MAMMALIA 453 



CHAPTER XXVI. 



GENERAL FEAl^URES OF MAMMALIA. 

 Class Vf. — Mammalia. 



The Mammalia are the last class of the Vertebrata^ 

 and as they indubitably stand at the head of the animal 

 kingdom, both structurally and intellectually, they will be 

 specially treated here. Special emphasis is laid upon 

 the skeleton, because the skeleton of a vertebrate is always 

 a permanent embodiment of the part played by its former 

 possessor in the arena of life. 



Skin. — The skin of mammals conforms to that of 

 vertebrates in general, hence two layers of the epidermis 

 can be distinguished — the outer horny layer or stratum 

 corneum and the inner mucous layer or stratum mucosum. 

 The base of the mucous layer which rests upon the 

 dermis consists of a single layer of epithelial cells, the 

 basal epithelium, which by tangential divisions (parallel to 

 the surface) are perpetually giving rise to more cells 

 in layers above them. The lower of these cells are 

 still living and protoplasmic, but those nearer the surface 

 have undergone a cornification, by which the proto- 

 plasm is replaced by horn or ceratin. The cells thus 

 cornified are no longer living, but are continually being 

 shed in detail upon the surface. Thus the whole surface 

 of the mammal is enveloped in a thin, flexible layer of 

 ceratin., the corneous layer, produced by the underlying 

 mucous layer of living protoplasmic cells. The dermis, 

 as in other vertebrates, consists of a dense mass of con- 

 nective tissue, blood-vessels, nerves, muscles, fat and skin- 

 glands. With the first three we are not here concerned, 

 but one of the essential features of the class Mammalia 

 is the development of the three latter. The muscle is 

 present beneath the skin, connecting it tightly with the 

 body below, as a thin sheet known as the pa?inicuhis 



