RESPIRATION. 



blood can go but one way. The stream behind, and the valves 

 in the veins of the extremities, determine the effects of the 

 pressure to be in the course of the circulation. The compressed 

 vessels are at once nearly emptied, and the instant that the pres- 

 sure is alternately removed are again filled ; and the momentary 

 impediment during the compression is immaterial, on account of 

 the innumerable venous anastomoses. The progress of the blood 

 cannot but be accelerated. The dyspnoea that is felt arises from 

 the force with which the blood drives through the lungs, and 

 which renders frequent respiration necessary. 



In the foetus the ease is analogous, although Dr. Carson has 

 imagined it different, and thought it necessary to frame a little 

 hypothesis to reconcile circumstances. The foetal lungs, expe- 

 riencing no atmospheric pressure, are contracted to the utmost, 

 and the diaphragm, suffering no stimulus from the will on account 

 of uneasy sensation arising from want of breath, is completely 

 relaxed, and forced upwards to remove the vacuum ; and the 

 venous blood without the thorax must be drawn forcibly into the 

 right auricle, preventing the vacuum which the discharges of blood 

 from the left ventricle tend to produce. In the foetus, moreover, 

 the blood is propelled into the aorta by both ventricles, as Mr. 

 John Bell remarks, and, therefore, the circulation less requires 

 other assistance. The vacuum from the dilatation of the cavities 

 of the heart occurs in the foetus and all animals which have a 

 heart : but, in those which have no such respiration as the human, 

 there can be no assistance to the circulation by thoracic vacuum. 



The ordinary cause of the first inspiration appears to be the 

 novel impression of cool air upon the surface ; for, if at any time 

 we are suddenly exposed to a cold wind, or plunge into cold 

 water, the diaphragm and intercostal muscles instantly contract, 

 and a sudden inspiration takes place. The blood rushes into the 

 expanded lungs, and, being afterwards obstructed when the in- 

 spiratory muscles cease to act, and the elastic lungs shrink, gives 

 rise to an uneasy sensation, which is instinctively removed by an- 

 other inspiration, and thus respiration afterwards continues through 

 life. The fact of respiration commencing before the chord is 

 tied, shows that neither congestion in the aorta, nor deficiency of 

 chemical changes, is the cause of the first inspiration. If an ani- 

 mal is born under warm water, its respiration begins at the mo- 

 ment you choose to bring it up into the air. Buffon proved this 

 by causing a bitch's delivery to take place in a tub of warm 

 water, and allowing the pups to remain there for half an hour. 



