146 



THE INTEGUMENT 



of these ducts in each nipple Aaries from one to many. They open 

 to the surface at the end of the nipple, where the stratified squamous 

 epithelium of the covering skin continues inward to line their outer- 

 most portion. Before reaching the nipple, the lactiferous ducts 

 dilate into reservoirs for the storage of the secreted milk. In the 

 inactive state the alveoli are collapsed and the gland is composed 

 mainly of connective tissue. 



Nails.— The nails are comparatively simple derivatives of the skin, 

 which cover the dorsal surface of the tips of the fingers and toes of 

 primates. (Fig. 88.) They are horny plates, slightly convex and 



Fig. 8S. — Cross-section of the finger tip and nail of human, showing nail bed, 

 grooves, nail, and skin of the finger. 



roughly rectangular. The skin underneath the nail bed rounds later- 

 ally and proximally into folds which form the nail wall. The over- 

 lapping of these walls on the nails make deep proximal and shallow 

 lateral nail grooves in which the nail lies. Under the proximal wall 

 the nail is soft and is composed of stratified squamous epitheliinn 

 corresponding to the stratum germinativum, proliferation from 

 which gives rise to cells that harden as they push outward. The nail 

 plate is formed of fused, flat, scale-like, cornified, ei)ithelium cells, 

 and corresponds to the stratum corneum of the skin. In the proximal 

 region the underlying epithelium is thicker and forms the matrix 

 from which new nail formation takes place. 



REFERENCES. 



Alvey, C. H. 1932. The epidermal "glands" of Ceratodus and Protopterus, 



Anat. Rec, 54, 91. 

 Cooper, Z. K. 1930. A histological study of the integument of the Armadillo, 



Tatusia novemcincta, Am. Jour. Anat., 45, 1. 

 David, L. T. 1932. Histology of the skin of the Mexican hairless swine (Sus 



scrofa), Am. Jour. Anat., 50, 283. 



