CONCLUSION 213 



concerned. Thus the free-nerve terminations concerned 

 with touch in the vertebrate skin require a much more vig- 

 orous mechanical stimulus than do the auditory hair cells 

 in the ear, cells that have been appropriated by just such 

 a system as that seen in the tactile terminations. The 

 same is true of the chemical senses. The free termina- 

 tions of the primary sensory neurones of the human 

 mouth (Fig. 53, A) are sensitive to solutions of ethyl al- 

 cohol not less in concentration than 5 molecular, whereas 

 those connected with taste buds (Fig. 53, B) are stimu- 



L iji<C3> l> & 



FIG. 53. The appropriation of secondary sensory cells by primary sensory neurones in 

 the vertebrates; A, primary sensory neurone associated with the mouth surface of a ver- 

 tebrate and concerned with the common chemical sense of that surface; B, a primary sen- 

 sory neurone that lias appropriated secondary sensory cells as represented by those in a 

 taste bud in the vertebrate mouth. In each example the peripheral end of the neurone is 

 toward the left, the central toward the right. 



lated by a solution of concentration 3 molecular (Parker 

 and Stabler, 1913). Hence wherever a primary sensory 

 neurone has appropriated secondary sensory cells, there 

 is apparently an increase in the sensitiveness of the re- 

 sulting 1 combination. 



Not only have the sensory or afferent neurones of the 

 differentiated nervous system appropriated cells not 

 originally a part of the system, but the efferent neurones 

 have also associated themselves with effectors not in the 

 beginning under nervous control. Just as muscle arose 

 as an independent effector, so other like elements have 

 made their appearance in the animal series. To these 

 categories belong cilia, nettle cells, chromatophores, 

 luminous organs, glands, and so forth. Electric organs 



