522 



COMPARATIVE ANATOMY 



muscles or with glands. The ganglion cells of both types lie within the 

 gray matter of the spinal cord; but those of the somatic motor nerves 

 lie in the ventral column, while the visceral motor fibers come from 

 ganglion cells in the intermediolateral column. 



EVOLUTION OF THE PERIPHERAL NERVOUS SYSTEM 



The peripheral nervous system consists of two main divisions, the 

 cerebrospinal nerves and the autonomic system. The cerebrospinal 

 nerves are also of two kinds; cranial, which have their foramina of exit 

 from the cranium, and spinal, which are connected with the cord and leave 

 the vertebral column by way of intervertebral foramina. 



MUCOUS EPITHELIUM 



SOMATIC MUSCLE 



SYMPATHETIC GANGLION 

 VISCERAL SMOOTH MUSCLE 



Fig. 435- — Neuron relations of motor and sensory nerves which have their centers in the 

 medulla oblongata. (Redrawn from Patten, after Ranson.) 



Nerve Components. In addition to the four kinds of nerve fibers 

 found in spinal nerves and distinguished by differences in function and 

 distribution, some of the cranial nerves may have special fibers which 

 supply sense organs or specialized muscles. Four special groups of nerves 

 must therefore be added to the four general kinds which have already been 

 mentioned, making a total of eight kinds of nerve components. (Fig. 435) 



I. General somatic efferent fibers are relatively large neurites, formed 

 as outgrowths from nerve cells located in the ventral column of the spinal 

 cord and brain. Somatic motor cells are multipolar and contain numerous 

 Nissl bodies. The neurite arises from a hillock or implantation core 

 free from Nissl granules. As the neurite emerges from the cord or brain 

 it is surrounded bv mvelin and neurilemma sheaths. Both sheaths are 



