MAMMALIA. 



687 



tion of the blood-vessels might of course be pointed out, a few 

 of which may require notice elsewhere ; but, generally speaking, the 

 arrangement of the vascular system in all quadrupeds is so similar, 

 that the anatomical student who may push his researches thus far 

 will never be at a loss in identifying the different vessels, and com- 

 paring them with those found in the human body. 



(788.) Although the respiration of Mammalia is inferior, as 

 regards the extent to which their blood is exposed to the influ- 

 ence of the atmosphere, to the perfection of this process in Birds, 

 nevertheless, such is the elevated temperature of the body in these 

 hot-blooded animals, that a warm covering of some non-conduct- 

 ing material is here absolutely requisite to retain the vital warmth, 

 and defend them against the thermometrical changes of the ele- 

 ment they inhabit. Their skin is generally, therefore, clothed 

 with a warm covering of hair ; a cuticular structure, the nature 

 and growth of which it behoves us now to examine. We must first, 

 however, notice the organization of the skin itself, and then the 

 nature of the various structures employed to defend it will be 

 readily understood. 



The skin of all Mammals, like that of the human body, con- 

 sists of the cutis, or vascular 

 true skin ; of the epidermis, 

 or cuticle ; and of a thin 

 layer of pigment interposed 

 between the two, which is a 

 diversely coloured secretion, 

 deposited like the cuticle 

 upon the surface of the cutis. 



The hairs that cover the 

 quadruped, whatever be their 

 form or thickness, are cylin- 

 ders of horny or cuticular 

 substance, that grow upon 

 so many minute vascular 

 pulps, from the surface of 

 which the corneous mate- 

 rial is perpetually secreted. 

 Some kinds of hair are per- 

 manent, and, if constantly 

 cut, will continue to grow 

 during the whole life of the 



318. 



