THE INTEGUMENT 



673 



(2) Sebaceous (acinous in shape), connected with each hair (Fig. 

 393). 



(3) Mammary (modified tubular glands producing milk (Fig. 399). 



(4) Tarsal or Meibomian (modified sebaceous glands in eyelid, 

 producing oil to keep tears from overflowing (Fig. 400). 



(5) Anal (acinous in form, commonly scent glands, secreting a sub^ 

 stance either for sexual attraction or for protection (Fig. 400). 



Glands are often also divided according to the method by which 

 they furnish their secretions. First, into necrobiotic glands if they burst 

 when liberating their fluids. The individual gland is then destroyed. 

 And, second, vitally secretory glands if the secretions are poured through 

 the walls of the gland while the gland itself remains functional for an 

 indefinite period. In fact, this physiological distinction is often used to 

 determine homologies when other methods cannot be used. 



Each animal class seems to develop integumental glands in its own 

 peculiar way, there being no definite and continuous history of gland 

 development found throughout the various groups. Those animals living 

 in the water, such as fish and amphibians, have glands that secrete pro- 

 tective substances which are often poisonous. The Sauropsida seldom 



Fig. 400. 



A, Sweat gland; B, Acinous gland. Complete gland and cross section. The 

 cross section is cut at the level of the arrow. (Compare with Fig. 393.) 



have any integumental glands at all, and snakes have characteristic 

 cloacal glands secreting a particular nauseating substance. Certain 

 turtles have so-called musk-glands, probably for sexual attraction. In 

 some lizards there is a row of so-called glands (really femoral pores) 

 along the inner portion of the femora that secrete a substance at breeding 

 time which hardens into short spines or teeth. In birds there are only 

 the uropygial glands in the caudal region which furnishes an oil for the 

 feathers. 



In mammals there are many and varying glands in the skin, but 

 they may all be placed into two groups (Fig. 400), namely, the sweat- 



