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 

 vertebrae 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 ex- 

 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 



