vii THE STAGES OF LIFE AND DEATH 269 



auricle ; the greater part passes through the ductus arteriosus 

 and mingles with the blood of the descending aorta. Thus much 

 arterial blood coming from the inferior vena cava enters the left 

 auricle, but very little venous blood from the pulmonary veins. 

 This mixed blood, which is mainly arterial, passes from the left 

 auricle into the ventricle below, flowing thence through the aorta 

 partly to the upper limbs and head, partly to the lower limbs. 

 The blood which goes to the upper limbs is more arterial than 

 that which flows into the lower parts, because the latter contains 

 a large admixture of the venous blood coming from the pulmonary 

 artery and passing into the descending aorta through the ductus 

 arteriosus Botalli. It is probably due to this fact that the head 

 and upper limbs of the foetus develop more rapidly than the 

 pelvis and lower limbs. 



In conclusion we may say that, contrasted with the adult, 

 in whom the arterial blood passing through the left heart is 

 absolutely separated from the venous blood passing into the 

 right heart, in the foetus, in most parts of the circulation, the 

 vessels conveying arterial blood laden with oxygen and histo- 

 genic materials are in communication with those conveying 

 venous blood deficient in oxygen and nutritive substances, so 

 that a mixture of the two kinds of blood takes place. This 

 difference in the circulatory system is in perfect harmony with 

 the needs of the foetus, which differ essentially from those of 

 the adult. The adult must be able to move and work, therefore 

 his tissues, and more especially his muscles, consume a large 

 quantity of oxygen, which is indispensable for the development 

 of heat and force. The chief requirements of the foetus, on the 

 contrary, are growth and development, for it has but little need of 

 heat and movement. During intra-uterine life the respiratory 

 muscles do not work ; the active movements of the foetus are 

 accomplished with a minimum expenditure of energy, for in the 

 maternal uterus it is immersed in the amniotic fluid, and has not 

 therefore even to support the weight of its own body. Moreover, 

 the foetus requires but little oxygen for the production of heat, 

 especially during the first months, for loss of heat is prevented 

 by the mother's body. The need of oxygen, however, increases 

 gradually in proportion to the growth of the foetus, and provision 

 is made for this need by the gradual development of the placenta! 

 vessels, and the proportionate increase in the intensity of the 

 exchange of the respiratory gases between the uterine and 

 placental blood. Although the blood of the foetus contains as a 

 rule less oxygen than that of the mother, because it does not need 

 to consume so much, still its temperature is always somewhat 

 higher than that of the maternal blood (on an average 0*5 C.) ; 

 this is due to the very limited loss of heat from the foetus and to 

 its own production of heat. 



