MAMMAEY GLANDS. 841 



vessels become obliterated. The ductus venosus is no longer permeable, 

 so that the portal system and that of the vence cavce are quite separated : 

 the foramen ovale closes, thus completely separating the right from the 

 left auricle : the ductus arteriosus is reduced to a mere ligament ; all 

 the blood, therefore, driven from the right side of the heart must now 

 pass into the expanded lungs, and be returned through the pulmonary 

 veins to the left side of the heart. Thus the pulmonary and systemic 

 circulations being rendered totally distinct, arterialized blood alone 

 enters the arterial system, to be distributed through the body, and, the 

 umbilical arteries disappearing, the highest form of the circulatory 

 apparatus is fully established. 



(2494.) After birth the mammary glands supply the first nutriment 

 to the still helpless offspring. These vary in number and position in 

 different species of placental Mammifers, their number being of course 

 greatest in the most prolific races. Where the arms or anterior limbs 

 can be used for supporting or clasping the feeble young, as in the 

 Quadrumana, the Bats, and the females of our own species, it is upon 

 the breast that these nutrient founts are placed; but in less gifted 

 tribes the mammce are situated beneath the abdomen or in the inguinal 

 region. Their structure, however, is similar throughout the entire class ; 

 each gland consisting of innumerable minute secreting cells, grouped 

 together in lobules and in lobes. Delicate excretory ducts, derived 

 from all these ultimate cells, unite together again and again until they 

 form capacious ducts, or rather reservoirs for milk. In the Human 

 female the lactiferous canals terminate by numerous orifices upon the 

 extremity of the nipple ; but where the nipples are of large size, 

 they generally contain a wide cavity wherein the milk accumulates in 

 considerable quantities, to be discharged through one or two orifices 

 only. Such are the modes by which Supreme Beneficence has provided 

 for the infant progeny of Mammiferous beings, and conferred the en- 

 dearments of maternity where He has bestowed intelligence to appre- 

 ciate affection. But even this is not all : from the superabundance of 

 the store provided there may be yet to spare ; and Man is privileged to 

 bid his lowing herds yield him their milk for food, and thus obtains no 

 slight addition to the bounteous table spread for his enjoyment. 



THE END. 



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