122 SENSE OF TOUCH. 



as will be seen in some respects from our appreciation of their 

 temperature, its consideration properly belongs to the sense we are 

 considering, in the acceptation here given to it, and adopted by -all the 

 French physiologists. According to them, tact is spread generally in 

 the organs, and especially in the cutaneous and mucous surfaces. It 

 exists in all animals ; whilst touch is exercised only by parts evidently 

 destined for that purpose, and is not present in every animal. It is 

 nothing more than tact joined to muscular contraction and directed 

 by volition. So that, in the exercise of tact, we may be esteemed pas- 

 sive; in that of touch, active. 



The organs concerned in touch, execute other functions besides ; and 

 in this respect touch differs from the other senses. Its chief organ, 

 however, is the skin ; and hence it is necessary to inquire into its struc- 

 ture, so far as is requisite for our purpose. 



1. ANATOMY OF THE SKIN, HAIR, NAILS, ETC. 



The upper classes of animals agree in possessing an outer envelope 

 or skin, through which the insensible perspiration passes ; a slight de- 

 gree of absorption takes place; the parts beneath are protected; and 

 the sense of touch is accomplished. In man, the skin is generally 

 considered to consist of four parts, the cuticle, rete mucosum, corpus 

 papillare, and corium ; but when reduced to its simplest expression, the 

 whole of the integument, with the mucous membrane, which is an ex- 

 tension of it, may be regarded as a continuous membrane, more or less 

 involuted, more or less modified by the elementary tissues which com- 

 pose it or are in connexion with it, and within which all the rest of the 

 animal is contained. It consists of two elements a basement tissue 

 or membrane, composed of simple membrane, uninterrupted, homo- 

 geneous, and transparent ; covered by an epithelium or pavement" of 

 nucleated particles. 1 



1. The epidermis or cuticle is the outermost layer. It is a dry, 

 membranous structure, devoid of vessels and nerves ; yet it is described 

 by some recent investigators as a tissue of a somewhat complex organiza- 

 tion, connected with the functions of exhalation and absorption ; but its 

 vitality is regarded to be on a par with that of vegetables. The absence 

 of nerves proper to it renders it insensible ; it is coloured ; and exhales 

 and absorbs in the manner of vegetables. It is, so far as we know, 

 entirely insensible ; resists putrefaction for a long time, and may be 

 easily obtained in a separate state from the other layers by maceration 

 in water. It is the thin pellicle raised by a blister. 



The cuticle is probably a secretion or exudation from the true skin, 

 which concretes on the surface ; becomes dried, and affords an efficient 

 protection to the corpus papillare beneath. It is composed, according 

 to some, of concrete albumen ; according to others, of mucus ; and is 

 pierced by oblique pores for the passage of hairs, and for the orifices of 

 exhalant and absorbent vessels. MM. Breschet and Roussel de Vauzeme 2 

 affirm, that there is a special "llennogenous or mucifie apparatus" for 



1 Todd and Bowman, The Physiological Anatomy and Physiology of Man, p. 404, London, 

 1845. 

 3 Nouvelles Recherches sur la Structure de la Peau, par M. Breschet, Paris, 1835. 



