EFFECTS OF RESPIRATION. 185 



their being constantly more and more filled by the influx 

 from the arteries. 



But strong expiratory efforts, as in straining and the 

 like, are not fairly comparable to ordinary expiration, inas- 

 much as they are instances of more or less interference 

 with expiration, and involve probably circumstances lead- 

 ing to obstruction of the circulation in the pulmonary 

 capillaries, such as are not present in the ordinary rhyth- 

 mical exit of air from the lungs. 



The act of inspiration is favourable to the venous circu- 

 lation, and its effect is not counterbalanced by its tendency 

 to draw the arterial, as well as the venous, blood towards 

 the cavity of the chest. When the chest is enlarged in 

 inspiration, the additional space within it is filled chiefly 

 by the fresh quantity of air which passes through the 

 trachea and bronchial passages to the vesicular structure 

 of the lungs. But the blood being, like the air, subject 

 to the atmospheric pressure, some of it also is at the same 

 time pressed towards the expanding cavity of the chest, 

 and therein towards the heart. The effect of this on the 

 arterial current is hindered by the aortic valves, while 

 they are closed, and by the forcible outward stream of 

 blood from the ventricles when they are open ; while, on 

 the other hand, there is nothing to prevent an increased 

 afflux of blood to the auricles through the large veins. 



Sir David Barry was the first who showed plainly this 

 effect of inspiration on the venous circulation ; and he 

 mentions the following experiment in proof of it. He 

 introduced one end of a bent glass tube into the jugular 

 vein of an animal, the vein being tied above the point 

 where the tube was inserted ; the inferior end of the tube 

 was immersed in some coloured fluid. He then observed 

 that at the time of each inspiration the fluid ascended in 

 the tube, while during expiration it either remained sta- 

 tionary, or even sank. Poiseuille confirmed the truth of 

 this observation, in a more accurate manner, by means of 



