796 



THE AFFABATUS OF TBE SENSES. 



Structure. — The epidermis comprises two layers, whicli are not very- 

 distinct from each other in the Horse. The deep layer, or rete mucosum. is 

 composed of soft, nucleated, pigmentary cells, which are round on the surface 

 of the derma, and polyhedric elsewhere. The superficial, or liorny layer, is 

 constituted by hard, horny, flattened cells, which still contain some pigment- 

 granules, and are insensibly confounded with those of the rete mucosum. 



(The theory of growth of the epidermis is believed to be as follows : — a 

 layer of plastic lymph is thrown out on the surface of the derma, and is 

 converted into granules, which are termed cell-germs, or cytoblasts. These 

 imbibe serum from the lymph and adjacent tissues, so that the outermost 

 covering of the cytoblast is gradually distended ; the latter becomes a cell, 

 and its solid portion, which always remains adherent to some point of the 

 inner surface of the cell membrane, forms the nucleus of the cell. Within 

 this nucleus one or more nuclei are developed ; these are named nucleoli. 

 The process of imbibition continuing, the cell becomes more or less spherical ; 

 so that, after a certain time, the papillary layer of the derma is covered by a 

 thin stratum of spherical cells pressed closely together, and corresponding 

 with every irregularity of the papillse. New cells being continually produced 

 before the formation of the others has been quite completed, these are removed 

 in layers further and further from the surface of the derma, and becoming 

 subjected to the influence of physical laws, their fluid contents evaporate : 

 they collapse, flatten, and gradually assume an elliptical shape ; then they 

 are a mass of completely flat cells, with an included 

 nucleolated nucleus, and finally become a thin mem- 

 branous scale, in which the nucleus is scarcely appa- 

 rent). 



In Solipeds and other animals, the epidermis is 

 generally dark-coloured, from the presence of pig- 

 ment corpuscles, the number of which increases with 

 their depth in the membrane. This coloration is 

 intended to prevent the rubefacient eifects of the 

 heat of the sun's rays, by augmenting the absorb- 

 ing and dispersing power of the cutaneous surface. 

 In the majority of cases, this coloration is absent in 



Fig. 369. 



^ 



^^©/sT®^© „ _ ____..., 



* MM^m^^ *^® Sheep, whose skin is protected by a thick fleece ; 

 _1_B f¥ ^ y^^ and also in the Pig, whose habits in the wild, as in 

 a domesticated condition, keep it out of the direct 

 action of the sun. 



(In some regions of the body of all animals, the 



skm forms folds, as at the junction of the fore-limb 



^'ith the body, the flank, and between the thighs. 



a. Nuclei resting upon the 7^ f ^°'^' ^* ^°^™s ^'^e large pendulous layer at 



surface of the derma, /; y"^ throat and breast, known as the " dewlap ;" and 



these nuclei arc gradu- m the Goat and Pig, it not unfrequently constitutes 



fllJiZ/tiZ *t-i'''' P^olo'ig^tions depending from the throat, 



cells' are flattened into ^ "^^^^^^ always contain a small cartilaginous 



nucleus and some muscular fasciculi. The thickness 



of the epidermis is sometimes greatly increased by 



wear and friction, as we frequently see in the skin 



™, „ covering the knees of Sheep, etc.) 



^aa\- tmctions of the skin are, as we have seen, tactile and secretory ; in 



MdTtion, it IS eminently protective. Its secretory action is always more or 



lees active, but the production of perspiration is greatest when the body is 



OBWQUB SECTION OF EPI- 

 DEEMIS, SHOWINQ THE 

 PROGRESSIVE DEVELOP- 

 MENT OP ITS COMPO- 

 MENT CELLS. 



lamellie, forming the 

 outer surface of the epi- 



dermis, e. 



