290 



PHYSIOLOGY 



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 ad- 

 jacent sensory and muscular cells, and the response of the muscle cells would 

 be greatest near the stimulated spot, gradually dying away as the area of the 

 excitation widened. A further step in the development of such a hypotheti- 

 cal elementary nervous system would occur when certain of the sensory cells 

 (Fig. 132, 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 would act as relays of force, picking 



up the excitations arriving from 

 the undifferentiated sensory cell-. 

 and sending them on with increased 

 vigour along the nerve network. 

 In such a manner a stimulus 

 applied at one point could be sent 

 on in successive relays from cell to 

 cell throughout the whole reactive 

 tissue on the surface of the body. 

 We cannot point to any par- 

 ticular animal as presenting in- 

 stances of either of the two types 

 of elementary nervous system 

 just described. If such exist, t hex- 

 have not yet been investigated, 

 or the undifferentiated character 

 of their nervous tissues has 

 thwarted the efforts of zoologists 

 to display their specific characters 

 .by staining reagents. In the 

 lowest definite nervous system 

 with which we are acquainted, 

 namely, that of the jelly-fish, 



all three types of cell, the sensory cell, the reactive or central cell, and 

 the motor cell, are already developed and have undergone among 

 themselves a considerable degree of differentiation. In a j elly-fish or medusa, 

 such as aurelia or sarsia (Fig. 133), the reactive tissue of the body is confined 

 to the under-surface of the so-called umbrella with the tentacles and manu- 

 brium. A section through the umbrella shows a layer of epithelium contain- 

 ing differentiated sense cells, below which is a plexus or rather network 

 of fine nerve til>iv> with a certain number of nerve cells at the nodes of the 

 netxvork. From this network fibres pass more deeply to end in a finer net- 

 work situated among a layer ot muscle fibres formed, like the sensory cells, by 

 a differentiation of the primitive epithelium or epiblast (Fig. 134). Besides 



Fio. 133. Diagrammatic view of a jelly-fish. 



(HEBTWIO.) 



U, umbrella ; M, manubrium ; T L , T 2 , tentacles ; 

 v, velum ; N, nerve ring ; R. ' marginal body.' 



