362 THE ANATOMY OF THE HORSE. 



nerve, and is then expended in the skin of the anus and root of 

 the tail. 



As in other regions of the spine, each of the inferior primary 

 branches just considered communicates with the contiguous ganglion 

 of the sympathetic 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 Gangliated 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 points of detachment, are as follows : — 



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

 ternal iliac at its root. It behaves like the lumbar branches of the aorta. 

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

 the intervertebral foramen between the 5th and 6th lumbar vertebra?. 



2. The Internal Pudic Artery. This is a considerable vessel, which 

 has 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 



