146 THE CIRCULATION. 



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, strain- 

 ing, and similar violent expiratory efforts ; the effect 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, inasmuch as 

 they are instances of more or less interference with expiration, 

 and involve probably circumstances leading to obstruction of 

 the circulation in the pulmonary capillaries, such as are not 

 present in the ordinary rhythmical exit of air from the lungs. 



The act of inspiration is favorable to the venous circulation, 

 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 ad- 

 ditional 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 in- 

 serted ; the inferior end of the tube was immersed in some 

 colored fluid. He then observed that at the time of each in- 

 spiration the fluid ascended in the tube, while during expiration 

 it either remained stationary, or even sank. Poiseuille con- 

 firmed the truth of this observation, in a more accurate manner, 

 by means of his haemadynamometer. And a like confirmation 

 has been since furnished by Valentin, and in minute details 

 by Ludwig. 



The effect of inspiration on the veins is observable only in 

 the large ones near the thorax. Poiseuille could not detect it 

 by means of his instrument in veins more distant from the 



