?3 ANIMAL PHYSIOLOGY. 



is adherent thereto. Hence it happens that the nail is 

 stronger the farther from the root, and that its outer surface 

 is harder than the deep part where the recent and soft addi- 

 tions to the thickness are placed. 



The substance of a nail is an instance of the texture called 

 horn. But it is always to be kept in mind that the word 

 horn has a double meaning, which has descended to it from 

 the Latin. This has obviously arisen from the structure 

 of the horns of the sheep and cxen, which consist of an outer 

 coating of the texture called horn investing a core of bone. 

 But on the horn of the stag there is no horny covering, the 

 structure being a growth of bone, which, when young, is 

 covered with integument, but afterwards becomes denuded. 

 Instances of morbid growth of solid horny texture occur 

 occasionally in man, but rarely grow to any considerable size. 

 In bedridden persons, whose nails have been neglected, it 

 sometimes happens that some of them project like claws, 

 curving over beyond the digit, and become nearly as thick as 

 they are broad. 



47. A Hair consists of a bulb or root imbedded in a follicle, 

 and a shaft or stem ascending therefrom. The follicle is an 

 invagination of the integument lined with a thin prolongation 

 of the cuticle, divisible into two layers, sometimes called the 

 inner and outer root-sheath, arid so adherent to the root of 

 the hair that it is liable to be removed with it when a hair is 

 pulled out. At the base of the follicle is an enlarged 

 papilla ; and the hair itself may be considered as the ex- 

 aggerated cuticular investment of this papilla. The root 

 of the hair forms a bulbous enlargement round the papilla, 

 and consists in greater part of polygonal cells, which in 

 dark hairs are loaded with pigment ; but towards the upper 

 part, round about, the cellular substance is changed to 

 iibrous, and on the surface there is an imbricated epithelium 

 continuous, below, with the innermost layer of 'the cuticular 

 lining of the follicle. All these three elements may be repre- 

 sented in the stem. The epithelium on the surface of the 

 root, traced upwards, is seen to form on the stem an ex- 

 tremely thin coat, of which the most easily discernible part 

 is a network formed by the thickened edges of the scales. In 

 many of the larger hairs, the cells which constitute the main 



