BOOK VII. 



THE SENSORY APPARATUSES. 



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

 designated sensory nerves have for their principal, or even exclusive function, the 

 transmission to the brain of the impressions derived from the surrounding 

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

 tion, and the organs to which they are distributed constitute the sensory 

 apparatuses. These are admirably disposed for the reception of the cerebral 

 •stimuli, and are five in number— the apparatuses of touch, taste, smell, vision, and 

 hearing. The principal characteristics of these will be briefly enumerated. 



CHAPTER I. 

 APPARATUS OP TOUCH. 



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

 incidentally, those resulting from variations of temperature. The apparatus 

 which ministers to it, is formed by the peripheral radicles of the nerves of general 

 sensibility distributed in the skin — the resisting membrane closely investing the 



entire body, and continuous, at the margin of the natural 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 limbs and the lips. 



The structure of the skin, though pertaining to general anatomy, will 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. 



Preparation. — See treatises on histology. 



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

 epidermis. 



The Derma. — The derma or coriiim (corium cutis, cutis vera), forms nearly 

 the entire thickness of the membrane. Its inner face (stratum subcutaneum) 

 adheres more or less closely to the subjacent parts, through the medium of a 

 cellulo-adipose expansion {panniculus adiposus). Its external face, covered by 

 the epidermis, is perforated by openings through which the hairs pass, or through 

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



