326 



PHYSIOLOGY 



just mentioned, into special contractile structures. If in the course 

 of development the protoplasmic continuity between these two 

 sets of cells had not become interrupted (and we have no ground 

 for assuming that such an interruption occurs under normal circum- 

 stances), it is evident that we should have so produced the simplest 

 form of a reflex arc (Fig. 133, B), namely, a sensory cell, which is stimu- 

 lated by slight physical changes in its surroundings and is thereby 

 thrown into a state of activity similar to that which we have already 

 studied in muscle and nerve. This state of activity would be propa- 



u 



FIG. 134. Diagrammatic view of a jelly-fish. (HERTWTG.) 



u, umbrella ; M, rnanubriuin ; T I; T. 2 , tentacles ; V, velum ; N, nerve 

 ring ; n, ' marginal body.' 



gated by the protoplasmic channels to the muscular cell and arouse 

 there the specific function of the muscle, namely, contraction. In such 

 a simple reactive tissue, lines of less resistance would be rapidly laid 

 down through the protoplasmic continuum, and these lines, acquiring 

 a specific structure or composition, would form a network uniting 

 sensory and muscular cells. Thus a stimulus applied to any sensory 

 cell would spread to the adjacent sensory and muscular cells, and 

 the response of the muscle-cells would be greatest near the stimu- 

 lated spot, gradually dying away as the area of the excitation widened. 

 A further step in the development of such a hypothetical elementary 

 nervous system would occur when certain of the sensory cells 

 (Fig. 133, c) developed a special sensitiveness, not to mechanical 

 changes in the environment, but to the protoplasmic excitatory 

 process arriving at them along the nerve network. These cells 



