CHAP. XXVIII.] THE VENOUS CIRCULATION. 377 



but has been most frequently noticed in observations on the circu- 

 lation of cold-blooded animals. 



Pulsations synchronous with the heart's beats may be not un- 

 commonly noticed in the human subject, which it is important 

 that the observer should not confound with those resulting from 

 the extension of the heart's impulse, as above referred to. These 

 may be called the regurgitant, and the communicated venous 

 pulse. The former is caused by the regurgitation which takes 

 place from the right auricle into the large venous trunks connected 

 with it at every systole of that cavity. In a state of health, 

 the regurgitation is so small that its influence extends only to 

 the larger veins but when the right cavities of the heart are 

 dilated, a much larger quantity of blood is regurgitated, and a 

 distinct venous pulse is visible in the superficial jugular veins, and 

 sometimes in all the superficial veins which are distributed over 

 the neck and upper part of the chest. The communicated venous 

 pulse results simply from the proximity of some large artery which, 

 in its pulsations, communicates to the vein a movement of a similar 

 nature. 



Hales and Poiseuille estimated the force of the current of blood 

 in the veins; the former by the introduction of tubes into the 

 large veins, as in his experiments upon arteries, the latter by the 

 haemadynamometer, and their observations have lately been re. 

 peated by Valentin and Mogk. Hales found that the blood rose 

 to four feet two inches above the level of the heart, in a tube 

 inserted towards the head into the jugular vein of a mare, the 

 blood rising several inches when the animal strained, but subsiding 

 again when it became quiet ; hence it is plain that the force of the 

 heart, competent as it is to maintain a column of such a height,, 

 must be amply sufficient to return the blood to the heart. Valentin 

 and Mogk's observations show that the force of the blood in the 

 veins of dogs is equal to one-eleventh or one-twelfth of that in the 

 corresponding arteries. 



The venous circulation is influenced a good deal by the respira- 

 tory movements, which tend partly to promote, partly to retard it. 

 These effects are produced most plainly by the forced movements 

 of respiration. Thus a deep inspiration, by enlarging the capacity 

 of the chest, generates a tendency to a vacuum which, under the 

 pressure of the surrounding atmosphere, is filled chiefly by the rush of 

 air into the trachea, and through it to the lungs, but partly by the 

 afflux of the blood, which must be principally venous, since the semi- 

 lunar valves would oppose any reflux in both the great arteries. Sir 



VOL. II. C C 



