78 HISTORY OF THE HUMAN BODY 



is eaten by the owner. In birds and mammals there is no 

 periodic moult, so far as the skin is concerned, and no con- 

 tinuous layer cast off, but the dead and dried cells are con- 

 stantly being worn from the surface and pass away unnoticed. 

 In these animals, however, there is usually a definite period 

 for the renewal of the accessory parts, the feathers and hairs, 

 a form of moult to be carefully distinguished from the fore- 

 going. 



The corium, in common with other connective tissues and 

 in contrast to the epidermis, is not composed wholly of cells, 

 but consists in great measure of fibers, which run in all direc- 

 tions between the cells and are produced through their agency. 

 These fibers, which, though not the formative element of the 

 corium, are the most important structural ones, form a rather 

 loose and often very elastic felting, which, in many vertebrates, 

 notably mammals, forms the main bulk of the skin. In fact, 

 it is this layer alone, which, artificially thickened by the action 

 of tannin, is used for leather, the epidermis being first removed 

 by maceration. The corium is the thickest in mammals, but 

 is also fairly thick in amphibians and in many fishes. In rep- 

 tiles and birds it is thin, the amount of protection thus lost 

 being compensated for by the dense and firm covering afforded 

 by the accessory epidermic structure, scales and feathers re- 

 spectively. Birds have the thinnest corium of all vertebrates, 

 a condition undoubtedly correlated with the development of 

 the feather coat, which renders the protection of a thick corium 

 superfluous. 



In the formation of the accessory organs each of the two 

 layers furnishes materials characteristic of itself, and, although 

 in later growth a structure that originates in one layer can, 

 and generally does, invade the province of the other, there is 

 a definite place of origin for each element involved. Thus 

 from the epidermis come intppnm*u.tn.l gland* of all sorts, al- 

 though they usually dip down into the corium from which they 

 receive a fibrous investment. Pigment may_ba-.deriveH from 

 cither_layer, but more usually from the corium, and when 

 found in the epidermis, as it commonly is, it is more likely to 



