BOOK VII. 



ApPAKATUS of SlNSE. 



Among the nerves described in the preceding book, those which have been 

 designated sensory nerves have for their principal, or even exclusive f anction, 

 the carrying to the brain the excitations derived from the surrounding 

 physical world. These nerves are, therefore, the essential instruments of 

 sensation, and the organs to which they are distributed constitute the 

 APPABATUS or THE SENSES. These are admirably disposed for the re- 

 ception of the cerebral excitations, and are five in number : the apparatus 

 of tQucl, taste, smell, vision, and hearing. The principal charateristics of these 

 will be briefly enumerated 



CHAPTEE I. 



APPAEATTJS OF TOUCH. 



The sense of touch is destined for the appreciation of tactile sensations, 

 and, incidentally, those resulting from variations of temperature. The 

 apparatus composing it is formed by the peripheric radicles of the nerves 

 of general sensibility distributed in the skin: the resisting membrane 

 closely investing the whole of the body, and continuous, at the margin 

 of the natilral openings, with the mucous or internal membrane. 



The entire skin, therefore, represents the organ of touch ; but, as in Man, 

 this membrane has certain privileged regions which are more active than 

 others in the exercise of this faculty : these are the four extremities and the 

 lips. 



The structure of the skin, though pertaining to general anatomy, shall be 

 studied here somewhat in detail, and then the arrangement of its appendages 

 — the hair and horny productions — will be examined. 



Article I. — The Skin Peoper. 



The skin, properly speaking, is composed of two layers : the derma and 

 epidermis. 



The Derma. — The derma or chorion (corium cutis), forms nearly the 

 entire thickness of the membrane. Its inner face adheres more or less 

 closely to the subjacent parts, through the medium of a cellulo-adipose 

 expansion. Its external face, covered by the epidermis, which it secretes (or 

 forms), is perforated by openings through which the hairs pass, or through 

 which the secretion of the sudoriparous and sebaceous glands is thrown out 

 upon the surface of the skin ; tlus external face also shows a multitude of 

 little elevations termed, papilloe, in which the majority of the nerves terminate 



