COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 339 



to the ganglia and plexi already noted. At the posterior end of the peritoneal 

 cavity the sympathetic trunks gradually diminish and disappear. 



g) The lumbar and sacral spinal nerves and the lumbosacral plexus: There 

 are seven pairs of lumbar nerves and four (rabbit) or three (cat) pairs of sacral 

 nerves. The ventral rami of the last four lumbar nerves form a lumbar plexus, 

 those of the sacral nerves a sacral plexus; but since the two plexi are united with 

 each other, they may be considered together as the lumbosacral plexus. 



Remove all viscera from the peritoneal cavity, including the postcaval vein 

 and aorta. In the dorsal wall is a muscular mass extending from the vertebrae 

 to the pelvic girdle. This consists of a lateral larger muscle, the iliopsoas, and a 

 smaller medial one, the psoas minor. In the rabbit the psoas minor is a slender 

 muscle which occupies only the posterior part of the mid-dorsal region ; its stout 

 shining tendon passes to the dorsal side of the inguinal ligament. In the cat the 

 psoas minor extends nearly as far anteriorly as the iliopsoas; it narrows pos- 

 teriorly to a tendon, which passes obliquely laterally on the ventral surface of 

 the iliopsoas which is thus exposed both medially and laterally to the tendon of 

 the psoas minor. The psoas minor covers a part of the iliopsoas in both animals, 

 and the greater part of the lumbar plexus is situated between the two muscles. 

 Note the abdominal parts of the sympathetic cords between the posterior portions 

 of these muscles. 



Locate the last thoracic spinal nerve. It lies about one-half inch posterior 

 to the last rib. The first nerve posterior to this on the dorsal wall is the ventral 

 ramus of the first lumbar nerve. Shortly posterior to this is the second lumbar 

 nerve. These two nerves pass to the muscles and skin of the abdominal wall; 

 in the cat each divides into two branches. The third lumbar nerve emerges dorsal 

 to the iliopsoas muscle and divides into a larger lateral branch to the abdominal 

 wall and a more slender medial branch, which passes obliquely caudad, reaching 

 and following the course of the iliolumbar artery and vein. The fourth lumbar 

 nerve is the first of the lumbar plexus. It has two main branches, the lateral 

 cutaneous nerve and the genitofemoral nerve. The former is the stout trunk 

 which emerges between the iliopsoas and psoas minor muscles and accompanies 

 the course of the iliolumbar artery and vein, passing to the thigh. The genito- 

 femoral nerve is a long slender nerve which runs along the medial border of the 

 psoas minor muscle, lateral to the sympathetic cords. In the posterior part of 

 its course it accompanies the external iliac artery. It supplies the thigh and 

 abdominal wall of and adjacent to the inguinal region. After locating these 

 two branches of the fourth lumbar trace them toward the vertebral column, 

 removing the psoas minor as far as necessary. Find the point of emergence of 

 the fourth lumbar from the vertebral column and note the connection, very 

 stout in the cat, between the fourth lumbar and the fifth. 



The fifth lumbar contributes by means of its connection with the fourth 

 lumbar to the lateral cutaneous branch named above and also forms a strong 

 union with the sixth lumbar. To expose these remove the rest of the psoas 



