PARTURITION 1093 



birth. At an early period of development the embryo also contains 

 much more water than the adult; the specific gravity of its tissues 

 increases as development goes on. 



The remarkable vitality of the foetus, and its resistance to 

 asphyxia, are related not to the feebleness of its metabolism, but to 

 the comparatively slight excitability and high endurance of nervous 

 centres like the respiratory, vaso-motor, and cardio-inhibitory. 

 Even when totally deprived of oxygen, as by pressure on the um- 

 bilical cord during delivery, the child does not perish in the two or 

 three minutes which decide the fate of the asphyxiated adult ; nor are 

 the convulsions, rise of blood-pressure, and slowing of the heart-beat 

 associated with asphyxia in the latter, so readily induced, nor 

 premature and fatal efforts at respiration easily excited in utero. 

 But although in such a case the embryo behaves as a separate 

 organism, governed by its own laws, there are circumstances in 

 which it becomes merely a part of the mother and participates in her 

 fate. Thus, the stream of oxygen which normally passes from the 

 maternal to the foetal blood is turned back if asphyxia threatens 

 the mother; the blood of the umbilical arteries, instead of being 

 purified in the placenta, loses the little oxygen it holds to the 

 blood of the uterine sinuses, and the tissues of the embryo are 

 impoverished to support the metabolism of the maternal organs. 

 In the same way, the phenomena of starvation have taught us 

 that the nutrition of the organism is not subject to the rules of 

 red tape. In normal circumstances the flow of nutriment follows 

 definite lines: the blood feeds the tissues through its intermediary, 

 the lymph, and recoups itself from the contents of the 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 im- 

 portant still are drawn upon for supplies. 



Parturition. The period of gestation is abruptly closed about 

 280 days after the last menstruation, usually in what would have 

 been the tenth intermenstrual period had menstruation been occur- 

 ring. There is necessarily a considerable variation in the time when 

 reckoned in this way, since the cessation of the menses merely an- 

 nounces that conception has occurred some time after the last 

 period. It may even be disputed whether the fertilized ovum 

 corresponds to the last menstruation or to the first absent 

 period. Parturition, or the expulsion of the foetus, is accom- 

 plished by periodical contractions, the ' pains ' of labour, at first 

 confined to the uterus. Soon the os uteri begins to soften and 

 dilate, the walls of the vagina become congested, and its secretions 

 are augmented. The uterine contractions increase in frequency 

 and force, and are now accompanied by reflex contractions of the 



