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served, that the vena azygos, which was distended with blood, was of 

 the size of the little linger ; the termination downward of this vessel, in 

 the right renal vein, and above in the superior cava, were most distinct, 

 and by compressing it from above downward, or from below upward, the 

 blood flowed into one or the other of these vessels. 



As the causes which determine the circulation of the venous blood, 

 communicate to it an impulse which is far from rapid, and as this fluid 

 meets with only trifling obstacles, and such as are easily overcome, the 

 pressure against the parietes of the veins is very inconsiderable, and these 

 vessels do not pulsate, as the arteries. There is observed, however, 

 near the heart, an undulatory motion, which the blood communicates to 

 the parietes of the vessels. These kinds of alternate pulsations depend on 

 the rapidity with which the blood, whose course is progressively accele- 

 rated, flows towards the heart, and on the reflux of the blood, during the 

 contraction of the right auricle. The contraction of this cavity, forces 

 back the blood into the veins which open into it; this retrograde course 

 is manifest in the superior cava, and is the more readily occasioned, as the 

 orifice of this vein is not furnished with any valve that might prevent it. 

 It does not, however, extend very far towards the brain, the blood having 

 to ascend against its own weight, and the jugulars admitting of considera- 

 ble dilatation. This regurgitation is still more marked in the inferior cava, 

 the orifice of which is but imperfectly closed by the valve of Eustachius ; 

 it is felt in the abdominal veins, and extends even to the external iliacs, ac- 

 . cording to the testimony of Haller. 



LXV. The orifice of the great coronary being exactly covered over by 

 its valve, the blood does not return into the tissue of the heart, which 

 being a contractile organ, would have had its irritability impaired by the 

 presence of venous blood. It is of consequence to observe, that this re- 

 flux never extends to the veins which bring back the blood from the mus- 

 cles, and that it is never felt in the veins of the limbs which are furnished 

 internally with valvular folds. The case is very different, between our or- 

 gans of motion and these secretory glands : towards these the blood re- 

 quired to be sent back, so as to be the longer exposed to their action : ve- 

 nous blood diminishes and even destroys muscular irritability, and is truly 

 oppressive, as may be ascertained by injecting some, in the arteries of a 

 living animal, or else by tying the veins, so as to prevent its return, or 

 by observing what happens, when the course of the blood is interrupted, 

 either by applyingfirm ligatures round the limbs, or by wearing confined 

 clothes. 



I am satisfied, that it was from observing the oscillatory undulations of 

 the venous blood, in the great vessels, that the ancients were led to the 

 opinions they entertained on the course of the blood, which they com- 

 pared to the Euripus, whose waves are represented by the poets, as un- 

 certain in their course, and in currents running in contrary directions. 



The internal veins in which this reflux is observed, show this motion of 

 the blood most distinctly of any; their sides which are thin and semi- 

 transparent, not being, as in other parts, surrounded by an adipose cellu- 

 lar tissue. To give a complete notion of the doctrine of the ancients, on 

 the subject of the circulation, it will merely be necessary to add to the 

 above idea, the opinion which they entertained, that the chyle taken up 

 by the meseraic veins, was carried to the liver, in which its sanguification 

 was effected ; and lastly, that the arteries were filled with vital spirit, and 



