322 



MAN, A MAMMAL 



The Glands of the Skin. Scattered through the derails, and 

 usually connected with the hair follicles, are tiny oil-secreting 

 glands, the sebaceous glands. The function of the sebaceous gland 

 is to keep the hair and surface of the skin soft. The other glands, 

 known as sweat glands, are to be found in profusion, over 2,500,000 

 being present in the skin of a normal man. These glands carry 

 off certain wastes from the blood in the water they pass off. 

 Thus the skin not only protects the body, but also serves as 

 an excretory organ. Its most important function, however, is 

 the regulation of the heat of the body. How it does this, we 

 shall learn later. (See Chapter XXVII.) 



Connective Tissue. The layer immediately beneath the der- 

 mis is known as the subcutaneous layer. It is an important 

 storage place for fat. Underneath this layer we find a mass of 

 flesh or muscle. Intermixed with this is a considerable amount 

 of fat. The fat, muscle, in fact, all the tissues in the body, 

 are held together by fibrous threads called connective tissue. 



Muscles and Movement. We are all aware that motion in any 

 of the higher animals is caused by the action of 

 the muscles. These contract to cause move- 

 ment. In man and the other vertebrate ani- 

 mals, the muscles are almost always fastened 

 to bones, which, acting as levers, give wide 

 range of motion. 



Arrangement of Voluntary Muscles in the 

 Human Body. Muscles are usually placed 

 in pairs; one, called the extensor, serves to 

 straighten the joint; the other, the flexor, 

 bends the joint. Locate, by means of feeling 

 the muscles when expanded and when con- 

 tracted, the extensors and flexors in your 

 own arm. Use the leg of a frog to deter- 

 mine which muscles are extensors and which 

 flexors (see the Figure) . This paired arrange- 

 Muscles of the left leg of ment of muscles is of obvious importance, a 

 the frog; b, M. biceps; fl exor musc i e balancing the action of an ex- 



g, M. gastrocnemius : ., ,, . , , . . ,, 



an, M. semimembran- tenSOr n the other Slde of the J omt The 



osus; tr, M. triceps. end of the muscle that has the wider move- 



