The Integumentary System 81 



Glands. — All the glands of the integument have their origin in 

 the stratum germinativum although they are found in the dermis. Glands 

 of some sort are found in the skin of every vertebrate class. (Fig. 5.) 



The sweat glands are examples of a simple tubular gland, and 

 in human beings are found all over the skin surface. They are most 

 numerous on the palms and soles, and largest under the arms. In 

 mammals that possess a large amount of hair, the sweat glands are 

 confined to certain restricted areas. In deer, they are found only 

 at the base of the tail; in cats, rats, and mice only on the soles of the 

 feet. The ability of any mammal to withstand high temperatures may 

 be correlated with the number of sweat glands its possesses. 



Closely associated with the hair follicles are the sebaceous glands 

 which are examples of compound alveolar glands. These secrete the 

 lubricating material sebum. Their chief function appears to be to 

 keep the surface of the skin soft and pliable. During adolescence, these 

 glands are stimulated to greater activity. If the sebum fails to dis- 

 charge, the pores become engorged and are known as whiteheads. The 

 enclosed material may become darkened to become a blackhead or 

 infected to become a pimple. 



Along the edges of the eyelids are modified sebaceous glands, the 

 meibomian glands, which produce an oily film over the surface of the 

 eyeball. Associated with the eyelashes are small tubular sebaceous 

 glands. When these become infected, they are known as sties. In 

 the external ear are special wax glands, which are found in many verte- 

 brates. Structurally they resemble tubular glands, but functionally they 

 are like sebaceous glands. Their secretions are useful to prevent harm- 

 ful materials from getting into the inner portion of the ear. 



The most characteristic glands of the mammals are those which 

 give the class its name, the mammary glands. These are inactive 

 in the male, and active in the female only after the birth of young. 

 The milk secreted by these glands is a natural food for the young. 

 It contains a large amount of water, milk sugar, albumin, butter fat, 

 and salts. The milk secreted by any one species is adapted to the 

 needs of its young. The position of these glands varies with different 

 species. In those that hold their young in the arms, the mammary 

 glands are located in the pectoral region. Among those animals that 

 stand while nursing, the glands are usually located between the legs 

 either anteriorly as in the elephant or posteriorly as in cows and horses. 



Among the fishes, the commonest type of gland is the mucus- 

 secreting single-celled glands that are found all over the surface. These 



