APPENDAGES OF THE SKIN. 81 



surface of the hand and fingers, and upon the sole, these lines are very distinct, 

 and are disposed in curves. They depend upon the large size and peculiar 

 arrangement of the papillae upon which the epidermis is placed. The deep 

 surface of the epidermis is accurately moulded upon the papillary layer of the 

 derma, each papilla being invested by its epidermic sheath ; so that when this 

 layer is removed by maceration, it presents a number of pits or depressions 

 corresponding to the elevations of the papillae, as well as the furrows left in 

 the intervals between them. Fine tubular prolongations from this layer are 

 continued into the ducts of the sudoriferous and sebaceous glands. In struc- 

 ture, the epidermis consists of flattened cells, agglutinated together, and having 

 a laminated arrangement. In the deeper layers the cells are large, rounded, 

 or columnar, and filled with soft opaque contents. In the superficial layers 

 the cells are flattened, transparent, dry, and firm, and their contents converted 

 into a kind of horny matter. The difference in the structure of these layers is 

 dependent upon the mode of growth of the epidermis. As the external layers 

 desquamate, from their being constantly subjected to attrition, they are repro- 

 duced from beneath, successive layers gradually approaching towards the free 

 surface, which, in their turn, die and are cast off. 



These cells are developed in the liquor sanguinis, which is poured out on 

 the free surface of the derma ; they contain nuclei, and form a thin stratum of 

 closely-aggregated nucleated cells, which cover the entire extent of the papil- 

 lary layer. The deepest layer of cells, according to Kolliker, are of a columnar 

 form, and are arranged perpendicularly to the free surface of the derma, form- 

 ing either a single or a double, or even triple, layer; the laminae succeeding 

 these are composed of cells of a more rounded form, the contents of which are 

 soft, opaque, granular, and soluble in acetic acid. As these cells successively 

 approach the surface by the development of fresh layers from beneath, they 

 assume a flattened form from the evaporation of their fluid contents, and finally 

 form a transparent, dry, membranous scale, lose their nuclei, and apparently 

 become changed in their chemical composition, as they are unaffected now by 

 acetic acid. 



The black color of the skin in the negro, and the tawny color among some 

 of the white races, is due to the presence of pigment in the cells of the cuticle. 

 This pigment is more especially distinct in the cells of the deeper layer, or rete 

 mucosum, and is similar to that found in the choroid. As the cells approach 

 the surface and desiccate, the color becomes partially lost. 



The arteries which supply the skin divide into numerous branches in the 

 subcutaneous tissue; they then pass through the areolae of the corium, and 

 divide into a dense capillary plexus, which supplies the sudoriferous and seba- 

 ceous glands and the hair-follicles, terminating in the superficial layers of the 

 corium, by forming a capillary network, from which numerous fine branches 

 ascend to the papillae. 



The lymphatic vessels are arranged in a minute plexiform network in the 

 superficial layers of the corium, where they become interwoven with the capil- 

 lary and nervous plexuses; they are especially abundant in the scrotum and 

 round the nipple. 



The nerves which supply the skin ascend with the vessels through the areolas 

 of the deep layers of the corium to the more superficial layers, where they 

 form a minute plexiform mesh. From this plexus the primitive nerve-fibres 

 pass to be distributed to the papillae. The nerves are most, numerous in those 

 parts which are provided with the greatest sensibility. 



The appendages of the skin are, the Nails, the Hairs, the Sudoriferous and 

 Sebaceous Glands, and their ducts. 



The nails and hairs are peculiar modifications of the epidermis, consisting 

 essentially of the same cellular structure as that membrane. 



The Nails are flattened, elastic structures, of a horny texture, placed upon 

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