78 Journal of Comparative Neurology. 



raxis of a neuron may come in very close proximity to the end 

 branches of the dendrites of one or several other neurons. By 

 this contiguity of end-brush and cell body or end-brush and den- 

 drites, neurons, while not losing their identity, are linked into 

 chains, so that a physiological continuity exists between them. 

 Such nerve-chains, or neuron-chains, as we may call them, vary 

 greatly in complexity, and in the number of neurons which 

 enter into their construction. They may be very simple, con- 

 sisting of only two neurons, or very complex indeed, embrac- 

 ing a large number of them. 



The entire nervous system may therefore be said to be 

 made up of such neuron chains, and the tracing of them is a 

 problem, which, perhaps more than any other, engages the at- 

 tention of the neurologist at the present time. For it is only 

 as the respective positions of the several links in such chains or 

 paths become established, that the results of the many investi- 

 gations become of practical value. 



It may be seen from what has been said, that the cells of 

 the peripheral ganglia, the various sympathetic ganglia included, 

 the peripheral nerves, as well as the neurons in the central nerv- 

 ous system, form each in its respective place, a link or a 

 portion of a link in such a chain. These neuron-chains are 

 paths along which nerve impulses travel from the periphery to 

 the nerve centers ; from one nerve center to another ; and from 

 the centers to the peripheral tissues, such as muscles, glands, 

 etc. The anatomical mechanism which probably underlies a 

 volitional muscular contraction, and a simple reflex may here 

 be briefly described, as illustrations of neuron-chains. If we 

 take first the example of a volitional muscular contraction we 

 will find that a chain consisting of two neurons is involved. 

 The cell body and dendrites of one of these is situated in the 

 motor, cortical center of the brain, its neuraxis passes through 

 the internal capsule, the crura and one of the pyramidal tracts of 

 the medulla and spinal cord. Somewhere in the cord it ends in 

 the grey matter of the anterior horn, terminating in an end-brush 

 which is in close proximity to the cell body or the dendrites of 

 a neuron, which forms the second link in the chain. The near- 



