CHAPTER XXII 



THE PHYSIOLOGY OF THE NERVE CELL AND OF THE SPINAL CORD 



1. GENERAL CONSIDERATIONS CONCERNING THE FINER 

 STRUCTURE OF THE NERVOUS SYSTEM 



IT has for a long time been customary to divide nerve tissue into two 

 elements : nerve cells and nerve fibers. The nerve cells were first seen by 

 Ehrenberg (1833) in the spinal ganglia. Eemak (1835) first pointed out 

 that the processes of nerve cells are continued in the sympathetic nerves of 

 the vertebrates as an integral part of the nerve fibers. This was shown to be 

 true also for the invertebrates by Helmholtz and Hanover (1842). Deiters 

 (1863) demonstrated that all central nerve cells have two kinds of processes: 

 first, axis-cylinder processes, which connect with the medullated nerve fibers 

 and become directly continuous with this axis cylinder; and secondly, proto- 

 plasmic processes, which break up into very fine branches whose ultimate fate 

 Deiters was unable to ascertain. 



By the introduction (1873) of Golgi's method of impregnating the nerve 

 elements with silver, a fresh impetus was given to research in the field of 

 neuro-histology. With the application of this method, which has given us such 

 a rich and comprehensive view of the structure of the nervous system, are 

 associated preeminently such names apart from that of Golgi himself as 

 those of Cajal, Kolliker, Eetzius and von Lenhossek. 



According to the view represented by these investigators the nervous sys- 

 tem is to be regarded as made up of genetically separate and distinct nervous 

 units to which the general term neurons is now applied. The most essential 

 and important part of the neuron is the nerve cell. Several processes are 

 given off from this, one or two of which (from some cells more) are con- 

 tinued as the axis cylinders of nerve fibers and consequently are termed axis- 

 cylinder processes or axons. As for the remaining processes, the so-called 

 dendrites or protoplasmic processes of Deiters, they divide into numerous 

 branches which become exceedingly attenuated, and in this way greatly increase 

 the superficies of the nerve cell. 



The nerve process continues as an integral part of the nerve fiber to its 

 final distribution, where it generally breaks up into a small terminal arboriza- 

 tion. At different points along the course of the nerve fiber a variable number 

 of side twigs or collaterals are usually given off, which, in turn, after a shorter 

 or longer course, like the nerve fibers themselves, end in delicate ramifications. 



According to the original conception of the neuron theory the individual 

 nerve units do not form a continuous network, but are anatomically separate 



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