112 



HISTORY OF THE HUMAN BODY 



all be referred to two primary forms, each originally de- 

 veloped in association with a hair. This primitive condition 

 is still common, though often with some slight modifications, 

 and is shown in diagrammatic form in Fig. 30. The glands 

 arise as outpushings from the wall of the follicle, into which 

 they empty. One of these types is a long, slender tube, often 

 convoluted at its free end, a tubular gland; the other short 

 and somewhat lobed, an acinous gland. Aside from this 

 morphological distinction the tubular gland is vitally secre- 

 tory, the acinous necrobiotic. As a secondary modification 

 either type may exist without a hair, but such cases are excep- 

 tional. It seems likely that such a complex as that represented 

 in the figure was originally associated, as are the hairs, with 

 the primary scales, one for each, the glands being associated 

 with the median hair only, but this cannot as yet be definitely 

 asserted. 



Each of the two glandular elements is capable of great mod- 

 ifications, both morphologically and physiologically. The 

 tubular type is not always convoluted, but may be straight 

 and simple, or branched. Its characteristic secretion is a thin, 

 colorless, watery fluid, the perspiration or sweat, but it is 

 viscous and reddish in the hippopotamus and albuminous and 

 of a blue color in Cephalophus, a South African antelope. A 

 much modified form of these glands furnishes the thick and 

 oily ear-wax. In distribution these glands are often found 

 over the entire body (hippopotamus, bear), but may be strictly 

 localized, as in most rodents, where they are found mostly on 

 the ventral surface of the paws. They fail entirely in the 

 two aquatic orders of Sirenia and Cetacea, also in Manis, in a 

 sloth (Chol&pus), and an insectivore (Chrysochloris). They 

 are usually found on the palmar and plantar surfaces, where, 

 in man and the monkeys, their openings are readily seen at 

 regular intervals along the middle line of the friction ridges. 

 In this location, by moistening the ridges, they undoubtedly 

 assist the firmness of the grasp, often a factor of vital im- 

 portance in an arboreal animal. When distributed over the 

 general surface and yielding the customary colorless watery 



