AIDS TO THE VENOUS CURRENT. 679 



already mentioned, of course suppose it to be superadded to that of the 

 heart in the venous circulation. 



The motion of the blood in the veins, and its consequent return, 

 through them, to the heart, are aided, in Man and the higher ani- 

 mals, by certain secondary or so-called adjuvant causes, such as the 

 pressure of the muscles, and the thoracic respiratory movements. 



In a few exceptional cases the veins themselves possess a power of rhythmic 

 contraction ; the veins in the delicate ears of the rabbit have been seen to pul- 

 sate ; in the bat's wings, the veins contract from 8 to 10 times in a minute 

 (Wharton Jones). The caudal vein of the eel, the portal veins of the myxine, 

 and some of the abdominal veins of the amphioxus, are also pulsatile at cer- 

 tain points. 



The effects of muscular pressure, considered as an aid to the circu- 

 lation, are entirely due to the presence and direction of the valves in 

 the interior of the veins. These are found chiefly in the veins of the 

 limbs, especially in the superficial veins, and also in the large veins at 

 the root of the neck. Furthermore the free edges of the valve-seg- 

 ments being turned towards the heart, in the direction of the venous 

 blood-current, the valves allow the return of blood to the heart, but 

 are speedily closed, when any obstacle to the onward flow of the blood 

 occurs, as when a vein is compressed between a valve and the heart. 

 Under such circumstances, the reflux of the blood in the veins, from 

 its trunk to its branches, is checked, and on any additional pressure, 

 the blood contained in the veins, is urged on toward the heart. More- 

 over, owing to the frequent anastomoses between neighboring veins, 

 some of the blood may also be pressed into collateral channels, which 

 are not subjected to pressure, and so be aided in its progress to the 

 heart. This is exemplified by the increased quantity of blood forced 

 into the superficial veins of the limbs, during muscular efforts which 

 compress the deep-seated veins. In the actions of different muscles in 

 the various movements of the body, sometimes one set of veins, some- 

 times another, must be compressed ; and the varying degrees of com- 

 pression to which the deep-seated veins especially are subjected, must 

 assist or hasten the return of blood to the heart. But this is not an 

 essential cause of the venous circulation, for that is perfectly per- 

 formed during the most complete rest of the muscles of the limbs, as 

 in the state of repose, sleep, and paralysis ; moreover the circulation 

 through the brain is performed altogether independently of muscular 

 pressure, and of the presence of valves in its veins. When, owing to 

 muscular exertion, a larger quantity of blood is returned to the heart in 

 a given time, the frequency of the heart's beats is always increased, a 

 mutual adaptation being thus evidenced, between the rapidity with 

 which the blood is returned to the heart, and that with which the heart 

 endeavors to transmit it onwards. 



The respiratory movements have been long believed to aid in the 

 systemic circulation of the blood. Unlike the pulmonary circulation, 

 the systemic circulation is partly performed within, but partly, and 

 chiefly, without, the thorax ; hence, different portions of it are un- 

 equally affected or disturbed by the thoracic movements. It has al- 



