CHAPTER 25 



Protection, Support and Movement 



The preceding chapters traced the main currents of vertebrate evolution 

 and discussed the major changes made by the various groups of verte- 

 brates as they became adapted to the changing environment. With this 

 as a background, the succeeding chapters will present the morphologic 

 and physiologic aspects of each of the organ systems in turn. In these 

 the major emphasis will be placed on the mammalian condition and on 

 those transformations that have occurred in the line of evolution that 

 leads to mammals. 



214. The Integument 



The skin, or integument, is the outermost layer of the body and 

 separates the organism from its external environment. It helps to main- 

 tain a constant internal environment and protects the body against a 

 variety of mechanical and chemical injuries. Yet the skin does not com- 

 pletely isolate the organism from its environment, for many sensory 

 stimuli are received by the skin and some exchange of gases, water and 

 excretory products may occur through it. In addition, a variety of bony 

 plates, scales, feathers, hair, pigment cells and glands develop from the 

 skin and serve a variety of purposes. The skin is truly a "jack-of-all 

 trades." 



In general it may be said that the greater the difference between the 

 internal and external environments, the greater is the importance of this 

 organ in protecting the underlying tissues, and the more elaborate is its 

 structure. Lower chordates, for example, whose internal environment is 

 very similar to the sea water in which they are living, have a very 

 delicate skin consisting of a single layer of columnar epithelium sup- 

 ported by a few connective tissue fibers. In all vertebrates, the skin is 

 more highly developed and is made up of an outer stratified epithelium 

 (the epidermis) and a deeper, rather thick layer of dense connective 

 tissue (the dermis). 



The epidermis of fishes and amphibians contains relatively little 

 horny material, but a large amount of horny keratin is deposited in the 

 outer cells of the epidermis of the higher terrestrial vertebrates. These 

 flattened, cornified cells are dead, and in mammals form a thick, water- 

 proofing stratum corneum that is clearly demarcated from the deeper, 

 proliferating layers of the epidermis known as the stratum germinativum 



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