GANGLION-CELL AND NERVE. 23 



esses of a ganglion-cell are not at all absolutely and fundamentally different 

 from each other. 



The developmental unit — which comprises ganglion-cell, neuraxon, 

 dendrites, and their ramifications — is called a neuron. It is probable that the 

 entire nervous system is composed of numerous neurons, built one upon 

 another. The majority of these nexirons appear to stand isolated, only con- 

 nected to neighboring neurons through a contact so intimate as easily to 

 make possible the transmission of physiological processes. Purely morpho- 

 logical studies here lead to no conclusions. 



The labyrinth of 'fibers found in almost every part of the nervous system 

 and the uncertainty of our present methods admit too easily of false conclu- 

 sions; but the observations of experimental pathology and of pathological 

 anatomy all teach that, if a ganglion-cell be diseased or injured, the changes 

 will not be propagated farther than the processes of that cell reach. This 

 is demonstrable on the axis-cylinder of a peripheral nerve, which can often 

 be studied through many centimeters of its course. Its condition is abso- 

 lutely dependent upon the condition of the cell from which it springs. 



These circumstances also indicate that each ganglion-cell stands 

 isolated, — not directly joined with any other. It must be mentioned, how- 

 ever, that conscientious observers have repeatedly described links of con- 

 nection between different cells. 



These are fundamental facts. They will be better comprehended after 

 a review of what is known regarding the origin and course of a single, 

 thoroughly studied tract. 



Many motor nerves arise from large ganglion-cells which lie in the 

 anterior horn of the spinal cord. From each of these cells there arises one 

 neuraxon. It passes out of the spinal cord as a nerve-root, and then passes 

 into a nerve-trunk, within which its course lies until in a muscle it branches 

 off to the end-plate (Fig. 7). 



That part of the system which reaches from the periphery to its first 

 ending in the central system is designated as a tract of the first order. These 

 tracts of the first order — in this case including anterior horn, motor nerve, 

 and muscle-ending — have been, because of their peculiar relation in disease, 

 for years classified together in pathology and separated from the tracts of a 

 higher order. 



The further transmission of the nervous impulse takes place in this 

 manner among the mammals: to tracts of the first order connect tracts of 

 the second order or even of the third and fourth in succession. All consist 

 of these parts: ganglion-cell, neuraxon, and ramification. If we turn to the 

 chosen example we find that, around the numerous dendrites which the 

 ganglion-cell of the anterior horn sends oiit, there are many fine fibrillse. 

 These fibrillae surroxmd them without, so far as we know, coming into 



