TEE APPENDAGES OF TEE SKIN. 903 



the other very close, and lodged in the papillary layer, which is traversed by 

 recurrent fibres giving off tubes that pass into the nerve-corpuscles of the papillae. 



The superficial network detaches the terminal fibres that pass into these 

 papillge or into the stratum mucosum of the epidermis. The intra-epidermic 

 nerve terminations have been particularly observed in the snout of the Pig. 



Epidermis. — The epidermis is a thin pellicle, covering the superficial face of 

 the derma ; it is destitute of blood-vessels, and is formed of cells which are being 

 continually deposited on the derma ; these cells become flattened in layers as they 

 are pushed up from the latter, and are destroyed by friction on the surface of the 

 skin. The deep face of the epidermis is moulded on the upper surface of the 

 derma ; consequently, it lodges the papillae, and dips into the follicles and ex- 

 cretory ducts of the glands of the skin ; its external face is not a very exact 

 repetition of the surface of the derma, and is covered with hair. The epidermis 

 tends to equalize, and to fill up, the depressions existing between the papillse. 



Structure. — The epidermis comprises two layers, which are not very distinct 

 from each other in the Horse. The deep layer, or rete Malpighi {stratum mucosum), 

 is composed of soft, nucleated, pigmented, denticulated 

 cells, which are sometimes attached by their fine pro- ^'s- '*^^- 



longations {prickle-cells) to other ceils more or less 

 distant ; there are spaces between them filled with an ^^ 

 amorphous semifluid substance. The superficicd or horny 

 layer {stratum corneum), is constituted by hard, horny, 

 flattened cells, which still contain some pigment-granules, -^^^^^ 



and are insensibly confounded with those of the rete ^^^t-S-^. 

 mucosum. ^ "^ ^ ""^-S 



(Where the epidermis is thick, there is seen between ^.j'^ 9 n%j^--p 

 the two layers just mentioned, a third — the stratum ^ <3> ^ g,; 'C®#-<* 

 lucidum — the nature of which has not yet been defined. p^g^j©^^' 

 It is transparent, and apparently amorphous. The theory "^^^S:,:^^^^^ 

 of growth of the epidermis is believed to be as follows : o^^ic^^j^ section of epi- 



5 1 , • 1 J- o DERMIS, SHOWING THE 



a layer of plastic lymph is thrown out on the surface ot progressive develop- 



the derma, and is converted into granules, which are ment of its compo- 



termed cell-germs, or cytoblasts. These imbibe serum ^^'^^ cells. 



from the lymph and adjacent tissues, so that the outer- «' ^",tl ^flKra,') • 



most covering of the cytoblast is gradually distended ; these nuclei are giadu- 



the latter becomes a cell, and its solid portion, which ally developed into cells 



. /• . . at 0, c, and a, and the 



always remains adherent to some point of the inner sur- cells are flattened into 

 face of the cell membrane, forms the nucleus of the cell. lamellae, forming the 

 Within this nucleus one or more nuclei are developed ; epiderm^i^l e^^ 

 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 demia 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 membranous scale, in which the 

 nucleus is scarcely apparent.) 



