NEURONS AND NEURON-CHAINS 



53 



4. The neuron is also a trophic unit, as is seen (a) in the degeneration of a 

 portion of an axon severed from its cell of origin, (6) in the phenomenon of 

 chromatolysis or axon reaction, and (c) in the regeneration of the degenerated 

 portion of the axon by an outgrowth from that part of the axon still in con- 

 tact with its cell of origin. 



5. Neurons are the only elements concerned in the conduction of nerve 

 impulses. The nervous system is composed of untold numbers of such units 

 linked together in conduction systems. 



While a majority of neurologists now accept the neuron doctrine as pre- 

 sented here, there are dissenters (Marui, 1918). In his very interesting book, 



Fig. 31. Diagrammatic section through the spinal cord and a spinal nerve to illustrate a 

 simple reflex arc: a, b, c, and d, Branches of sensory fibers of the dorsal roots; e, association neuron; 

 /, commissural neuron. 



"Allgemeine Anatomic und Physiologic des Nervensystems," Bethe has vigor- 

 ously controverted every one of the five cardinal points just presented. 



We will next examine some of the simpler chains of neurons to see how they 

 enter into the formation of the conduction pathways. 



Neuron-chains. The simplest functional combination of neurons is seen in 

 the reflex arc, and this again in its simplest form is illustrated in Fig. 31. Such 

 an arc may consist of but two neurons, one of which is afferent and conducts 

 toward the spinal cord; the other is efferent and conducts the impulses to the 

 organ of response. The arc consists of the following parts: (1) the receptor, 

 the ramification of the sensory fiber in the skin or other sensory end organ; 

 (2) the first conductor, which includes both branches of the axon of the spinal 

 ganglion cell; (3) a center including the synapse; (4) the second conductor, which 



