676 



CIRCULATION. 



blood in the jugular vein, and to draw it in 

 some degree towards the heart. In many persons, 

 particularly the young and those of a thin habit 

 of body, the jugular veins in the neck are fre- 

 quently very distinctly seen to become full 

 during expiration, and to be rapidly emptied 

 and collapsed during inspiration, a fact which 

 shews clearly enough that the blood passing 

 through this vein enters the chest most easily 

 when that cavity is dilated. The position, 

 however, of the body has a very considerable 

 influence on this rapid evacuation of the jugular 

 veins in such instances. Again, there are 

 several direct experiments upon animals which 

 are much opposed to the views at present un- 

 der consideration. 



Dr. Arnott* has shewn very successfully that 

 such a power as that supposed to aid the venous 

 circulation could have very little effect in pro- 

 moting the flow of fluids through soft tubes, 

 which collapse as easily as the larger veins do, 

 because not more than an inch of fluid at the 

 most can be drawn through one of them by a 

 syringe, without its sides being brought toge- 

 ther so as to close the mouth of the syringe, 

 and this objection is in no way removed by the 

 circumstance that the veins are kept open by 

 the vis a tergo of the heart, because even al- 

 though they should be open, a force from be- 

 Jore, to adopt the incorrect expression frequently 

 applied to a suction power, if strong enough to 

 make any impression on the flow of the blood, 

 would act, to a certain amount, just in the 

 same way as if no force from behind existed ; 

 that is, it would tend to make the sides of the 

 vessel come together, and would thus offer an 

 obstacle to the further progress of the blood. 



In repeating some of Barry's experiments, 

 Mr. Ellerbyf found that when he introduced 

 a tube into the jugular vein of an ass for two 

 or three inches only, there was no suction ex- 

 erted through it, but that the fluid in which its 

 further extremity was immersed rose only when 

 the tube was thrust eight or nine inches into 

 the vein so as to reach the chest, in which case, 

 of course, the vein was held open by the rigid 

 tube, and the suction power was enabled to act 

 through it to an extent which does not take 

 place in the natural state of the jugular vein. 

 Messrs. Ellerby and DaviesJ also found that 

 the venous circulation was for a short time not 

 materially impeded by opening the chest or 

 the introduction of tubes into it through the 

 parietes. It must be apparent to every one that 

 the suction power or vis a fronte can exert lit- 

 tle, if any, force of traction on the blood in the 

 large or superficial veins of the limbs, for on 

 making pressure upon the trunks of one of 

 these, so as to prevent the action of the vis a 

 tergo, we find that if the limb is at rest the 

 motion of the blood in the part next "the heart 

 is wholly arrested. But if, while we maintain 

 the pressure on the vein at one place we empty 

 the vein for some way towards the heart, 

 close the vein on the side next the heart, and 

 then remove the pressure from the remote 



* Elements of Physics, vol. i. 

 t Lancet, vol. xi. p. 326. 

 } Lancet, vol. xi. 606. 



situation, the blood is at once impelled through 

 the portion of the vein which had been emptied, 

 by the force of the heart alone. Messrs. Ellerby 

 and Davies have shewn that the same pheno- 

 mena, or the absence of a vis a fronte and evi- 

 dence of a vis d, tergo, attend the flow of blood 

 in the largest veins even, which are situated in 

 the immediate neighbourhood of the chest; for 

 after the application of a ligature upon the vena 

 cava inferior, it was found that the part of this 

 vein between the ligature and the chest was not 

 emptied towards the heart, and that when the 

 part of the vena cava in the immediate vicinity 

 of the chest was emptied, and pressure then 

 applied at the entrance of the vena cava into 

 the auricle, the blood rose to fill the emptied 

 portion of the vena cava, although no suction 

 power could in this place operate. It was also 

 found that no fluid rose in the remote extremity 

 of a tube introduced into the femoral vein.* 

 These experiments shew that a suction power, 

 whether produced in the way supposed by Dr. 

 Carson, or in that stated by Sir D. Barry, can 

 have very little effect in promoting the flow 

 of blood in the veins, a conclusion which is 

 rendered still more certain from some other ge- 

 neral considerations, such as the following : 



1 . The whole of the vessels belonging to the 

 pulmonary circulation are placed within the 

 chest, and consequently the flow of blood in 

 the pulmonary veins must be independent of 

 any suction power connected with respiration. f 



2. In the foetus, as there is no pulmonary 

 respiration, both the pulmonary and systemic 

 venous circulations go on without any assist- 

 ance from a suction power. And 



3. In the portal circulation of the higher 

 animals and in the venous circulation of fishes 

 breathing by gills, as well as of those reptiles 

 in which air is forced into the lungs by a process 

 of deglutition, there can be no aid derived from 

 a suction power. 



We have already, in our description of the 

 varieties of form in the circulatory organs of 

 animals, adverted to the intimate relation which 

 very generally subsists between the structure 

 and functions of the organs of circulation and 

 respiration. We shall now mention a few 

 other circumstances connected with the func- 

 tions of circulation in the adult human body, 

 which seem to depend upon this relation of the 

 motion of the blood to the respiration. 



The influence of the mechanical operations 

 of respiration is not confined to the venous cir- 

 culation, for it has been shewn by direct expe- 

 riment that the force of the blood in the arteries 

 varies also from the same cause, being greater 

 during expiration than during inspiration. This 

 greater force of the blood in the arteries during 

 expiration, known to Haller, Lamure, and 

 Lorry, was proved by the experiments of Hales, 

 Poiseuille, and MagendieJ formerly mentioned. 



* See also Macfadyen's Remarks, Edin. Med. 

 and Suig. Journal, vol. xxii. p. 271 ', Cams in 

 Meckel's Archiv. iv. p. 413 ; and Remarks in the 

 Edin. Journ. of Med. Science, vol. ii. p. 462. 



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

 tions and Sounds of the Heart. Med. Chir. Trans, 

 of Edin. vol. iii. 



i Journ. de Physiol. vol. i. 



