TEH APPENDAGES OF THE SKTN. 



903 



Fig. 488. 



the other vei7 close, and lodged in the papillary la\-er, which is traversed by 

 recurrent fibres giving off tubes that pass into the nerve-eorpuscles of the papilla?. 



The superficial network detaches the terminal fibres that pass into these 

 papilhe or into the stratum mucomm of the epidermis. The intra-epiderraic 

 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 layere 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 papilhe. 



Structure. — The epidermis comprises tw'o layers, which are not very distinct 

 from each other in the Horse. The deep laj/er, or rete Malpighi {stratum mvrosum), 

 is composed of soft, nucleated, pigmented, denticulated 

 cells, which are sometimes attached by their fine pro- 

 longations {pricJcle-cells) to other cells more or less 

 distant ; there are spaces between them filled with an 

 amorphous semifluid substance. The superficial or fioni// 

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

 flattened cells, which still contain some pigment-granules, 

 and are insensibly confounded with those of the rete 

 mucosum. 



(Where the epidermis is thick, there is seen between 

 the two layers just mentioned, a third — the stratum 

 lucidum — the nature of which has not yet been defined. 

 It is transparent, and apparently amorphous. 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 dei-ma, and is converted into granules, which are 

 termed ceJl-ijenns, or cytohlasts. These imbibe serum 

 from the lymph and adjacent tissues, so that the outer- 

 most 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 sur- 

 face 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 papill*. 

 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.) 



OBLIQUE SECTION OF EPI- 

 DKRMIS, SHOWING THK 

 PROGRESSIVE DEVELOP- 

 MENT OF ITS COMPO- 

 NENT CELLS. 



a, Nuclei resting upon the 

 surface of the derma, /; 

 these nuclei are gradu- 

 ally developed into cells 

 at 6, c, and d, and the 

 cells are flattened into 

 lamellae, forming the 

 outer surface of the 

 epidermis, e. 



