174 THE CIRCULATION. 



apart from what may be effected by tlie presence of the 

 valves of the venous system. 



It is true that in violent expiratory efforts there is a 

 certain retardation of the circulation in the veins. The 

 effect of such retardation is shown in the swelling-up of 

 the veins of the head and neck, and the lividity of the face, 

 during coughing, straining, and similar violent expiratory 

 efforts ; the effects shown in these instances being due both 

 to some actual regurgitation of the blood in the great 

 veins, and to the accumulation of blood in all the veins, from 

 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- 



