876 A MANUAL OF PHYSIOLOGY 



alimentary canal. But when the normal sources of nutrient material 

 fail, the body falls back upon its stores. The organs immediately 

 necessary to life are kept, as far as possible, on full diet ; organs of 

 secondary importance have to be content with half-rations ; organs 

 less important still are drawn upon for supplies. 



At birth, great changes take place in the circulation, and these 

 are intimately connected with the commencement of the respiratory 

 activity of the lungs. The causes of the first respiration are : (i) The 

 increasing venosity of the blood circulating in the bulb, which 

 stimulates the respiratory centre when the umbilical cord has been 

 cut or tied and the placental circulation thus interfered with 5(2) the 

 stimulation 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 new-born 

 child is mainly costal) sucks blood into the thorax, and expands the 

 vessels of the lungs for its reception ; 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 pulmonary 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 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 vein and the ductus 

 venosus become impervious soon after the interruption of the placental 

 circulation. 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, indeed, represents in large part the fragments of cells lining 

 the alveoli of the mammary glands, which have undergone a fatty 

 change and been bodily broken down. This is particularly the case 

 with the first milk of each lactation, the colostrum as it is called, 

 which consists of little else than the debris of fattily degenerated 

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

 rises to the top as cream, milk contains a considerable quantity of 

 a nucleo-proteid, casein, 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 proteid, lact-albumin 



