THE MECHANISM OF LIFE 581 



The manner in which the elements of the nervous system are 

 united with one another anatomically and functionally deserves 

 special attention, since the later researches upon the finer structure 

 of the central nervous system, which have been made possible by 

 the extraordinary development of the microscopic technique, 

 especially by Golgi, Weigert, Ehrlich, and others, have led to the 

 discovery of very peculiar but fixed relations. The element of 

 the central nervous system is the ganglion-cell, but the ganglion- 

 cell with its characteristic differentiations. From the body of the 

 cell there extend processes, more or less numerous according to 

 the function of the cell, among which two kinds may be distinguished 

 sharply from one another. Some form a more or less richly 

 branched structure, and are, therefore, appropriately termed 

 dendrites. The older histologists termed these protoplasmic 

 processes. The others are the nerve-processes. So far as we now 

 know, as regards the number of the latter there are only two 

 varieties of ganglion-cells: unipolar (previously called multipolar 

 on account of the numerous dendrites), provided with only one 

 nerve-process, and bipolar, with two nerve-processes. These nerve- 

 processes are simply the beginning of the nerve-fibres, which not 

 rarely reach a length of one metre and more. The conducting 

 nerve puts even the most distant cells of the animal body into 

 physical connection with the ganglion-cells, and transmits the 

 impulses that go out from the bodies of the ganglion-cells to the 

 tissue-cells, or in specific cases to other ganglion-cells. In its 

 course from the body of the ganglion-cell to the cell that it inner- 

 vates, the nerve-process appears different at different points. 

 It sends off here and there collateral branches, and a little beyond its 

 origin is surrounded by a sheath consisting of myelin, the 

 medullary sheath. The latter is divided into segments by the 

 so-called nodes of Ranvier, and disappears shortly before the cell 

 which the nerve supplies is reached. The medullary sheath, in 

 which the nerve-fibre runs as the axis-cylinder, is itself usually 

 surrounded by a membranous sheath, the neuriletnma. The end of 

 the nerve shows very characteristic differentiations according to the 

 kind of cell which it innervates. Such a complete cell, i.e., a 

 ganglion-cell with all its appendages, represents the elementary 

 constituent of the nervous system, and can fittingly be termed with 

 Waldeyer a neuron (Fig. 282). The combination of the innumerable 

 neurons with one another constitutes the nervous system of the 

 animal. According to the later researches of Golgi, KoUiker, His, 

 Ramon y Cajal, and others, the connection of the neurons with one 

 another appears to be everywhere of such a kind that the dendrites 

 of the ganglion-cells receive the stimulating impulses, while the 

 nerve-process transmits them from one ganglion-cell to the den- 

 drites of another. The bipolar ganglion-cells, which are contained 

 chiefly in the spinal ganglia lying at the two sides of the spinal 



