106 



THE INDIVIDUAL ORGANISM 



longer than the rest. They are, however, all built on the same common 

 pattern. Each nerve has two connections with the spinal cord, a dorsal 

 root and a ventral root. The roots of the spinal nerves are quite short and 

 are protected by the bone of the vertebrae. Along the course of the dorsal 

 root, still protected by the bony tissues of the vertebrae, there occurs on 

 each a slight enlargement, the dorsal-root ganglion. Each spinal nerve is 

 formed by the union of its two roots at the point where the nerve issues 



cerebrum 



cerebellum 



cord 



Fig. 7.2. The left side of the brain. 



from between the vertebrae. The common trunk thus formed is very 

 short, for immediately beyond the spinal column it divides into three 

 main branches, the rami. Each ramus is a great nerve trunk which, like a 

 main artery, divides again and again until the remotest parts of the body 

 are supplied with minute nerve endings. The dorsal ramus of each nerve 

 supplies the muscles and skin of the back; the ventral ramus (usually 

 much the largest of the three) supplies the muscles and skin of the 

 lateral and ventral parts of the body wall and the arms and legs. The 

 third branch, the ramus communicans, supplies the smooth muscles, 



