INTEGUMENT 23 



A periodic casting of feathers, or moulting, takes place in 

 all Birds, and corresponds to the similar process of the casting of 

 the horny layer of the skin in Amphibians and Eeptiles. 



The feather-covering of Birds must have been acquired in very early 

 geological periods, for Archseopteryx, found in the Jurassic strata of Bavaria, 

 possessed well-formed feathers with a very delicate shaft and vane. Paloeonto- 

 logical researches have not brought to light any definite intermediate stages 

 between scales and feathers, but that they must once have existed is shown 

 by the development of these structures. 



Mammals. — The integument of Mammals gives rise to hairs, 

 ■which are characteristic of and confined to this Class. They may be 

 almost uniformly developed all over the body and even on the soles 

 ■of the feet, or may become reduced in more or less extensive regions. 

 They are most scanty in the Cetacea, vehere only a few occur on 

 the lips, and even these may disappear in the adult. The 

 ■first to appear are certain tactile-hairs (vihrissce) on the head, along 

 the course of the trigeminal nerve ; all the hairs, however, serve 

 as tactile oi'gans as well as for keeping the body warm. 



Nothing definite can be at present stated as regards the 

 phylogenj' of hairs, but it seems at any rate probable that they 

 are not directly comparable to the scales of Eeptiles and feathers 

 ■of Birds : ^ the arrangement of the hairs in alternating groups is 

 probably the last indication of the former possession of scales. 



Each hair first arises as a proliferation of the epidermic cells 

 in the region of the Malpighian layer which comes to project 

 inwards towards the dermis (Fig. 15, A, B and C). In this 

 manner the hair-germ is formed. Thus the epidermic portion is the 

 primary one ; a corresponding dermal papilla is formed secondarily, 

 and is the homologue of the papilla which forms the first trace of 

 the scale in a Reptile or the feather in a Bird. 



The thickening of the epidermis then grows downwards in the form of a 

 papilla and becomes surrounded by the cells of the dermis, so that, as in the 

 case of the feather, it comes to lie within a kind of pocket, the hair-follicle 

 (Fig. 14, C). The originally uniform mass of cells of the hair-germ later 

 becomes differentiated into a peripheral and a central portion. The latter 

 consists of more elongated cells, and gives rise later to the hair-shaft with its 

 medulla or pith, and to the cortex, as well as to the cuticle of the shaft and 

 tlie so-called inner rout-sheath; the former gives rise the outer root-sheath 

 (comp. Fig. 16 A, which represents the fully-formed hair). The base of the 

 liair-shaft which fills up the bottom of the follicle is broadened out to form 

 the Iwir-bulb (Fig. 15, D), which grows round the later formed and highly 

 vascular hair-papilla like a cap (C, D). At Dr, in D, the sebaceous glands 

 (p. 27) are seen arising by a proliferation of the Malpighian cells. The hair 

 usually breaks through the skin in an oblique direction ; the direction difi'er- 

 ing in different parts of the body. 



The hair or hair-shaft embedded at its base in the hair- 

 follicle, is more or less cylindrical: it consists of three parts — 



^ It has been suggested that tlie hairs correspond to modified integumentarj' 

 .sense-organs such as occur in the lower Vertebrates (comp. Figs. 15 and 150). 



