THE INTEGUMENTARY SYSTEM 



121 



Hair Structure. The hair of all mammals is essentially similar. There 

 are, however, such differences of detail as enable an expert to identify 

 different species. 



Each hair consists of a "root" buried in the skin, and an external 

 shaft. Microscopic examination shows a multicellular structure, with 

 the cells in three layers, an outer cuticle, a cortex, and a central medulla. 

 The cells of the cuticle are scale-like, overlapping one another like shingles 

 on a roof. Cortex cells, greatly elongated, make up the greater portion 

 of each hair. The medulla occurs only in the "contour hairs" and is 

 wanting in the finer hairs. It is made up of cuboidal cells usually in a 

 double row. 



Fig. 114. — Diagram of structure of hair, b, blood-vessels; ct, cuticle of hair; ex, 

 cortex; g, gland; h, hair; he, Henle's layer; hf, hair follicle; hx, Huxley's layer; w, medulla; 

 p, papilla; sg, stratum germinativum of epidermis. (From Kingsley's "Comparative 

 Anatomy of Vertebrates.") 



The root is surrounded by epithelial and connective-tissue sheaths. 

 It ends in a swollen "bulb," from which it grows and which contains a 

 connective-tissue papilla, with capillaries which feed the hair. 



Hairs of different human races differ in cross section. In general, the 

 rounder the hair, the straighter it is; the more compressed, the curlier. 

 It has not been shown that these differences have been developed either 

 by natural or by sexual selection. 



Hair Direction. Hairs, instead of projecting vertically from the skin, 

 emerge at an acute angle, have a slant in some special direction, and thus 

 form streams in various parts of the body. Where such currents meet, 

 either "rhomboids" or "vortices" may form, the latter being commonly 

 called "cowlicks." The fact that such rhomboids and vortices appear 

 on the human body in regions where the hair is short, has been interpreted 

 to mean that man's hairy covering was once longer than at present. 



Although in general the direction of hair growth is such as to make 

 gravity the determining influence, it is a curious fact that the hair on 



