3io 



EXCRETION 



The connections of the nails with the true skin resemble those of the 

 epidermis ; but the relations of these structures to the epidermis itself 

 are somewhat peculiar. Before the fourth month of foetal life, the epi- 

 dermis covering the dorsal surfaces of the last phalanges of the fingers 

 and toes does not present any marked peculiarities ; but at about the 

 A. B fourth month, the hard cells 



^-^^~^ of the horny layer of the nails 

 make their appearance be- 

 tween the Malpighian and 

 the horny layers of the epi- 

 dermis, and at the same time 

 the Malpighian layer beneath 

 this plate, which is destined to 

 become the Malpighian layer 

 of the nails, is thickened and 

 the cells assume a more elon- 

 gated form. The horny layer 

 of the nails constantly thickens 

 from this time ; but until the 

 end of the fifth month, it is 

 covered with the horny layer 

 of the epidermis. After the 

 fifth month, the epidermis 

 breaks away and disappears 

 from the surface ; and at the 

 seventh month, the nails begin 

 to increase in length. Thus, 

 at one time the nails are actu- 

 ally included between the two 

 layers of the epidermis ; but 

 after they have become devel- 

 oped, they simply are covered 

 at their roots by a narrow 

 border of the horny layer. The nails are therefore to be regarded as 

 modifications of the horny layer of the epidermis, possessing certain 

 anatomical and chemical peculiarities. The Malpighian layer of the nails 

 is continuous with the corresponding layer of the epidermis, but the 

 horny layers are distinct. 



Physiological Anatomy of the Hairs. Hairs, varying greatly in size, 

 cover nearly every portion of the cutaneous surface. The only parts in 

 which they are not found are the palms of the hands and soles of the 

 feet, the palmar surfaces of the fingers and toes, the dorsal surfaces of 



Fig. 66. Section of the nail (Sappey). 



A, section of the nail: i, i, superficial layer; 2, deep 

 layer ; 3, 3, 4, 4, section of the grooves on the attached sur- 

 face ; 5, 5, union of the superficial with the deep layer ; 

 6, dark line between the two layers, 



B, cells of the superficial layer, lateral view. 



C, cells of the superficial layer, flat view. 



D, cells of the deep layer. 



