41 



the change in the blood during respiration. This 

 was afterwards still further examined by EM- 

 MERT, with all the accuracy we could wish ift 

 a skilful experimenter, and he has satisfactorily 

 proved, that the dividing of the eighth pair of 

 nerves has no immediate influence on the change 

 of the blood in the lungs, but that it affects the 

 respiration, which, by degrees, becomes more 

 and more uneasy ; after which, the blood in the 

 arteries begins to become dark, so that, when 

 the animal, after several hours, dies, it is found 

 to have venous blood in the arteries, because the 

 respiration has ceased, 



Of the difference in the blood in different ages, 

 and during different diseases, we know hardly 

 any thing. It has been supposed, that the blood 

 of the foetus underwent, in the placenta, a pro- 

 cess not unlike that, which takes place in the 

 lungs after birth, and that it was returned through 

 the veins of the navel-string; but credible authors 

 have asserted, that the eye cannot distinguish be- 

 tween the arterial and venous blood of the foetus. 

 The chief object of the process in the lungs, is 

 the maintaining of the animal heat, but the foetus 

 derives its temperature from the surrounding me- 

 dium, and consequently wants no source of heat 



