410 Circulation cf the Blocd in the Fcetus. [Book IX. 

 anterior auricle, whence there is a paffage into the 

 pofterior auricle, -which is clofed up after birth, but 

 \vhjdi now turns the'greater part of the blood received 

 by the anterior auricle, from the anterior to the pofte- 

 rior cavities of the heart. A confiderable quantity of 

 blood, however, notwithftanding this palfage, does 

 pafs into the anterior ventricle j but all the blood 

 which is received by the anterior ventricle is not fent 

 K> the kings, which before birth are too much con- 

 dcnfed to tranfmit fo large a quantity ; part of it is" 

 turned afide by a veflel called the ductus arteriofus, 

 which paries from the pulmonary artery to the aorta. 

 Tims, befides the blood which efcapes paffing through 

 the lungs by means of the pafiage from the anterior 

 to the pofterior auricle, a fecond portion efcapes by 

 the velTel which leads from the pulmonary artery to 

 the aorta, fo that perhaps not more than a fixth part 

 of the blood which paffes through the reft of the body 

 paffts through the lungs before birth, whereas, after thefe 

 paffages are clofed, every drop which is circulated in 

 the body muft neceffarily circulate alfo through the 

 lungs. Thefe pafiages, which are peculiar to the 

 foetus, from caufes not afcertained clofe up very 

 quickly after birth. The blood is returned from the 

 fcetus by the arterias umbilicales which are the in- 

 ternal iliac arteries of the adult, but which in ' the 

 fcetus pafs out at the navel, and are continued to the 

 placenta, 



The fcetus, which in the early periods of gefta- 

 tion was almoft all head, is ftill at the time of birth 

 of very different proportions from thofe of the adult 

 body ; the head is remarkably large, and the lower 

 extremities remarkably fmall. The growth iif the 

 fcetus in the uterus is by no means uniform. The 

 weight of children a when born at' the full time, varies 



from 



