of 

 its course it is separated from the external iliac artery by the common 

 iliac vein. The collateral branches of the internal iliac, taken in the 

 order of their point of detachment, are as follows : — ■ 



1. The second last of the series of lumbar arteries arises from the in- \ 

 temal iliac at its root. It behaves like the lumbar branches of the aorta, i 

 Its upper division, much the larger of the two, passes upwards through 

 the intervertebral foramen between the 5th and 6th lumbar vertebrae. 



2. The Internal Pudic Artery. This is a considerable .vessel having 

 its origin at the last lumbar vertebra. Entering the pelvis, it descends 

 at the ischiatic edge of the ilium, and then passes backwards in the 

 texture of the sacro-sciatic ligament, or on its inner face. 



3. The Lateral Sacral Artery leaves the parent trunk at the sacro- 

 lumbar articulation, and passes backwards on the lower face of the 

 sacrum, beneath or at the inner side of the inferior sacral foramina. 

 A little behind the middle of the sacrum it divides into the ischiatic and 

 lateral coccygeal arteries. The former, much the larger of the two, passes 

 out through the edge of the sacro-sciatic ligament to reach the hip 

 (Plate 16); the latter continues the direction of the lateral sacral artery 

 to the tail. The inferior division of the 3rd sacral nerve appears in 

 the angle of separation between these two arteries. The collateral 

 branches of the lateral sacral artery are : — (1) Branches entering the 



