i;: 



Basic Structure oi' Vertebrates 





GftWJlA>flCELL 

 © Ci S----EPIDERM 



, ( ,^!^"i, 



A. AMPHIOXUS 



B PETROMYZON 



Fig. 11. Sections of the skin of four chordates, Arnphioxus, Petromyzon, Squalus, 

 and liana, showing the fundamental differentiation of the skin into dermis (corium ) 

 and epidermis. (Mainly after Plate and Schimkewitsch. Courtesy, Neal and Rand: 

 "Chordate Anatomy," Philadelphia, The Blakiston Company.) 



arthropods, or calcareous as in mollusks — which forms a tough or hard 

 external protective layer. Vertebrate skin is much more elaborate. It 

 consists of two layers (Fig. 11). The outer one, the epidermis, is a 

 cellular layer several or many cells in thickness. The deeper epidermal 

 cells are actively alive, but in most vertebrates (fishes being excep- 

 tional) the more superficial cells become more or less completely filled 

 with an inert nitrogenous substance, keratin, which accumulates at 

 the expense of the protoplasm. Thus is formed the characteristic horny 

 layer (stratum corneum), whose cells ultimately become quite dead 

 horny particles which are so agglutinated together as to form a hard, 

 tough, and somewhat elastic external protective layer. 



The deeper layer of the skin, the dermis or corium (Fig. 11), con- 

 sists for the most part of a fibrous tissue. The dermal cells which pro- 

 duce the fibers constitute a very small proportion of the bulk of the 

 layer. Between the dermis and the underlying muscle or bone is a fi- 

 brous subcutaneous connective tissue, less compact and more elas- 

 tic than the dermis. 



Blood-vessels rarely occur in the epidermis but are abundantly 

 present in the dermis, which is therefore the nutritive layer. The larger 

 vessels supplying the skin lie in the subcutaneous connective tissue. 



