MAMMALIA— PHYSIOLOGY 427 



flattened. Each cell has an oval or rod-shaped nucleus. The cell 

 substance is longitudinally striated but has no cross striations. It is 

 found in the walls of the intestine, trachea, and bronchi, the urino- 

 genital system, blood vessels and hair follicles of mammals and in 

 the foot of the mollusc. Striated muscle consists of long cylin- 

 drical fibers made up of fibrils arranged end to end in small bundles 

 and enclosed by a sheath or sarcolemma. This outer connective 

 tissue covering is continued into tendons which attach to the skele- 

 ton. Nuclei are situated in the liquid protoplasm under the sar- 

 colemma. The striated appearance is due to the presence in muscle 

 substance of different chemical compounds of protoplasm arranged 

 in light and dark bars which give a different staining reaction. It is 

 supposed that the wider bar is the contractile substance. 



Cardiac muscle is a peculiar type of involuntary striated muscle. 

 The fibers consist of primitive fibrils which are aggregated into 

 oblong masses each with a nucleus in the center. Muscles are well 

 provided with nerves and blood vessels. In cardiac muscle inter- 

 cellular bridges are found. (Clark, 1927.) 



Skin. — The skin covers the body completely, the epithelium 

 changing at the openings of the internal passages to a soft delicate 

 mucous membrane. It varies in thickness, becoming extremely 

 heavy in man on the palms of the hands and the soles of the feet. 

 In the cat it is well developed on the pads of the feet and in the 

 neck region, where it is protective against the bites of enemies. 

 The skin consists of the epidermis, cuticle or outer skin, and the 

 dermis, corium or true skin. 



The epidermis consists of two layers, the stratum Malpighi and 

 the horny layer, or stratum corneum. Although bony plates are 

 lost in the mammals, one finds heavy armor in some forms, the 

 rhinoceros having a skin over three inches thick. The dermis^ 

 cutis vera or true skin, consists of fibrous connective tissue with 

 many glands, blood vessels, lymphatics and nerves. The append- 

 ages of the skin are the nails, and the hairs, with their sebaceous 

 glands and sweat glands. They are all developed as thickenings 

 and down growths of the Malpighian layer of the epidermis. The 

 sebaceous glands are small saccular glands, the ducts from which 

 open into the mouths of the hair follicles. Both the ducts and the 

 saccules are lined by epithelium which becomes charged with fatty 

 material. The sweat glands, so well developed in terrestrial forms, 

 are much reduced in aquatic mammals. 



