676 CIRCULATION. 



blood in the jugular vein, and to draw it in situation, the blood is at once impelled through 



some degree towards the heart. In many persons, the portion of the vein which had been emptied, 



particularly the young and those of a thin habit by the force of the heart alone. Messrs. Ellerby 



of body, the jugular veins in the neck are fre- and Davies have shewn that the same pheno- 



quently very distinctly seen to become full mena, or the absence of a vis aj'ronte and evi- 



during expiration, and to be rapidly emptied dence of a vis a tergo, attend the flow of blood 



and collapsed during inspiration, a fact which in the largest veins even, which are situated in 



shews clearly enough that the blood passing the immediate neighbourhood of the chest ; for 



through this vein enters the chest most easily after the application of a ligature upon the vena 



when that cavity is dilated. The position, cava inferior, it was found that the part of this 



however, of the body has a very considerable vein between the ligature and the chest was not 



influence on this rapid evacuation of the jugular emptied towards the heart, and that when the 



veins in such instances. Again, there are part of the vena cava in the immediate vicinity 



several direct experiments upon animals which of the chest was emptied, and pressure then 



are much opposed to the views at present un- applied at the entrance of the vena cava into 



der consideration. the auricle, the blood rose to fill the emptied 



Dr. Arnott* has shewn very successfully that portion of the vena cava, although no suction 



such a power as that supposed to aid the venous power could in this place operate. It was also 



circulation could have very little effect in pro- found that no fluid rose in the remote extremity 



moling the flow of fluids through soft tubes, of a tube introduced into the femoral vein.* 



which collapse as easily as the larger veins do, These experiments shew that a suction power, 



because not more than an inch of fluid at the whether produced in the way supposed by Dr. 



most can be drawn through one of them by a Carson, or in that stated by Sir D. Barry, can 



syringe, without its sides being brought toge- have very little effect in promoting the flow 



ther so as to close the mouth of the syringe, of blood in the veins, a conclusion which is 



and this objection is in no way removed by the rendered still more certain from some other ge- 



circumstance that the veins are kept open by neral considerations, such as the following : 



the vis a tergo of the heart, because even al- 1. The whole of the vessels belonging to the 



though they should be open, & force from be- pulmonary circulation are placed within the 



J'ore, to adopt the incorrect expression frequently chest, and consequently the flow of blood in 



applied to a suction power, if strong enough to the pulmonary veins must be independent of 



make any impression on the flow of the blood, any suction power connected with respiration. f 



would act, to a certain amount, just in the 2. In the foetus, as there is no pulmonary 



same way as if no force from behind existed ; respiration, both the pulmonary and systemic 



that is, it would tend to make the sides of the venous circulations go on without any assist- 



vessel come together, and would thus offer an ance from a suction power. And 



obstacle to the further progress of the blood. 3. In the portal circulation of the higher 



In repeating some of Barry's experiments, animals and in the venous circulation of fishes 



Mr. Ellerbyf found that when he introduced breathing by gills, as well as of those reptiles 



a tube into the jugular vein of an ass for two in which air is forced into the lungs by a process 



or three inches only, there was no suction ex- of deglutition, there can be no aid derived from 



erted through it, but that the fluid in which its a suction power. 



further extremity was immersed rose only when We have already, in our description of the 



the tube was thrust eight or nine inches into varieties of form in the circulatory organs of 



the vein so as to reach the chest, in which case, animals, adverted to the intimate relation which 



of course, the vein was held open by the rigid very generally subsists between the structure 



tube, and the suction power was enabled to act and functions of the organs of circulation and 



through it to an extent which does not take respiration. We shall now mention a few 



place in the natural state of the jugular vein, other circumstances connected with the func- 



Messrs. Ellerby and Davies \ also found that tions of circulation in the adult human body, 



the venous circulation was for a short time not which seem to depend upon this relation of the 



materially impeded by opening the chest or motion of the blood to the respiration, 



the introduction of tubes into it through the The influence of the mechanical operations 



parietes. It must be apparent to every one that of respiration is not confined to the venous cir- 



the suction power or vis a fronte can exert lit- culation, for it has been shewn by direct expe- 



tle, if any, force of traction on the blood in the riment that the force of the blood in the arteries 



large or superficial veins of the limbs, for on varies also from the same cause, being greater 



making pressure upon the trunks of one of during expiration than during inspiration. This 



these, so as to prevent the action of the vis a greater force of the blood in the arteries during 



tergo, we find that if the limb is at rest the expiration, known to Haller, Lamure, and 



motion of the blood in the part next the heart Lorry, was proved by the experiments of Hales, 



is wholly arrested. But if, while we maintain Poiseuille, and MagendieJ formerly mentioned, 



the pressure on the vein at one place we empty , gee also Macfad > s R ema rk s , Edin. Med. 



the vein for some way towards the heart, and Surg . Journa i, vol. xxii. p. 271 ; Carus in 



close the vein on the side next the heart, and Meckel's Archiv. iv. p. 413 ; and Remarks in the 



then remove the pressure from the remote Edin. Journ. of Med. Science, vol. ii. p. 462. 



t See the late Prof. Turner's Essay on the Mo- 



* Elements of Physics, vol. i. tions and Sounds of the Heart. Med. Chir. Trans. 



t Lancet, vol. xi. p. 326. of Edin. vol. iii. 



$ Lancet, vol. xi. 606. t Journ. do Physiol. vol. i. 



