ccvi THE SKIK 



beneath the net of superficial blood-capillaries ; in certain parts on the 

 palm and sole, lymphatics pass into the papillae, but do not reach their 

 summits. 



Nerves. Nerves are supplied in very different proportions to different 

 regions of the skin, and according to the degree of sensibility. They pa?s 

 upwards towards the papillary surface, where they form plexuses, of which 

 the meshes become closer as they approach the surface, and the constituent 

 branches finer, so that the latter come at last to consist of only one or two 

 primitive fibres. The fibres also become more attenuated the further they 

 proceed towards their final destination. In the finest and most superficial 

 part of the plexus, the ultimate fibres, or at least some of them, undergo 

 actual division. Little more can be said of the termination of nerves on the 

 general cutaneous surface. A large share of the cutaneous nerves is dis- 

 tributed to the hair- follicles, whilst some end in special terminal organs, 

 namely, end-bulbs, tactile corpuscles, and Pacinian bodies. The last named 

 bodies are seated in the subcutaneous tissue. End-bulbs are found on the 

 glans penis and glaus clitoridis, and in some of the papillse on the red 

 border of the lips. The tactile corpuscles of the skin are more numerous ; 

 they are found in certain papilla of the palm and sole, more sparingly in 

 those of the back of the hand and foot, the palmar surface of the fore-arm 

 and the nipple. Such papillse commonly contain no blood-vessels, and are 

 named "tactile," as distinguished from the "vascular" papillse which 

 receive no nerves. Sometimes, however, a tactile and a vascular papilla 

 may spring from the same stem. The structure of these different terminal 

 corpuscles has been already described (pages cl to civ). 



Chemical composition. The corium being composed chiefly of white fibrous tissue, 

 has a corresponding chemical composition. It is, accordingly, in a great measure, 

 resolved into gelatin by boiling, and hence, also, its conversion into leather by the 

 tanning process. 



Development of the cutis. The cutis consists at first of cells which may in animals 

 be traced back to the first formative cells of the embryo. Many of them give rise to 

 connective tissue ; others to vessels and nerves ; and a third portion is converted into 

 fat-cells. No doubt the muscular tissue also originates from cells. The mode of 

 formation of these several elementary tissues has been already described. Progressive 

 development takes place from within outwards, so that the papillae are formed 

 latest. 



The cuticle at first differs in no point from the cutis, but consists of the earliest 

 formative cells. Their subsequent metamorphoses and the mode of production of new 

 cells have not been accurately determined ; the question has been already considered 

 at page Iv. 



Nails and hairs. The nails and hairs are growths of the epidermis, 

 agreeing essentially in nature with that membrane ; their epidermic tissue 

 is destitute of vessels and nerves, and separable from the cutis. 



Nails. The posterior part of the nail which is concealed in a groove of 

 the skin is named its "root," the uncovered part is the "body," which 

 terminates in front by the "free edge." A small portion of the nail near 

 the root, named from its shape the lunula, is whiter than the rest. This 

 appearance is due partly to some degree of opacity of the substance of the 

 nail at this point, and partly to the skin beneath being less vascular than 

 in front. 



The part of the coriurn to which the nail is attached, and by which in 

 fact it is secreted or generated, is named the matrix. This portion of the 

 skin is highly vascular and thickly covered with large vascular papillse. 



