MILK iu<;5 



increasing venosity of the blood circulating in the bulb, which ^i "' - 

 lates the respiratory centre when the umbilical cord has been cut r 

 tied and the placental circulation thus interfered with; (2) the stimula- 

 tion of the skin by the air, which, as we have seen, acts reflexly upon the 

 respiratory centre. That both of these factors may be involved is 

 shown by the fact that either compression of the umbilical cord alone, 

 or exposure of the foetus by opening the uterus of an animal without 

 interference with the circulation, has been observed to be followed 

 by attempts at breathing. Once distended, the lungs never again 

 completely collapse not even after death, nor when the chest is 

 opened. The aspiration caused by the elevation of the chest-walls in 

 inspiration (for the respiration of the newborn child is mainly costal) 

 sucks blood into the thorax, and expands the vessels of the lungs for 

 its recepti&n ; and in the measure in which the blood passing through the 

 pulmonary trunk finds an easy way through the lungs, the quantity 

 which takes the route of the ductus arteriosus diminishes. The pul- 

 monary veins, and consequently the left auricle, are better filled; and 

 the increasing pressure on this side of the septum tends to oppose the 

 passage of the blood through the foramen ovale, to approximate its 

 valve, and to close its orifice. 



By the second or third day the ductus arteriosus has usually become 

 obliterated. The umbilical arteries and veins and the ductus venosus 

 become impervious soon after the interruption of the placental circula- 

 tion. The vein and venous duct remain in the adult as the round 

 ligament of the liver, the arteries as the lateral ligaments of the bladder. 



Although from birth onwards the young mammal obtains its 

 oxygen and gets rid of its carbon dioxide through its own pulmonary 

 surface instead of through the placenta, it still lives, as regards its 

 food proper, on the tissues of the mother, and that in as literal a 

 sense as when it drew its supplies directly from the maternal blood. 



Milk. The milk secreted during the first few days of each lacta- 

 tion, the colostrum, as it is called, indeed may represent in part the 

 fragments of cells lining the alveoli of the mammary glands, which 

 have undergone a fatty change and been bodily broken down. The 

 colostrum corpuscles are leucocytes filled with fat globules taken up 

 from the contents of the alveoli. The chief chemical difference 

 between colostrum and ordinary milk is the greater richness of the 

 former in protein. It has been supposed that it is of special impor- 

 tance for the nutrition of the suckling, perhaps in virtue of the 

 enzymes contained in it, and it is said that young animals bear 

 artificial feeding much better if they have been allowed to suckle the 

 mother for the colostrum. 



In addition to the fat, which when milk is allowed to stand rises to 

 the top as cream, milk contains a considerable quantity of caseinogen, 

 to whose coagulation, under the influence of the lactic acid produced 

 from the lactose, or milk-sugar, by certain bacteria, spontaneous 

 curdling is due. Another protein, lact-albumin (Halliburton), a large 

 amount of water, and some inorganic salts, are the most important of its 

 remaining constituents. The molecular concentration (p. 420) of milk, 

 as measured by its freezing-point, is almost exactly the same as that of 

 blood-serum. Its electrical conductivity varies extremely, since it 



