62 



THE NERVOUS SYSTEM 



nerves. These have their cells of origin in the spinal ganglia and are continued 

 through the dorsal roots into the spinal cord (Fig. 37). We shall have much 

 more to say about the visceral components of the spinal nerves in the chapter 

 on the Sympathetic Nervous System. In the remaining pages of this chapter 

 we will confine our attention to the somatic components, i. e., to those fibers which 

 innervate the various parts of the body exclusive of the visceral and vascular 

 systems. 



Somatic Efferent Components. The skeletal muscles are innervated by 

 myelinated fibers, which are, for the most part, of large caliber. The axis- 

 cylinders of these fibers are the axons of cells located in the ventral part of the 

 gray matter of the spinal cord, and they end on the muscle-fibers in special 



Fig. 38. Nerve-ending in muscular fiber of a lizard (Lacerta viridis). Highly magnified: a, 

 End-organ seen in profile; b, from the surface; s, s, sarcolemma; p, p, expansion of axis-cylinder. 

 Beneath this is granular protoplasm containing a number of large clear nuclei and constituting 

 the "bed" or "sole" of the end-organ. In b the expansion of the axis-cylinder appears as a clear 

 network, branching from the divisions of the medullated fiber. (Kiihne in Quain's Anatomy.) 



motor end-plates. Such a primary motor neuron is illustrated in Fig. 25. A 

 motor fiber undergoes repeated division as it approaches its termination, but 

 each branch retains its myelin sheath until in contact with the muscle-fiber. 

 At this point this sheath terminates abruptly, and the neurilemma becomes 

 continuous with the sarcolemma (Fig. 38). The terminal branches of the 

 axon are short, thick, and irregular. They lie immediately under the sarcolemma 

 in a bed of specialized sarcoplasm containing a number of large clear nuclei. 

 The wave of activation, which travels down an axon as a nerve impulse, is 

 transmitted through these motor nerve endings to the muscle and initiates a 

 contraction. 



The Spinal Ganglia. Since the afferent fibers in the spinal nerves take their 



