THE NERVOUS SYSTEM. 211 
remaining thoracic nerves encircle the body, supplying the 
muscles and skin of those regions. 
The lumbar plexus (Fig. 106) is composed of the an- 
astomosing of the ventral branches of the four caudal lum- 
bar nerves. As in the cervical and thoracic nerves, the 
lumbar nerves divide into dorsal and ventral branches im- 
mediately without the intervertebral foramen. The former 
supply the muscles and skin of the back. In order to dis- 
play the lumbar nerves, the entire ventral and_ lateral 
abdominal wall should be cut away and the specimen 
securely nailed to the tray on its back. The adipose tissue 
and muscles lying on either side of the bodies of the lumbar 
vertebrze must be carefully picked away until the roots of 
the nerves are apparent. They may then be easily followed 
distad. 
The first three lumbar nerves of the cat are represented 
in man by the ilio-hypogastric and ilio-inguinal. The first 
nerve supplies the rectus muscle and skin of the abdomen. 
This nerve and the two next described are scarcely as large 
in diameter as an ordinary pin. The ventral branch of the 
second lumbar nerve divides into two branches, and sup- 
plies the skin of the caudal part of the abdomen, and struc- 
tures in the inguinal region. The ventral branch of the 
third lumbar nerve supplies the inguinal region and is also 
distributed to the transverse and rectus muscles. The 
fourth lumbar nerve divides into two parts, one of which is 
the genito-crural nerve, supplying the skin and other struc- 
tures of the ventral abdominal wall and thigh, the other 
branch together with part of the fifth nerve forms the exr- 
ternal cutaneous, supplying the lateral surface of the thigh 
region. 
The anterior crural nerve is composed mainly of branches 
of the fifth and sixth lumbar. It receives a small branch 
from the fourth. It supplies the psoas muscles, which it 
