THE INTEGUMENT AND THE EXOSKELETON 99 



placed by the permanent coat, which usually shows but slight 

 development save in certain definite localities. The lanugo 

 persists in a reduced condition on the face, especially in females, 

 forming the down which gives to the cheeks their character- 

 istic bloom. Abnormal hairiness in man, or hypertrichosis, 

 is fortunately rare, and is of two kinds ; the one, hypertricho- 

 sis vera, is due to an excessive growth of the permanent coat 

 which replaces the lanugo; the other, psendohypertrichosis, is 

 the result of the persistence of the lanugo. 



Localized hypertrophy in various mammals in the form of 

 manes, crests or tufts of hair, is of frequent occurrence and 

 is used for various purposes, such as defense from flies or 

 other noxious insects, attraction of the other sex, or as a pro- 

 tection from the teeth of rivals. Under this general head 

 come also the beard of man, which corresponds in position 

 and direction to that found in other primates, and the long 

 hair of the head. The other locations in Man in which long 

 hair occurs, the axillary and pubic regions, do not seem to 

 belong here, and probably represent portions that escaped 

 reduction rather than hypertrophy. The obvious function of 

 the cranial hair is a protection from the sun, and its location 

 suggests that it is developed with reference to the erect and 

 not the quadrupedal position, in which latter case it would 

 have extended farther down the back. The axillary and pubic 

 tufts may be for lessening the friction between the limbs dur- 

 ing motion ; it has been also suggested that they possessed a use 

 in transitional forms in furnishing places to which the infant 

 might cling, thus leaving the arms of the parent free for 

 climbing. In support of this latter view it may be noticed that 

 the distances between these locations correspond approxi- 

 mately to the proportions of a normal infant, and that an in- 

 fant thus attached is also in the right position for nursing. 



Aside from differences in caliber and length, the hair of vari- 

 ous mammals differs markedly in structure, in color, in the 

 shape of its cross-sections in various places and in the shape 

 assumed by each hair. In structure a hair consists of a firmer 

 cortex of varying thickness enclosing a softer medulla; a 



