TACTILE HAIR OF THE WHITE RAT 19 



All comparative study has shown that neither the size and 

 development of the tactile hair follicle nor the nerve endings 

 are proportional to the size of the animal. The rat, for instance, 

 possesses a far more highly developed tactile hair than the horse 

 or the ox or even the rabbit among the rodents. Leydig ('59) 

 fifty years ago, noted that in the horse the follicle was smaller 

 than in the dog or ox and the walls were thinner. In the polar 

 bear he found only a slightly developed tactile hair with a small 

 follicle and a sinus much more tender than a dog. The weasel 

 and otter, on the contrary, have longer follicles with a greater 

 ring sinus than the dog. The porcupine's follicle is egg shaped 

 and unusually large, but the greatest development of all the 

 trabecular filled sinuses is in the seal where it may be seen with 

 the naked eye. 



Of the investigations of the tactile hairs of the apes we shall 

 mention only one, that of Frederick ('05). He says that in some 

 apes the tactile hairs are so strong that they can be seen as such 

 by the eye alone. On the upper lip are many longer hairs which 

 are the longest and strongest laterally. As a rule the hairs are 

 weaker on the under lip. On microscopic study it is seen that 

 these hairs on the lips and in the supra-orbital regions are real 

 tactile hairs. The follicle is enclosed from without by a fibrous 

 capsule of connective tissue and between the external and inter- 

 nal lamina are cavernous spaces filled with blood. These extend 

 from the mouths of the sebaceous glands to the papilla. Fre- 

 quently they extend beneath the papilla, between it and the 

 base of the follicle, the whole space being filled with a close mass 

 of interpolated cavernous tissue. In none of the apes studied 

 was there any differentiation between the ring sinus and the 

 spongiform body; instead the whole cavernous space in its whole 

 length was crossed by numerous closely radiating connective 

 tissue bundles which connected the lamina externa with the 

 lamina interna. He found the ring sinus lacking in apes, which 

 in this respect are like hoofed animals. 



Szymonowicz ('09) finds the endings described in the tactile 

 hairs of other animals on the beard hairs of men. The nerve 

 ring contains more medullated fibers than is usually found in 



