CHAPTER XLIX 



THE INTEGUMENT. 



As has been the custom of this book throughout, in examining any 

 organism, we first observe its external appearance. It is thus the outer 

 covering of the body which becomes the first object of our study. In 

 fishes we therefore study scales ; in the frog and the human being, the 

 skin, while on other mammals, fur, and on birds, feathers. Yet, what- 

 ever forms such external parts may assume they are a covering of the 

 body, and as such form what is called an integument ( ). 



This term includes the skin or cutis and all the structures derived from 

 it. If an animal lives in water the effect of water upon such covering 

 must be considered; likewise, consideration must be given to whether 

 it lives in a cold climate or in a warm, and whether it lives in the air or 

 burrows beneath the earth. All these things are bound to have their 

 effects upon modifying an animal's outer covering. 



Microscopically the integument of vertebrates consists of two layers 

 (Fig. 393), an outer, epidermis, which is the remainder of the ectoderm 

 after the nervous system has been separated from it, and a deeper layer, 

 the corium or derma composed of mesenchyme which has been derived 

 from the somatic portion of the somite. It is into this deeper structure 

 that the nerves and blood vessels extend. 



Accessory organs are developed in both layers, and may begin 

 growth in one and extend through the other. In all cases, however, 

 each element of the accessory organs has a very definite place of origin. 



The integumental glands thus arise from the epidermis, though 

 dipping down into the corium to receive a fibrous covering. 



Pigment usually develops in the corium and often then migrates 

 to the epidermis, although it does sometimes develop in the latter layer 

 itself. 



Blood vessels (except in the mucous membrane of the pharynx of 

 lungless salamanders) develop in the corium. 



Sensory nerve endings are quite freely distributed throughout the 

 epidermis, but the more specialized forms remain in the corium, often 

 pushed up into the epidermic zone in the form of papillae. The epider- 

 mis is thus a bloodless, protective, covering with but slight sensitive- 

 ness, all the more delicate structures being found in the lower layer. 



Both skin layers have the power to form hard parts known as 

 cxoskeleton. 



True bone, for example, develops from the corium, while horn and 

 enamel originate in the epidermis. 



Horny structures, such as hairs or feathers (Fig. 394), are formed 

 from the epidermis alone, but, dip down into the richly vascular corium 



