BLOOD SUPPLY 223 



are embedded in the subcutaneous fat or in the deeper part of the 

 corium near the hair follicle. The glands are so disposed as to be 

 included within a triangular space beneath the corium,, which is 

 bounded by the arrector pili muscle and the hair follicle. The 

 saccules are lined by several layers of polygonal epithelial cells 

 the outermost of which are cylindrical and rest upon the basement 

 membrane. 



In the peripheral layers the lining epithelial cells multiply so 

 actively that the daughter cells are pushed inward until they fill 

 the entire saccule. During this excursion they are progressively 

 farther and farther removed from their source of nutrition, and 

 as they approach the outlet or duct of the saccule a process of 

 fatty degeneration appears within the cell, by which its protoplasm 

 becomes changed into a granulo-fatty mass. The accumulated 

 product of this degeneration and final disintegration of the epi- 

 thelial cells forms the secretion of the gland. Continued cell 

 multiplication at the periphery maintains the integrity of the 

 organ. 



Development. The sebaceous glands are developed as minute 

 epithelial buds from the sides of the hair columns or from the 

 deeper surface of the epidermis. These buds soon assume the 

 characteristic flask-like shape and later become hollowed out by 

 fatty degeneration of their central cells. By this process also their 

 lumen is eventually made continuous with that of the follicle. 

 Secondary saccules of the sebaceous glands are developed in a 

 similar manner by outgrowing germs which appear near the con- 

 stricted neck portion of the primary saccule. 



BLOOD SUPPLY OF THE SKIN 



The larger arteries supplying the skin lie in the subcutaneous 

 tissue. From these vessels branches pass toward the surface, 

 giving off lateral twigs to the rich capillary plexuses in the sub- 

 cutaneous connective and adipose tissues and about the sweat 

 glands, hair follicles, and sebaceous glands. These arteries con- 

 tinue their course to the deeper part of the corium, where they 

 form an anastomosing plexus of small vessels. Branches from 

 this plexus pass to the papillary layer, where they form a second 

 plexus from which terminal arteries are distributed to the capil- 

 laries of the papillae. 



The distribution of the veins is similar to that of the arteries. 

 The primary plexus is found in the papillary layer ; occasionally 



