THORAX AND ABDOMEN OF THE HORSE 179 



dorsal branch of the fifth sacral assists in the formation of the dorsal 

 nerve. The two nerves follow the dorsal and ventral caudal arteries. 



Pars pelvina svsTEMiE sympathici. — The pelvic part of the 

 sympathetic may be said to begin after the last lumbar ganglion. 

 From this point the nerve-trunk passes into the pelvis dorsal to the 

 hypogastric artery and the common iliac vein. Traversing the pelvic 

 surface of the sacrum medial to the ventral branches of the sacral 

 nerves, the nerve-cord develops a ganglion opposite each of the first 

 three ventral sacral foramina, and finally divides into medial and 

 lateral branches. The medial branch joins its fellow from the opposite 

 side of the body, and the single nerve thus formed accompanies the 

 coccygeal artery into the tail. There is often a small unpaired ganglion 

 (ganglion impar) at the point of union of the two branches. The 

 lateral branch is connected with the last two sacral nerves, and joins 

 the trunk formed by the union of the ventral branches of the coccygeal 

 nerves. 



The sacral ganglia (ganglia sacralia) are connected by rami 

 communicantes with the first three sacral nerves. Efferent filaments 

 pass from them to the rectum, bladder, uterus and penis. 



The dissection of the pelvis should be completed by an examination 

 of the connection of the hip-bone with the vertebral column. 



The sacro-iliac articulation (Articulatio sacroiliaca). — Though 

 the joint between the sacrum and the ilium is usually described as a 

 diarthrosis, the amount of movement possible in it is exceedingly 

 slight. This might be deduced from an examination of the articular 

 surfaces ; for these are flattened, roughened by eminences and de- 

 pressions, and covered by a thia layer of cartilage. Not infrequently, 

 the very narrow joint-cavity is crossed by fibrous bands passing from 

 one bone to the other. Moreover, the ligaments disposed in the 

 neighbourhood of the joint all tend to prevent movement. A very 

 short joint capsule encloses the joint. 



The fibres of the ventral sacro-iliac ligament (ligamentum 

 sacroiliacuni ventrale) are short and surround the joint-capsule so 

 closely that they might be regarded as thickenings thereof. The 

 ligament is especially strong dorsal to the joint, where it forms a 

 collection of almost vertical bundles in the angle between the ilium and 

 the transverse process of the first sacral vertebra. 



The short dorsal sacro-iliac ligament (ligamentum sacroiliacum 

 dorsale breve) is a rounded cord stretching from the sacral tuber of the 

 iliunti to the summits of the sacral spinous processes. 



