380 THE CIRCULATION OF THE BLOOD. [cHAP.XXVIH. 



from the regurgitation which takes place, into the large venous 

 trunks from the auricles at each systole. 



The veins possess a certain tonic influence similar to that of 

 arteries, by which they can adapt themselves to the varying quan- 

 tity of their contained blood. This is doubtless due to the presence 

 of muscular fibres in the tunics of veins already described; the 

 power of these fibres to alter the calibre of the vein is clearly 

 demonstrable by the influence of galvanism,* which causes an ap- 

 preciable diminution in the size of the vessel at the point of transit 

 of the current.-j- 



The flow of blood in the veins, then, it may be concluded, is 

 maintained chiefly by that same force through which it is driven 

 through the arteries and capillaries, aided by the sort of suction 

 in the centripetal direction, which is caused by inspiration and by 

 the diastole of the auricles, and promoted likewise by the contrac- 

 tion of the various muscles, among or through which the veins 

 pass, and by the position and mechanism of the valves. 



It is proper to observe, that the venous circulation being moved 

 by less force than the arterial (the heart's power having already 

 very much expended itself on the arteries, and capillaries), is 

 more influenced by gravity either favourably or otherwise than 

 the arterial. Hence, in dependent positions, as in the lower ex- 

 tremities, when the blood has to ascend against gravity, the 

 veins are apt to swell, and to acquire a permanent dilatation 

 and thickening of their coats from the retardation of the current 



* See Kolliker's experiments Subold and Kolliker's Journal. 



t While these pages were passing through the press (Feb. 1852), Mr. Wharton 

 Jones announced, in a paper read to the Royal Society, the discovery that the 

 veins of the bat's wings contract and dilate rhythmically, and that they are pro- 

 vided with valves, some of which completely, others only partially, oppose 

 regurgitation of blood. The rhythmical contractions and dilatations are con- 

 stantly going on, and that at the rate of ten contractions in the minute. During 

 contraction, the flow of blood in the vein is accelerated, and on the cessation of 

 the contraction, the flow is checked, with a tendency to regurgitation. But 

 this check is usually only momentary ; already, even while the vein is in the 

 act of again becoming dilated, the onward flow recommences and goes on, though 

 with comparative slowness, until the vein contracts again. It is the heart's 

 action which maintains the onward flow of blood during the dilatation of the 

 vein, whilst it is the contraction of the vein coming in aid of the heart's action 

 which causes the acceleration. Mr. Jones states that he has not been able to 

 observe unequivocal evidence of tonic contractility in veins, as Kolliker's ex- 

 periments indicate ; he likewise affirms, in opposition to a statement of Mr. 

 Paget, quoted at p. 330, that nowhere do the arteries and veins of the bat's 

 wing communicate, the only communication being the usual one through the 

 medium of capillaries. Proceedings of the Royal Society, Feb. 1852. 



