io 1 8 A MANUAL OF PHYSIOLOGY 



body-weight per hour is approximately two and a half times that of 

 the mother under the same conditions. (Carpenter and Murlin.) 



The foetal heart beats at the rate of about 140 times a minute at 

 full term.* The blood-pressure in the umbilical artery of the 

 mature embryo (sheep) varies from 60 to 80 mm. of mercury ; 

 but at the beginning of the aorta it will be more. The pressure in 

 the pulmonary trunk must be about equal to that in the aorta, since 

 the comparatively short and easy circuit through the lungs does 

 not as yet exist ; and in accordance with this equality of pressure 

 (of work to be done) is the equality of thickness (of working power) 

 in the walls of the two sides of the heart. 



Suppose, now, that the embryo contains Go grammes of blood for 

 every kilo of body-weight, and that the whole of the blood passes 

 through the circulation in twenty seconds. Then in twenty-four 

 hours 259*2 kilos of blood will be forced through the heart for every 

 kilo of body-weight against a pressure of, say, 80 mm. of mercury, 

 or i metre of blood. This is equivalent, in round numbers, to 260 

 kilogramme-metres of work, or o'6 calories. Now, taking the total 

 heat-production of the heart at three times the equivalent of its 

 mechanical work, we get r8 calories per kilo of body -weight in 

 twenty-four hours (see p. 585), or about ^ of the heat-production 

 of a resting adult. 



Such movements of the skeletal muscles as occur cannot account for 

 any large proportion of the total metabolism, since they are executed 

 in a medium (the amniotic fluid) of nearly the same specific gravity 

 as that of the body, and therefore require the expenditure of a very 

 limited amount of energy. The ordinary functional activity of the 

 embryo, then, is quite incapable of accounting for the intensity of 

 the foetal metabolic processes. Still less can it be due to an active 

 combustion in the tissues to compensate for a rapid loss of heat, 

 for the foetus lies sheltered in the uterus as in a thermostat at its 

 own temperature, and can lose practically no heat unless its tempera- 

 ture be kept a little above that of the maternal blood. The only 

 remaining explanation of the magnitude of the foetal metabolism 

 is that the growth processes are associated with a large amount of 

 oxidation (and cleavage) . 



Notwithstanding the intensity of metabolism in the embryo, not 

 only is even the purest blood, as has already been stated, far from 

 saturated with oxygen, but the relative proportion of haemoglobin, 

 the oxygen-carrier, is less than in the adult ; and although constantly 

 increasing in amount from the moment of its first appearance, it is 

 still somewhat deficient, even at full term, but leaps sharply up at 

 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 com- 

 paratively slight excitability and high endurance of nervous centres 



* It has not been finally determined whether the rate of the heart 

 varies with the size or, what probably comes to the same thing, with the 

 sex of the foetus. As we have seen, the variation of the rate in the adult 

 with the size of the body is associated with a corresponding variation in 

 the metabolism and heat-loss, which are proportionally greater in a small 

 than in a large animal. If this is a causal connection we should not 

 expect that in the embryo in utero, where the conditions as regards heat- 

 loss are entirely different, such a relation should exist, at any rate within 

 the same species. 



