TACTILE TISSUES 



201 



is the tactile cell, whose afferent process terminates in a nerve-ending 

 in the stratified epithelium of the cornea of mammals. It was formerly 

 thought that the branching fibrils of this ending lay free upon the corneal 

 surface. They probably reach almost if not quite to it. The least touch 

 of a foreign solid practically reaches these endings directly and causes 

 them to transmit a sensation of pain to the nerve centers. They come 

 very near, if not entirely, to being sensory tactile endings that operate 

 directly in response to the touch of solids and not through the agency of 

 any intermediate cells or substance. 



This form is little removed in similarity of structure and operation 

 from the great mass of tactile perceptory organs found, in the majority of 

 animals, at the periphery. These 

 organs utilize the living or (less 

 often) dead cells, among which 

 they branch, as intermediate or- 

 gans, through which they receive 

 their motion stimuli. When the 

 intermediate tissue is removed, 

 by accident or otherwise, the tac- 

 tile sensation is intensified to 

 one of pain. A good example 

 of a large and widely distributed 

 group is to be seen in the affer- 

 ent sensory endings of nerve cells 

 in the skin on the snout of the 

 pig (Fig. 179). This figure needs 

 but little explanation, showing 

 the end-organ fibrils passing in angular paths among the stratified 

 epithelial cells to within two or three cells of the surface. These 

 epithelial cells are not sensory. They can feel nothing. But the slight- 

 est touch on the surface pushes them against the nerve-endings, and 

 the sensation is carried to the nerve centers. Nor are these epithelial 

 cells specialized to do their work in any way that can be detected. Any 

 other cells in this position would do just as well. In fact, many of them 

 are dead cells. 



In the columnar cells of many animals which have no stratified epi- 

 thelium in the epidermis we find a similar arrangement. In the earth- 

 worm such a sensory structure is found (Fig. 180). Here the sensory 

 fibrils are placed around and between the proximal ends of the epithelial 

 cells which convey the motion stimulus to them in exactly the same way 

 that the stratified cells did in the pig's snout. We must realize the 

 difficulty, however, of distinguishing these sensory endings from motor 

 endings used to stimulate the gland cells of this epithelium into secretion. 



FIG. 179. Sensory nerve end-organ in the exter- 

 nal epithelium of a pig's snout. (After RETZIUS.) 



