1020 A MANUAL OF PHYSIOLOGY 



periodical contractions when its bloodvessels are perfused with such 

 an artificial fluid as Locke's solution, or, indeed, when it is simply 

 immersed in the oxygenated solution (Kurdinowski) (Practical 

 Exercises, p. 1025). 



It is a question of great interest how the uterine contractions are 

 started so abruptly at full term after so long a period of quiescence. 

 It can hardly be that the increasing mechanical distension of the 

 uterus, tolerated for so many months, should suddenly, in an hour, 

 become intolerable. For if the fcetus c^es before full term it is 

 expelled without reference to the bulk which the uterus has reached. 

 It is more likely that some chemical change associated with the 

 completion of intra-uterine development, a change which leads, 

 perhaps, to the production of some specific substance in the placenta 

 or the fcetus, is the determining event. The placenta is a structure 

 whose function is strictly limited to the term of intra-uterine develop- 

 ment. The fcetus is to live on, and so is the mother. May it not 

 be that the placenta or essential elements in it are timed to die, or 

 to begin to die, at full term, and that in their death or degeneration 

 the substance or substances are produced which start, and later 

 sustain, the uterine contractions ? And may not the contractions of 

 the uterus, by exciting its afferent nerves, or through the pressure 

 of the foetus the afferent nerves of the vagina, in turn evoke the 

 associated reflex contractions of the abdominal walls ? These are ques- 

 tions which have been asked, but not as yet satisfactorily answered. 



At birth, great changes take place in the fcetal 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 ; 

 (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 eleva- 

 tion 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 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 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 



