TACTILE TISSUES 



205 



which end in a number of telodendria on the surfaces of the rudimentary 

 muscle and tendon fibers (Fig. 185). 



nv.end, 



FIG. 185. Neurotendinous nerve end-organ in the rabbit. (From HUBER and DE WITT in 

 Journ. of Comp. Neurology.) 



All the above organs were found placed in the soft tissues of animals, 

 and no hard parts were developed in connec- 

 tion with them. It is true that the cuticle 

 of the earthworm intervened between the 

 source of the motion and the simple epithe- 

 lium cells that more directly acted to pass the 

 movement on to the nerve-ending, but it can- 

 not be said that this cuticle was developed 

 in any way to perform this as a duty. The two 

 structures to be demonstrated now will each 

 show such a rigid organ, in the one case a 

 cuticle which is a cell-product, and in the 

 other a rigid hair made from the dead and 

 hardened bodies of the cells themselves, 

 which is developed solely to act as an inter- 

 mediate structure in transmitting the motion 

 stimulus. These are the tactile hairs of the 

 crustacean, Palamonetes, and the tactile hairs 

 or vibrissae of the cat and other mammals. 



The crustacean in question has much the 

 same sort of integument that the earthworm 

 had, a layer of simple columnar epithelial 

 cells that produce a cuticle. One of the chief 



differences is that the cuticle is thicker and, FlG . l86 ._ Tactile end-organ of 

 in addition, is usually impregnated with salts 

 of lime. This makes it much harder and 

 more resistant to the perception of outside 

 movements through its boundaries. The sen- 

 sory nerve supply comes to the periphery as 

 it did in the earthworm, and could receive 

 the tactile stimuli much as the earthworm's nerve-endings did were it 

 not for the thick and hard shell. A simple modification of the whole 



a nerve fiber in the tactile hair 

 of a shrimp, Palamonetes. cu., 

 cuticle; hyp., hypodermis, which 

 is invaginated at (h.c.) into the 

 hair cells, h., outer structure of 

 the hair; nv.end., nerve-ending 

 in the lower part of the hair. 

 (After PRENTISS.) 



