THE MECHANISM OF LIFE 



579 



Uriila (Fig. 279, //). Here, likewise, a cell possesses, upon the one 

 side, ii sensory (■]<'iiieiit, ;uid, upon the other, a contractile fibre, 

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

 lated. What in all thes(> cases is differentiated within a single 

 cell, is m the iierxous s^'stem 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 ci^nducts 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 reHex 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 ease 

 the end-cell of the centri- 

 fugal path may be either 

 motor or secretory, or may 

 produce light or electricity. 

 Thus retlexly by the gang- 

 lion-cells parts of thi' cell- 

 community, wholly dift'er- 

 ent and far removed from 

 one another, are put into 

 union and activity b)- im- 

 pulses from the central 

 nervous system. 



If we start from the 

 scheme of the reHex arc, 

 the further factors that 



come into consideration in the mechanism of the central ner- 

 vous system are vei-y simple. Thej' consist only in the facts 

 that, ujjon the one hand, betwet'u 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 untler certain circumstances by 

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

 and 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 !• 2 



, 'JSd. — Schomoa of tlio rutlox ;ire. A, Simple 

 Hfhomo of roflex arc. At the left, below, ii wensory 

 LcU ; in the middle, above, a central ganfflion-L-cll ; 

 at the right, below, a muscle-cell. 7f, Scheme nf ji 

 rollcx arc ill vertebrates. At the left, below, ii 

 Huiisory coll, at the left, above, a sensory ganglion- 

 CL'll. At the right, above, a motor ganglion-cell, 

 at the right, bcluw, a muaolG-coU. (After Gegen- 

 baiu'.) 



