MAMMALIA. 731 



the foramen ovale ; and by means of the ductus arteriosus the 

 greater portion of the blood driven from the right ventricle during 

 the systole of that cavity passes into the aorta, a very small pro- 

 portion only finding its way into the pulmonary arteries. Such a 

 heart therefore supplies a mixed fluid to the fetal system ; of which 

 a portion, having passed through the arterial trunks, finds its way 

 back to the placenta through the two umbilical arteries, there to 

 recommence the same circle. 



(863.) Immediately after birth, however, the whole arrangement 

 is altered, and the adult condition fully established. The lungs 

 assume their functions, and the pulmonary arteries attain their full 

 proportions ; while the placenta at once ceases from its office, and 

 all the umbilical vessels become obliterated. The ductus venosus 

 is no longer permeable, so that the portal system and that of the 

 vena; cava 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 ex- 

 panded lungs, and be returned through the pulmonary veins to the 

 left side of the heart. Thus the pulmonary and systemic circula- 

 tions 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 circula- 

 tory apparatus is fully established. 



(864.) After birth the mammary glands supply the first nutri- 

 ment 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 mammte are situated beneath 

 the abdomen or in the inguinal region. Their structure, however, 

 is similar throughout the entire class ; each gland consisting of in- 

 numerable 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 lac- 

 tiferous canals terminate by numerous orifices upon the extremity 

 of the nipple ; but, where the nipples are of large size, they gene- 

 rally contain a wide cavity wherein the milk accumulates in consi- 



