MAMMALIA 453 
CHAPTER XXVI. 
GENERAL FEATURES OF MAMMALTA. 
Crass VI.—Mammatia. 
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. 
Sxin.— 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 comification, by which the proto- 
plasm is replaced by horn or ceratim. 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 Mammala 
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 panniculus 
