STRUCTURE OF THE VERTEBRATES 



279 



With the shortening of the cord the spinal nerves become 

 telescoped, and plexuses of nerves leave the two enlargements. 

 This is most prominent in man and the anthropoids. In these 

 animals the cord has shortened until it reaches only to the mid- 

 dle of the lumbar region, and is anchored to the pelvis with a 

 connective tissue filimi terminale. From the cervical enlargement 

 arise the nerves supplying the pectoral girdle; and from the 

 sacral (lumbar) enlargement comes a great plexus of nerves, the 

 Cauda equina of human anatomy. The nerves of the thoracic 



Ascending tracts 

 Gray substance 



Dorsal root 



Dorsal 

 ganglion 



Fig. 158. Typical Section of the Spinal Cord of the Mammal. 



region retain, to a large degree, the primitive metameric arrange- 

 ment. 



The white substance of the cord is composed of the long 

 ascending fibers which pass from the spinal nerves up the cord 

 to the brain; and the descending motor fibers from the brain to 

 the motor cells of the cord. Recall that each impulse reaching the 

 cord through a sensory fiber is transferred to the brain through 

 a myelinated collateral or another sensory fiber; and it becomes 

 evident that the number of long ascending fibers increases at 

 each spinal nerve. Similarly, the motor fibers from the brain 

 decrease in numbers at each lower level due to the fibers w^hich 



