CHAPTER XXVIII 



THE MAMMALIA 



After studying the anatomy of Amphioxus, the dogfish and 

 the frog, in some detail, we have gained a good general idea 

 of vertebrate organisation. In studying a higher vertebrate, 

 a reptile, a bird, or a mammal, we should find the same 

 general plan of structure as in the frog. The same organs 

 are present having the same general relations to one another, 

 but differing often to a considerable extent in detail, and 

 in each case there are certain features characteristic of the 

 class to which the animal belongs. 



Space forbids any reference to reptiles and birds, and for 

 the same reason nothing more than enumeration of the most 

 characteristic features of a mammal can be attempted here. 



Any mammal of convenient size for dissection may be taken 

 as a type. A rabbit is commonly chosen because it is easy 

 to obtain, but a rat, or a small carnivore, such as a cat, 

 would serve equally well. 



The following external features are characteristic of a 

 mammal : — 



The body is furnished with an external covering of hair. 

 In some mammals — e.g. the porpoise — the hairy covering may 

 be so much reduced as to be practically absent, but even 

 in such cases a few scattered hairs are always discoverable. 

 True hair must be distinguished from the chfetee and brisdes 

 so commonly found in invertebrates. A hair is an epidermic, 

 structure implanted in an invagination of the skin known as 

 the hair-follicle. The projecting part of the hair is covered 

 by a so-called cuticle formed of a layer of imbricated scales. 

 Within this is a fibrous or cortical layer formed of very 

 elongated epidermic cells, and the centre is occupied by a 

 cellular medulla or pith. The root of a young hair is bulbous 

 and has a small vascular core derived from the mesoblast, but 

 the shaft of the hair is non-vascular. The general structure 



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