170 PHYSIOLOGY. 



CLXXXIX. The situation of the blood-vessels of the lungs 

 is such, that, in the contracted state of this viscus, these ves- 

 sels must be much folded and straitened ; and it appears, that, 

 in the foetus, where they are constantly in a contracted state, 

 their capacity is not sufficient to transmit, in the time required, 

 the whole of the blood returning to the heart by the vena cava ; 

 but, after respiration has been repeated for some time, by the 

 dilatation of the lungs to a certain degree in inspiration, their 

 blood-vessels are unfolded, lengthened, and enlarged, so as to be 

 capable of transmitting the whole blood of the cava. 



CXC. In the infant who has breathed for some time, the 

 whole blood of the vena cava passes into the right ventricle of 

 the heart, and from thence enters into the vessels of the lungs ; 

 but, in the contracted state of the lungs, which occurs at the 

 end of expiration, the blood cannot be properly transmitted ; 

 and, for that purpose, an inspiration becomes absolutely neces- 

 sary. 



CXC I. It is, however, under a certain degree of inspiration 

 only, that the blood is freely transmitted through the vessels of 

 the lungs ; for, if the inspiration is full and continued, so that 

 the lungs are thereby much distended, we find that this state 

 also interrupts the free passage of the blood, and renders expir- 

 ation necessary. 



Expiration becomes also necessary ; because, perhaps, the air 

 long retained in the lungs loses a part of its elasticity, and be- 

 comes thereby unfit to keep the lungs distended ; but, more cer- 

 tainly and more especially, because, in an animal that has 

 breathed for some time, there is a noxious vapour constantly 

 arising from the lungs, which, if not dissolved by the air, and 

 carried out of the lungs, proves pernicious to life. 



CXCII. From what has been said, it appears that the alter- 

 nate motions of inspiration and expiration are necessary to the 

 circulation of the blood, and otherwise also to the health of the 

 body ; and it appears also, that the more frequent the alternate 

 motions of respiration are, the more quickly is the blood trans- 

 mitted from the right to the left ventricle of the heart. 



CXCIII. We can now perceive also the causes exciting these 

 alternate motions ; and we find no occasion for supposing them 

 to arise from any causes alternately interrupting the motions of 



