74 HUMAN EMBRYOLOGY AND MOEPHOLOGY. 



directed downwards, and their epithelial outgrowths follow the 

 same direction, thus causing ingrowing nail. 



About the end of the 7th month the matrix of the nail root 

 becomes differentiated, active growth sets in and the terminal 

 margin of the nails become free ; it grows forwards over the 

 corneous layer which covers the terminal row of papillae of the 

 nail bed. The ridge of corneous epithelium under the nail-tip 

 represents the hoof of ungulates. 



Sweat Glands. — In the 5 th month solid processes of epidermis 

 grow into the dermis and produce sweat glands. They arise at 

 the same time and in the same manner as, and often in common 

 with, the buds of hair roots and sebaceous glands. They are pro- 

 duced within the papillary ridges, and hence the ducts of sweat 

 glands, as may be seen on the palms and fingers, open along the 

 summits of these. The sweat glands in the axillae are peculiar. 

 In section they resemble the acini of the mammary gland, also 

 believed to be highly modified sweat glands. The axillary glands 

 contain much epithelial debris. They appear to be sexual in 

 nature. 



Sebaceous Glands. — The sebaceous glands are outgrowths 

 from the more superficial part of hair buds (Fig. 55). Their 

 epithelial lining is derived from the germinal layer. In sheaths 

 which have become occluded from the loss of the hair, or when 

 the mouth of the gland is blocked, the secretion is retained, and 

 a sebaceous cyst or wen, so frequently seen in the scalp, is 

 produced. Round the mouth, on the lips and nose, the sebaceous 

 glands, especially in disorders of the sexual organs, are apt to 

 retain their secretions and become inflamed, small pustules being 

 thus produced. The Meibomian glands in the eyelids are modified 

 sebaceous glands. At birth the child is covered by the vernix 

 caseosa, which is composed of desquamated corneous epithelium 

 and the secretion of sebaceous glands. 



THE MAMMAE. 



The mamma is developed in the same manner in both sexes. 

 At puberty the female breast undergoes a great development, 

 while in the male it retains the infantile form. 



