OF RESPIRATION. 95 



former case, and ingress of additional air into the bronchiae in 

 the latter, but also by the flow of venous blood into the auricles : 

 for the venous blood, being subject to the full atmospheric pres- 

 sure without the chest, will necessarily be driven into the chest 

 to prevent a vacuum ; the arterial blood is under the same cir- 

 cumstances, but the propelling force of the ventricles prevents 

 its retrogression. The atmospheric pressure on the blood-vessels 

 creates a necessity for greater strength in the ventricles, as it 

 impedes the progress of blood from the heart, but it also faci- 

 litates the return. Thus the smaller pressure on the heart acts, 

 by the intervention of the blood, as an antagonist to its con- 

 tracting fibres, assisting to dilate them when they become relaxed. 



By the tendency to a vacuum in the cavity of the thorax, what 

 effect the heart loses by atmospheric resistance without the chest 

 is exactly compensated within, and thus on the whole the heart 

 neither gains nor loses by all the various directions of atmos- 

 pheric pressure. 



In the foetus the case is precisely the same, although Dr. Carson 

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

 little hypothesis to reconcile circumstances. The foetal lungs, 

 experiencing 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, for the same rea- 

 son, be drawn forcibly into the right auricle, preventing the 

 vacuum which the shrunk state of the lungs, and the discharges 

 of blood from the left ventricle, tend to produce. 



The cause of the first inspiration appears to be the novel im- 

 pression 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 quick 

 inspiration takes place. The blood rushes into the expanded 

 lungs, and being afterwards obstructed when the inspiratory 

 muscles cease to act, and the clastic lungs shrink, gives rise to 



