THE INTEGUMENTARY SYSTEM 



175 



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 

 the human forearm suggests his animal ancestry. The hair of the fore- 

 arm slants from the wrist toward the elbow, in the reverse direction to 

 the slant on the upper arm. Man shares this pecuHarity with the apes 

 alone. All other mammals have the same hair direction on both parts 

 of the limb. Why this resemblance of man to the apes unless they share 

 a common ancestry? The pecuHarity is not adaptive, and it is not easy 

 to see why, if man and apes were 

 independently created, they should re- 

 semble one another in this detail. 



Hair Arrangement. That the 

 arrangement of hairs on the human body 

 has any evolutionary meaning is, to say 

 the least, surprising. Indeed, since such 

 patterns can have no use, we should 

 hardly expect to find them at all. No 

 less surprising is an arrangement of hair 

 in mammals that indicates descent from 

 scaly ancestors. 



In most mammals, the hairs occur in 

 groups of three or more. These groups 

 are arranged in parallel rows in such wise ^^^ ,33. -Arrangement of the 



that each cluster lies opposite an interval hairs in groups of threes and fives 



in the rows in front and behind. In ^^ the human embryo, with the 



probable ancestral arrangement oi 



short, the arrangement is imbricated, the scales. (From Kingsiey, after 

 like the universal arrangement of scales. Stohr.) 



This arrangement, though quite useless, is precisely what we should 

 expect if mammals have descended from scaly ancestors. 



Histogenesis of Hairs. Hairs are, in origin, epidermal, and therefore 

 ectodermal. Each begins as a minute epidermal papilla, which has 

 arisen by local cell proliferation in the stratum germinativum. Continued 

 proUferation gradually converts this papilla into a cellular column, which 

 extends obhquely downward into the underlying mesenchyma which is 

 to become the corium. The growing end swells into a bulb, in which 

 later develops the corium papilla from which the hair is to grow. Cellular 

 differentiation of the hair column results in an inner sheath and the 

 hair-shaft, all surrounded by an outer sheath. From the bulb to the 

 point in the hair column where the sebaceous gland develops, the cells 

 of the hair-shaft become cornified. Above this point the central cells 

 degenerate to form a canal in which the hairshaft grows towards the 

 surface. Continued cell multipUcation of the stratimi germinativum 

 of the papilla elongates the central hair shaft to extend beyond the skin. 



