400 



THE NERVES AND THE SENSE ORGANS 



to form the bulb. The cord's lower end is continued in 

 large nerve trunks, which supply the hips and lower 

 limbs. 



There are 31 pairs of nerves attached to the spinal cord. Each of 

 these joins the cord by a dorsal (back) and a ventral (front) root. The 

 two roots come together in a single "nerve" before they pass out of 

 the vertebrae, but their fibers remain separate, and have different 

 functions. The fibers of the dorsal roots are afferent, and carry 



Nerve 



Ef fervent 

 Fibres 



Dorsal 



Fissure Spinal 



Cord 



Gray 

 Matter 



Fig. 295. 

 The Spinal Cord and its Connections. 



messages to the cord and brain; the fibers of the ventral roots are 

 efferent, and carry messages to the organs. 



The spinal cord and brain are admirably protected by the vertebrae 

 and the cranium. They are surrounded by 3 membranes, which come 

 between them and the bones themselves. One of the membranes con- 

 tains a watery liquid; this acts as a cushion to prevent injury from 

 sudden shocks. 



402. The Sympathetic System. We have seen that 

 the brain and spinal cord are the great nerve centers of 

 the body, and that they consist of a multitude of nerve 

 cells gathered together in large ganglia and nerve trunks. 

 Besides this central system, the body has another group 

 of nerve structures, called the sympathetic system. This 



