LECT. XVII. PRESSURE ON THE BLOOD. 329 



laid bare, and a ligature applied to the vein : an incision 

 was then made below the ligature, and a jet of blood 

 escaped. When the artery was pressed between the fin- 

 gers, in order to prevent the passage of the blood within it, 

 the jet of venous blood diminished, and was ultimately 

 stopped completely; but when the pressure was removed, 

 the jet re-appeared. These phenomena could be repro- 

 duced several times. The conclusion drawn from this fact 

 is evident : the blood traverses the capillaries, and circu- 

 lates in the veins by the sole forces which have propelled 

 it into the arteries, namely, by the contraction of the left 

 ventricle, and that of the arterial coats, which are the chief 

 powers, the only ones, in fact, of the circulation. 



The influence which atmospheric pressure exercises upon 

 this function is very limited. I have already mentioned, 

 that at each experiment the column of mercury, of the 

 haBmo-dynamometer, applied to the artery, rises; and at 

 each inspiration it falls. Poiseuille observed the same 

 phenomenon, and under the same circumstances, in large 

 venous trunks : thus, in the jugulars the column of mercury 

 rises during expiration and falls during inspiration. These 

 phenomena do not occur when we make the, experi- 

 ment upon venous trunks distant from the thoracic ca- 

 vity. We easily understand that when the latter dilates, 

 the pressure of the atmosphere must compress the veins, 

 and thus, by means of valves, so placed in these ves- 

 sels as to impede the return of this liquid, it assists in 

 making the blood move towards the heart. On the con- 

 trary, during expiration, when the thoracic cavity is con- 

 tracted, all the vessels contained within it are simulta- 

 neously compressed. Experiment has shown, that the va- 

 riations of the pressure of the blood in the arteries and the 

 veins, correspond to the respiratory movements, and cease 

 to manifest themselves in the sanguineous trunks situated 

 beyond this cavity. 



