THE INTEGUMENT AND THE EXOSKELETON 79 



have wandered in from the corium than to have originated 

 in place. Blood-vessels, with the single exception of the 

 pharyngeal mucous membrane of lungless salamanders,* are 

 entirely confined to the corium. Sensory nerve endings of the 

 simplest type are distributed freely through the epidermis, but 

 the more specialized forms remain in the corium, although 

 they may be located in papillae pushed up into the epidermic 

 zone. The epidermis thus forms a bloodless covering with but 

 slight sensitiveness, the main function of which is to protect 

 the more delicate structures included in the lower layer. Aside 

 from this general protection afforded by the unmodified epi- 

 dermis, both layers of the skin have the power of originating 

 hard parts, which enter into the formation of certain acces- 

 sory external structures that form a more or less complete 

 exoskeleton. Thus the corium produces true bone, with the 

 haversian canals and other osseous characters, while the epi- 

 dermis forms horn and enamel, the latter superficially resem- 

 bling bone, but harder and with a different structure. The 

 structures formed from these may be composed of one sub- 

 stance and involve but a single layer in their formation, al- 

 though the other usually cooperates in some other way, or 

 again may be composites formed from material furnished by 

 both layers 



Thus exclusively horny structures, such as hairs or feathers, 

 are formed from the epidermis alone, but, through the neces- 

 sity of nourishment, they dip down into the richly vascular 

 corium, which forms special organs to further this result. The 

 dermal scutes of ganoids, and the dermal bones of higher 

 forms arise wholly within the corium, while a tooth is a com- 

 posite structure composed of dentine, a hard sort of bone, 

 from the corium, overlaid with enamel from the epidermis. 



As exoskeletal structures are universal among vertebrates, 

 and often form their most obvious characteristics, and espe- 

 cially as they have interesting morphological histories of their 

 own, they deserve special treatment, and will be taken up in 

 the order of their appearance. 



* See Chapter VII, under Respiration. 



