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 cells, 



S S^Xn- 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 



.!*>Y^ ''^^- ce ll throughout the whole reactive 



il^^Cj """""""'^^^ tissue on the surface of the body. 



|| ** li^/ We cannot point to any par- 



ticular animal as presenting in- 

 stances of either of the two types 



1 



of elementary nervous system just 



described. If such exist, they 

 have not yet been investigated, 

 or the undifferentiated character 

 | of their nervous tissues has 

 thwarted the efforts of zoologists 

 to display their specific characters 

 FIG. 133. Diagrammatic jiew of a jelly-fish. by staining rea gents. In the 



u, umbrella; M, manubrium ; T X , T 2 , tentacles ; lowest definite nervous System 

 v, velum ; N, nerve ring; B, ' marginal body.' with which we ^ ac q uainte d, 



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 jelly-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 manubrium. 

 A section through the umbrella shows a layer of epithelium containing 

 differentiated sense-cells, below which is a plexus or rather network of fine 

 nerve fibres with a certain number of nerve-cells at the nodes of the network. 

 From this network fibres pass more deeply to end in a finer network situated 

 among a layer of muscle fibres formed, like the sensory cells, by a differentia- 

 tion of the primitive epithelium or epiblast (Fig. 134). Besides this diffuse 



