CHAPTER Vr. 



OP TOUCH. 



Touch is the most simple of all the senses, the least differen- 

 tiated. The tactile sensations result from a shock, a pressure, 

 an agitation bearing almost directly upon the extremities of the 

 sensitive fibres. Where these fibres are very numerous, tactile 

 sensibility is strongly developed. It is habitually at certain 

 points of the teguments, skin, or mucous membrane that these 

 agglomerations of sensitive nervous terminations are found. 

 Ordinarily then these terminal extremities are clothed with an 

 envelopment, a kind of hood; sometimes they are terminated 

 by a nervous cell. There are then what are called corpuscles of 

 touch. 



The sense of touch is in some measure distributed everywhere 

 among the higher mammifers ; but it exists, more or less 

 developed, in the whole of the animal series. As it is the least 

 perfect of the senses, it is also the one which offers the least 

 variety in the different animal classes, compai-ed amongst them- 

 selves. 



In the hydral polypi and the anthozoaries, thetentacula which 

 surround the mouth are conadered as tactile organs. The 

 annelates, the hii'udinates, have as organs of touch tegumentary 

 cells, which have the form of bristles, baculi, in connection' with 

 sensitive fibres. According to Leydig, these tactile cutaneous 

 organs are sometimes, in the hirudinates, grouped in large 

 numbers at the bottom of cupuliform depressions. 



