796 



THE APPAEATUS OF THE SENSES. 



STRUCTURE. The epidermis comprises two layers, which 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 horny 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 papillae. 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 effects 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 

 the Sheep, whose skin is protected by a thick fleece ; 

 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 

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

 with the body, the flank, and between the thighs. 



T A.1 /^i i / mm 



Fig. 369. 



_==*'^-^- -- ' 



<^~2i? 



' -^^r- J^~- * 



OBLIQUE SECTION OP EPI- 

 DERMIS, SHOWING THE 



KENT CELS 



Nuclei resting upon the J? th Cow ' {i forms the lar g e pendulous layer at 

 surface of the derma,/,- * throat and breast, known as the " dewlap ;" and 

 these nuclei are gradu- in the Goat and Pig, it not unfrequently constitutes 

 ally developed into cells teat-like prolongations depending from the throat, 

 Lilt' aVlttnTd in": ^ nea ^ alw ^ s contain a small cartilaginous 

 lamella, forming the nuc l e us and some muscular fasciculi. The thickness 

 outer surface of the epi- of the epidermis is sometimes greatly increased by 

 dermis ' e - wear and friction, as we frequently see in the skin 



covering the knees of Sheep, etc.) 



(ihe functions of the skin are, as we have seen, tactile and secretory ; in 

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

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



