362 



THE ANIMALS AND MAN 



body in one part to a cell body in another part, or from a 

 cell body in the cortex of the brain down to the cranial 

 nerves or into the spinal cord, lie in or form most of the 

 white matter of the brain. 



The spinal cord. — The spinal cord is an extension of the 

 medulla oblongata. It extends from the medulla (at the 

 foramen magnum or opening through the skull) through 

 the vertebral cavities to the lumbar vertebras (fig. 66 B). 

 It is slightly enlarged in the neck region and again in the 

 lumbar region. It is protected by three coats, like the brain. 

 The cord is partly divided into halves by two deep fissures 

 (fig. 181). In the cervical and lumbar regions the cord 



Fig. 181. Diagram of a simple reflex arc. 1, sensory surface; 2, afferent 

 nerve; 3, motor cell in gray matter of cord; 4, efferent nerve; S, mus- 

 cle. (After Morat and Dayton.) 



gives off a series of spinal nerves. These occur in pairs. 

 Each one arises from the cord by two roots, an anterior 

 and posterior (fig. i8i). These roots unite to form the 

 single nerve on either side. 



The posterior root of a spinal nerve is the sensory root, 

 the anterior one is the motor root. The posterior root 

 bears a ganglion, an enlargement of nerve tissue within which 

 is found a collection of sensory nerve cells. Fig. i8i shows 

 the relation of the gray and white matter within the spinal 

 cord. It shows also the path of impulses from the sensory 

 nerve ending in the skin (i) to the cell bodies in the gan- 

 glion- and spinal cord, and from them to the muscle (5). 



