APPENDAGES OF THE SKIN. 605 



evaporation of their fluid contents, and finally form a transparent, dry, mem- 

 branous 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 sub- 

 cutaneous tissue ; they then pass through the areolse of the corium, and divide 

 into a dense capillary plexus, which supplies the sudatory and sebaceous glands 

 and the hair follicles, terminating in the superficial layers of the corium, by form- 

 ing 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 capillary 

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

 the nipple. 



The nerve's 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 papilla?. The nerves are most numerous in those parts which 

 are provided with the greatest sensibility. 



APPENDAGES OF THE SKIN. 



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 the 

 dorsal surface of the terminal phalanges of the fingers and toes. Each nail is 

 convex on its outer surface, concave within, and is implanted by a portion called 

 the root into a groove of the skin; the exposed portion is called the body, and the 

 anterior extremity, the free edge. The nail has a very firm adhesion to the cutis, 

 being accurately moulded upon its surface, as the epidermis is in other parts. 

 The part of the cutis beneath the body and root of the nail is called the matrix, 

 from its being the part from which the nail is produced. Corresponding to the body 

 of the nail, the matrix is thick, and covered with large highly vascular papillae, 

 arranged in longitudinal rows, the color of which is seen through the transparent 

 tissue. Behind this, near the root of the nail, the papilla? are small, less vascular, 

 and have no regular arrangement ; hence the portion of the nail corresponding 

 to this part is of a whiter color, and called lunula from its form. 



The cuticle, as it passes forwards on the dorsal surface of the finger, is attached 

 to the surface of the nail, a little in advance of its root ; at the extremity of the 

 finger, it is connected with the under surface of the nail, a little behind its free edge. 

 The cuticle and horny structure of the nail, both epidermic structures, are thus 

 seen to be directly continuous with each other. The nails, in structure, consist of 

 cells having a laminated arrangement, and these are almost essentially similar to 

 those composing the epidermis. The deepest layer of cells which lie in contact 

 with the papilla3 at the root and under surface of the nail, are of elongated form, 

 arranged perpendicularly to the surface, and provided with nuclei ; those which 

 succeed these are of a rounded or polygonal form, the more superficial ones 

 becoming broad, thin, and flattened, and so closely compacted together as to make 

 the limits of each cell very indistinct. 



It is by the successive growth of new cells at the root and under surface of the 

 body of the nail, that it advances forwards, and maintains a due thickness, whilst, 

 at the same time, the growth of the nail in the proper direction is secured. As 

 these cells in their turn become displaced by the growth of new cells, they assume 



