206 



HISTOLOGY 



neighboring region takes place, where each tactile ending comes 

 to the periphery, so that the slightest stimulus can be received. 

 The simple epithelium at such a point is invaginated into a group 

 of hair cells (Fig. 186, h.c.) t whose distal ends are lengthened out 

 so as to make a long, thin, hairlike projection (h.) from the surface 

 of the body. These cells form a cuticle, as do all other epithelial 

 cells, but it is not as thick as that on the rest of the body, and on 

 account of the form of the cells from which it is derived it is shaped 

 like a hair. The nerve-ending is single and large. It probably rep- 

 resents the ending of a single cell, and not one of the numerous 

 branches, as do the endings of like nature in the pig's snout. It lies 

 in a position homologous to that occupied by the tactile endings in 



the other epithelia at the bases 

 of the epithelial cells. It ex- 

 tends up into the fiber in this 

 position for a considerable dis- 

 tance, and the latter part of 

 its course must represent a 

 passage between the sides of 

 the cells, while the earlier part 

 was undoubtedly a contact with 

 their bases. 



The second example is that 

 of the mammal's tactile hair. 

 For the structure of the hair 

 itself, see the part devoted to 

 this subject in Chapter XX. 

 The nerve supply consists of the 

 terminal branches of a single 



ctile, sensory nerve-endings placed fiber which approaches the hair 



n.f. 



nv. end. 



FIG. 187. Ta 



work end-organs at nv.end. (From EDINGER and branches that encircle it just 

 HALL, after VAN GEHUCHTEN.) bdow the mouths o f fog 



sebaceous glands. This ring gives off branches that are naked and vari- 

 cose and run a short distance distally before terminating. There are 

 a considerable number of these end fibrils which lie outside the glassy 

 layer of the follicle (Fig. 187). 



The impulse in this case is transmitted in almost exactly the same 

 way as in the crustacean's tactile hair, although the two hairs are so 

 differently formed. In both the hair acts as a lever, transmitting the 

 slightest contacts with its outer portions, as greatly intensified motion 

 stimuli to the nerve end-organs at its base. In one case the nerve ter- 

 mination is inside the hair, while in the other it is outside. Some mol- 



