122 STRUCTURE OF THE VERTEBRATES 



7. Other Structures. Under this heading are listed a few of the 

 epidermal structures which are limited in distribution. (1) The 

 horn of the rhinoceros has no core, and is composed of epidermal, 

 hair-like fibers. (2) The whales which lack teeth have an ecto- 

 dermal strainer in the mouth (page 91). The horny material is 

 tough and flexible and was of commercial value in a past gener- 

 ation. (3) The ischial callosities on the rump of many Old World 

 primates are callous-like thickenings of the epidermis. These 

 are genetic characters, and differ from the acquired callouses on 

 the hands and feet of humans. 



End of tail 



Rhinoceros Horn Rattle of Rattle-snake 



Fig. 60. Other Ectodermal Structures of the Integument. 



Located in the skin as ectodermal derivatives are a number 

 of nerve receptors and organs of special sense. Perhaps the most 

 prominent of these is the lateral line system of the fish and 

 amphibia. The structure and relationships of the organs is dis- 

 cussed in Chapter XVIII, under Organs of Special Sense. 



C. Human Skin 



The skin of man makes an excellent summary of the skin and 

 its derivatives. The illustration gives the relationships of the two 

 primary layers of the integument, and the position and structure 

 of its outgrowths. The dermis is fairly thick and is of a loose 

 fibrous structure. The stratum germinativum has 'a waved ap- 

 pearance, being several cell layers in thickness. Immediately 

 above it is the pigment layer, the number of granules varying 

 in different races and parts of the body. The thick outer layer is 

 the stratum corneum. This last may be very thin, as on the lips, 

 glans, etc. 



