STRUCTURE OF THE NAILS. 217 



tactile and a vascular papilla may spring from the same stem. The 



Fig. 144. 



■V 



r^r 



'^^^^\> \\ 





in 



^ nV 





, ^, 





Fig. 144. — Section op Skin showing two Papilla; and deeper Layers op Epidermis. 



(Biesiadecki. ) 



a, Vascular jiapilla with capillary loop passing from subjacent vessel c ; i, nerve papilla 

 with tactile corjmscle, t. The latter exhibits transverse fibrous markings : three nerve- 

 fibres, d, are represented as passing up to it : at // these are seen in oj)tical section. 



structure of these different terminal corpuscles has been already described 

 (pages 147 to 153). 



Chemical composition. —The cerium being- composed chiefly of white fibrous 

 tissue, has a coiTesponding- chemical composition. It is, accordingly, in a great 

 measui'e, resolved into gelatin by boiling', and hence, also, its conversion into 

 leather by the tanning process. 



Development of the cutis. — The cutis consists at fii'st of cells which may be 

 traced back to the fii-st formative cells of the embryo. Many of them give rise 

 to connective tissue ; others to vessels and nerves : and a thii'd portion is con- 

 verted into fat-cells. The mode of formation of these several elementary tissues 

 has been already described. Progressive development takes place from within 

 outwards, so that the papiUse ai-e fonned latest. 



NAIIiS 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 vas- 

 cular than in front. 



