IV. THE RELATION OF TIIE CIRCULATION TO OTHER FUNCTIONS.
1. Respiration.—Of the opinions of those who attribute the suction of the blood through the veins to powers vvithin the chest, there are chiefly two which have of late years at tracted attention,—those namely of Dr. Car son of Liverpool,"' and of the late Sir David Barry.t According to Dr. Carson the lungs are of a highly elastic nature, and are kept in a state of forced distension by the pressure of the atmo sphere which enters them when the chest dilates. The lungs would collapse or fall away from the walls of the chest but for the force with which they are distended, and there is thus a tendency to the production of a vacuum within the chest or to a diminution of the pressure on the exte rior of the heart, in consequence of which the blood is forced or drawn into the heart and chest on the same principle that fluid enters the mouth in the act of sucking.
According to Sir D. Barry, at each inspira tion of air into the chest the lungs are not suffi ciently expanded to till the whole of the chest, or there is, in consequence of the expansion of the walls of the chest, a. less pressure within the chest than on its exterior, and the blood is pro pelled through the veins communicating with the heart by the external atmospheric pressure.
Neither Dr. Carson nor Sir D. Barry state, in a sufficiently explicit manner, how much of the force impelling the blood through the veins they conceive to be of the nature of suction : they both admit that the greatest part of this force belongs to the heart or vis tergo, but they yet state distinctly their belief that the suction power is an important cause of the mo tion of the blood throughout the whole venous system. The works of both these authors are replete with interesting remarks on the circula tion in general, and more especially on the flow of blood through the veins. The direct expe riments, however, in support of their opinions are comparatively few and inconclusive. Dr. Carson shewed that the lungs are always during life in a state of forced expansion, and estimates the pressure which the lungs of the sheep are capable of sustaining, when in the expanded condition, as equal to a column of seven inches of water. Sir D. Barry observed, in experiments made upon horses, that when one end of a tube is introduced into the ju- - gular vein, and the other extremity rests in a vessel containing water, the water rose during each inspiration some length in the tube, and sank again during expiration, distinctly indi cating the diminished pressure existing within the chest at the time of the rise of the water, and proving that the flow of the blood in some parts of the veins may be accelerated during inspiration from the same cause. Poiseuille,t by the employment of the instrument for mea suring the pressure of the animal fluids, to which allusion has already frequently been made, has confirmed Sir D. Barry's statement, that the.di minished pressure within the chest, at the time of inspiration, is such as to affect the flow of 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 tergo of the heart, because even al though they should be open, a force from be fore, to adopt the incorrect expression frequently applied to a suction power, if strong enough to make anv 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. Ellerbyt 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 Davies 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 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 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 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 afronte and evi dence of a vis a 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 1,vhole 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.t 2. In the fcetus, as there is no pulmonary respiration, both the pulmonary and systeinic 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.