174 CIRCULATORY SYSTEM. 



marked ; but they are not so extensible in the latter direction as 

 the arteries, while they are more so transversely. There can be 

 no doubt of their spontaneous powers of contraction, for it is 

 abundantly proved by their diminishing much in volume upon 

 the application of cold; moreover, when a venous trunk, dis- 

 tended with blood, is intercepted by two ligatures, and then 

 punctured, it empties itself entirely and rapidly. 



The circulation in the veins is produced, in a principal degree, 

 by the-contraction of the heart; their own contraction may also 

 favour this motion, as well as lateral pressure from contiguous 

 parts. As the movement of the blood in the smaller arteries is 

 so uniform as to be almost without pulsation, so the latter disap- 

 pears entirely in the veins. It is not clear that this circumstance 

 depends exclusively on the friction experienced by tlie blood in 

 passing through the capillaries, but is probably rather owing, as 

 Mr. Hunter has suggested, to the veins receiving their blood from 

 different arteries, some of whose channels are more circuitous 

 than others, and, consequently, their blood arrives at different 

 times. The momentum of the heart, then, even if it did impinge 

 upon those channels, would not be synchronous upon the venous 

 trunk, but would be divided in such a way as to produce a tre- 

 mour or confused motion. The larger veins, however, have near 

 the heart a pulsation during the contraction of the auricles, 

 arising from the arrest of their circulation at the moment. 

 During inspiration, the vacuum created in the thorax hurries 

 on the blood to the heart, but in expiration it is somewhat im- 

 peded.* 



It has sometimes happened, that a large vein near the heart 

 being opened by an accident or an operation, p. strong inspira- 

 tion has caused the introduction of air, which, being carried to 

 the heart, has produced instant death. It lately occurred in Pa- 

 ris to the celebrated surgeon Dupivytren. 



* This ancient observation has lately been renewed, with additional interest 

 and details, by M. Barry of Paris. See a Report of MM. Cuvier and Durneril* 

 concerning the Influence of the Atmosphere on the Circulation of the Blood, in 

 the Philadelphia Journal of tl?e Medical and Physical Sciences, July, 1826. M. 

 Barry has probably assigned too much importance to this influence, as it is cer- 

 tain that the circulation may go on very well where no vacuum is produced at 

 intervals in the thorax; for example, in the foetus, in incubation, and in fish. 



