104 HISTORY OF THE HUMAN BODY 



The study of hair direction has excited an occasional inter- 

 est among morphologists, and a number of theories have been 

 advanced to explain the origin of the various features, but 

 there has been as yet too little morphological work in this field 

 to allow much theorizing or to serve as a basis for definite con- 

 clusions. The general tendency of the hair to slope backwards 

 from the point of the nose to the end of the tail suggests the 

 influence of the air-currents upon a rapidly moving body, or 

 at least an adaptation to them, the same phenomenon being 

 strikingly exhibited by the direction of feathers in birds, and 

 that of scales in reptiles; in the same way the general down- 

 ward slope of the hair along the sides of quadrupeds suggests 

 the influence of gravitation, especially when taken in connec- 

 tion with the apparent hair direction in the sloth, which shows 

 a parting along the mid-ventral line and is directed ventro- 

 dorsally, as if in correlation with the customary inverted posi- 

 tion of the animal. In opposition to this, however, it may be 

 pointed out that in certain areas the direction is the reverse of 

 that which either of the above forces would produce, and as 

 for the case of the sloth, the direction observed may be that 

 assumed by the long hair after emerging from the surface, 

 since the direction of the follicles seems never to have been 

 investigated. Darwin's well-known attempt to attribute the 

 hair direction on the human arms to the direct influence of 

 tropical rains upon the arms of simians, when held above the 

 head for protection, is at variance with the facts, and hence 

 must be dismissed from the discussion. 



Recently a new line of explanation has been sought in the 

 influence of underlying parts, especially that of the sub-cutane- 

 ous muscles, the constant traction of which influences the hair 

 follicles over definite areas, but this idea cannot as yet be con- 

 sidered to have passed the stage of a vague hypothesis, 

 especially since many of the observations are fallacious, and 

 hence have no weight in establishing the conclusions. 



It seems likely, since the hairs originated in association 

 with a complete coat of scales and at a time which must be 

 designated as premammalian, and since the original hair direc- 



