VIVIPAROUS ANIMALS. 405 



ed directly to the liver, the other portion goes to the pul- 

 monic auricle by the inferior vena cava. The septum be- 

 tween the pulmonic and systemic auricle is at this time in- 

 complete, there being a valvular aperture termed foramen 

 ovule. Through this opening part of the blood in the pul- 

 monic auricle escapes into the systemic auricle, and the re- 

 maining portion, passing into the pulmonary artery, instead 

 of going directly to the lungs, is conveyed to the aorta, by 

 a passage termed ductus arteriosus. The blood thus expelled 

 by the systemic ventricle into the aorta, is conv-eyed to the 

 different parts of the body, but a great part is conveyed 

 back to the placenta by the umbilical arteries, which take 

 their rise from the iliacs. 



Although the foetus is thus nourished by blood derived 

 from the mother, the communication is so indirect, that no 

 injection into the uterine vessels passes into the foetus, and 

 no injection of the foetus reaches the vessels of the mother. 

 The two sets of vessels, therefore, do not anastomose. The 

 imperfect connection of the vessels is still farther demon- 

 strated by the superior rapidity of circulation in the foetus, 

 when compared with the mother, the heart of the former, in 

 the human subject, beating about one hundred and twenty 

 times in a minute. The circulation of the foetus even con- 

 tinues for some time after the death of the mother. But 

 however indirect the communication between the two circu- 

 lating systems, the condition of the mother still exercises a 

 considerable influence over the growth of the foetus. A 

 ^derangement in the flow of her blood, by diminishing the 

 quantity sent to the placenta, must consequently reduce the 

 supply to the umbilical veins *. A diseased state of the 

 mother is likewise frequently communicated to the foetus. 



* Where more foetuses than usual are generated in the uterus, the supply 

 of nourishment being thus divided, they seldom reach the ordinary size, and 



