APPENDAGES OF THE SKIN. 525 



Vessels and Nerves. The Arteries of the dermis which enter its 

 structure through the areolae of the under surface of the corium, divide 

 into innumerable intermediate vessels, which form a rich capillary 

 plexus in the superficial strata of the skin and in its papillary layer. 

 In the papillae of some parts of the dermis, as in the longitudinal pli- 

 cations beneath the nail, the capillary vessels form simple loops, but 

 in other papillae they are convoluted to a greater or less degree in pro- 

 portion to the size and importance of the papillae. The Lymphatic 

 vessels probably form, in the superficial strata of the dermis, a plexus, 

 the meshes of which are interwoven with those of the capillary and 

 nervous plexus. No lymphatics have as yet been discovered in the 

 papillae. 



The Nerves of the dermis, after entering the areolae of the deeper 

 part of the corium, divide into minute fasciculi, and these quickly sepa- 

 rate into primitive fibres, which form loops in the papillae. In the less 

 sensitive parts of the skin the loops are simple, and more or less acute 

 in their bend, in conformity with the figure of the papilla. In the sen- 

 sitive parts, however, and especially in the tactile papillae of the pulps 

 of the fingers, the loop is convoluted to a greater or less extent, and 

 acts as a multiplier of sensation. 



APPENDAGES OF THE SKIN. 



The appendages of the skin are the nails, hairs, sebaceous glands, 

 and perspiratory glands and ducts. 



The NAILS are horny appendages of the skin, identical in formation 

 with the epidermis, of which they are a part. A nail is convex on its 

 external surface, concave within, and implanted by means of a thin 

 margin or root in a fold of the dermis (matrix), which is nearly two 

 lines in depth, and acts the part of a follicle to the nail. At the 

 bottom of the groove of the follicle are a number of filiform papillae, 

 which produce the margin of the root, and, by the successive formation 

 of new cells, push the nail onwards in its growth. The concave sur- 

 face of the nail is in contact with the dermis, and the latter is covered 

 by papillae, which perform the double office of retaining the nail in its 

 place, and giving it increased thickness by the addition of newly- 

 formed cells to its under surface. It is this constant change occurring 

 in the under surface of the nail, co-operating with the continual re-pro- 

 duction taking place along the margin of the root, which ensures the 

 growth of the nail in the proper direction. The nail derives a peculi- 

 arity of appearance from the disposition and form of the papillae on the 

 ungueal surface of the dermis Thus, beneath the root, and for a 

 short distance onwards towards its middle, the dermis is covered by 

 papillae which are more minute, and consequently less vascular than the 

 papillae somewhat farther on. This patch of papillae is bounded by a 

 semilunar line, and that part of the nail covering it being lighter in co- 

 lour than the rest, has been termed lunula. Beyond the hmula the 

 papillae are raised into longitudinal plaits, which are exceedingly vascu- 



