176 



COMPARATIVE ANATOMY 



Each hair thus formed continues to elongate throughout its Ufa of 

 some months or years, the rate of growth varying greatly in different 

 parts of the body. But finally, growth ceases, the hair dies, and is shed. 

 If the hair papilla retains its stratum germinativum, a new hair grows. 



Each hair column, in addition to producing a hair, may form as lateral 

 outgrowths one or more sweat or sebaceous glands. Muscle cells devel- 

 oped from the mesenchyma of the corium attach themselves to the hair- 

 roots and become arrectores pilorum. 



MESENCHYMA 



HAIR COLUMN 



CORN in ED 

 INNER SHEATH 



.- ARRBCTOR MUSCLE 



SEBACEOUS GLAND 

 EPITHELIAL BED 



■ROOT OF HAIR 



MESENCHYMA 



Fig. 134. — A vertical section of skin of a five month human embryo, showing four 

 early stages in the development of a hair. The growth of a hair is initiated by the forma- 

 tion of an epidermal papilla projecting (down) into the underlying corium. (Redrawn 

 from Bremer after Stohr.) 



The human foetus has before birth a hairy covering, the "lanugo," 

 which is shed shortly before or soon after birth. The coat persists, 

 however, in certain types of "hairy men." The evolution theory affords 

 the only rational explanation of the lanugo. 



PIGMENT 



Skin color in man is due in part to the blood in the capillaries of the 

 corium. In addition, there are two pigments in the skin and hair, a 

 brown, sometimes darkened to a black, both in granules, and a yellow, 

 that may strengthen to a red, both diffused in the tissues. All are prod- 

 ucts of cell metabolism. 



The pigments of the hair are confined to the cortex. The epidermis 

 and the outer parts of the corium are both pigmented. Not until shortly 

 after birth do pigment granules appear in the stratum germinativum, 

 so that even negroes are born white. 



Moles and freckles involve excessive local pigmentation. Freckles 

 are small local patches of excess pigmentation, which are more Hkely to 

 occur in light-skinned individuals who have been exposed to strong sun- 



