224 THE MAMMARY GLANDS. 



morphosis. They retain still, as it were, the traces of organization ; 

 they belong rather to the hydrocarbon family than to the nitrogenized. 

 It may be that, for the removal of these, cell action is necessary. 



Whatever importance may be attached to such a suggestion, it is very 

 Modeofremov- clear that, notwithstanding the extreme thinness of the walls 

 from\hVS of the tuft vessels the relaxation in the speed of the blood 

 pighian sac. current through them, and the pressure brought to bear upon 

 them, that water could not be separated by oozing through them unless 

 there was an additional provision. The sac into which the exudation is 

 to take place is already full, and it may be questioned whether ciliary mo- 

 tion in the uriniferous tubes would exert a sufficient exhaustion to relieve 

 the interior of the capsule from pressure ; but the introduction of a liquid 

 of a different nature into the uriniferous tube may call at once into oper- 

 ation the principle described at page 131 as acting in the capillary circu- 

 lation of the blood, and thus the contents of the Malpighian sac are drawn 

 forward into the uriniferous tube, just in the same manner that water is 

 drawn from the inside of a bladder through the pores thereof by alcohol 

 on the outside. 



THE MAMMARY GLANDS. 



The mammary glands are situated on various portions of the abdom- 

 inal and thoracic surfaces of animals of the class mammalia. 



Description of 



the mammary In the higher members of this class they present the appear- 

 ance of racemose glands, rudimentary in the males, but well 

 developed in the adult females, especially after parturition. They separ- 

 ate from the blood the white secretion, milk. 



In the ornithorynchus the mammary gland consists of an obtuse cone 

 of coecal follicles, ending upon an areolar surface. There is no nipple. 

 The milk is expelled, both in these and the marsupials, by direct mus- 

 cular pressure. In cetaceans the nipple is included in a cleft of the in- 

 its compara- tegument, but in the higher mammalia it projects, so that, be- 

 tive anatomy. ' n g rece i ve <j i n ^ o the mouth of the young, and suction being 

 made, the pressure of the air takes effect upon the surface of the gland 

 and expels the milk. 



In different cases the number of mammas differs. In the human spe- 

 cies there are but two, placed upon the thoracic surface, and from their 

 position favoring the care and nursing of the child. Among other ani- 

 mals the number seems to have a relation to the number of young brought 

 forth at a birth, there usually being a pair for each one. Many excep- 

 tions to this rule, however, occur. 



The mammary gland corresponds in anatomical structure to the paro- 



Its stru P t ^ anc * P ancreas - I* cons i sts of 15 or 20 lobes, each from J 



to 1 inch in width ; these are composed of lobules, and these, 



again, of coecal vesicles. The excretory ducts are lined with tesselated 



