1 84 THE CIRCULATION. 



veins ; and, much more, the contraction of muscles capable 

 of acting on veins provided with valves. 



It remains only to consider (4) the influence of the 

 respiratory movements on the circulation. 



Although the continuance of the respiratory movements 

 is essential to the circulation of the blood, and although 

 their cessation is followed, within a very few minutes, by 

 that of the heart's action also, jet their direct mechanical 

 influence on the movement of the current of blood is pro- 

 bably, under ordinary circumstances, but slight. The 

 effect of expiration in increasing the pressure of the blood 

 in the arteries has been already mentioned (page 167), and 

 is minutely illustrated by the experiments of Ludwig. It 

 acts as the pressure of contracting muscles does upon the 

 veins, and is advantageous to the movement of arterial 

 blood, inasmuch as all regurgitation into the heart is 

 prevented by the force of the onward stream of blood from 

 the contracting ventricle, and in the intervals of this con- 

 traction by the closure of the semilunar valves. Under 

 ordinary circumstances, and with a free passage through 

 the capillaries of the lungs, the effect of expiration on the 

 stream of blood in the veins is also probably to assist, 

 rather than retard its movement in the proper direction. 

 For, with no obstruction in front, there is the force of the 

 blood streaming into the heart from behind, to prevent 

 any tendency to a backward flow, even apart from what 

 may be effected by the 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 fane, 

 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 



