THE MECHANISM OF LIFE 



579 



terata (Fig. 279, II). Here, likewise, a cell possesses, upon the one 

 side, a sensory element, and, upon the other, a contractile fibre, 

 which contracts as soon as the sensory end-organoid is stimu- 

 lated. What in all these cases is differentiated within a single 

 cell, is in the nervous system of animals distributed to several 

 cells. In the simplest case of the latter, three different cells are 

 concerned. One cell, the sensory cell, receives the stimulus ; from 

 this a centripetal nerve-path conducts to a central cell, the ganglion- 

 cell, and from here a centrifugal nerve-path conducts to a cell that 

 performs the reaction, the motor end-cell (Fig. 280, A). But this 

 form of reflex arc is realised perhaps only in the invertebrates. 

 In vertebrates, so far as the conditions are known, a fourth cell at 

 least is interpolated in the arc, since in place of one ganglion-cell 

 at least two are present, one of which receives the stimulus from 

 the sensory-cell and conducts it to the other, while the other 

 transfers the impulse to 

 the motor end-cell (Fig. 

 280, B). In a given case 

 the end-cell of the centri- 

 fugal path may be either 

 motor or secretory, or may 

 produce light or electricity. 

 Thus reflexly by the gang- 

 lion-cells parts of the cell- 

 community, wholly differ- 

 ent and far removed from 

 one another, are put into 

 union and activity by im- 

 pulses from the central 

 nervous system. 



If we start from the 

 scheme of the reflex arc, 

 the further factors that 



come into consideration in the mechanism of the central ner- 

 vous system are very simple. They consist only in the facts 

 that, upon the one hand, between the sensory and the motor end- 

 organ more than two ganglion-cells possessing different functions 

 are interpolated, and, upon the other hand, certain ganglion- 

 cells are innervated not simply from one side, by a single other 

 ganglion-cell, but by several, and under certain circumstances by 

 many others. Thus, by means of their nerve-fibres very complex 

 arid intricate connections are formed between the ganglion-cells 

 and the individual systems of ganglion-cells, which latter are the 

 centres of definite vital processes and hence the seat of definite im- 

 pulses. A network of ganglion-cells and uniting nerve-fibres 

 results, which is apparently inextricable, but in reality insures a 

 very definite and unified co-operation of the various parts of the 

 organism that it binds together. By the proper innervation of all 



p p 2 



FIG. 280. Schemes of the reflex arc. A, Simple 

 scheme of reflex arc. At the left, below, a sensory 

 cell ; in the middle, above, a central ganglion-cell ; 

 at the right, below, a muscle-cell. B, Scheme of a 

 reflex arc in vertebrates. At the left, below, a 

 sensory cell, at the left, above, a sensory ganglion- 

 cell. At the right, above, a motor ganglion-cell, 

 at the right, below, a muscle-cell. (After Gegen- 

 baur.) 



