42 



INTRODUCTION TO NEUROLOGY 



Neurons can function only when connected together in chains, 

 so that the nervous impulse can be passed from one to the other. 

 In any such chain the neuron first to be excited is called the 

 neuron of the first order, and the succeeding members of the 

 series neurons of the second, third, fourth order, and so forth. 

 All reflexes require an afferent neuron which conducts the ner- 

 vous impulse from the receptor to the center, one or more effer- 

 ent neurons conducting from the center to the organ of response, 



W 



Fig. 7. The body of a pyramidal neuron from the cerebral cortex, 

 stained by Nissl's method, illustrating the arrangement of the chromophilic 

 substance and the form of the nucleus: a, Axon; 6, chromophilic bodies 

 surrounding the nucleus; c, a mass of chromophilic substance in the angle 

 formed by the branching of a dendrite; d, nucleus of a neuroglia cell (not a 

 part of the neuron). (After Ram6n y Cajal.) 



and usually one or more neurons intercalated between these 

 within the center itself (see pp. 25, 56, 109). Figure 1, p. 25, 

 illustrates the simplest possible connection of neurons in a reflex 

 arc of the spinal cord, involving only two elements. The 

 afferent neuron sends its dendrite to the skin and its axon into 

 the spinal cord, where the nervous impulse is taken up by the 

 dendrites of the efferent neuron, which in turn transmits it to a 

 muscle. Figures 5 to 9 illustrate the forms of other neurons. 



