"Mechanism of respiration"

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PDFs courtesy of Elsevier, via Health Sciences Center Library, Emory University. Included are John Goodman's paper "On the physiology of the mechanical action of the heart," Lancet 2 (29 December 1838): 515-19, and Snow's response. Only Snow's letter is transcribed below.

To the Editor of the Lancet

Sir:--As I consider the paper of Mr. Goodman in the Lancet of Dec. 29, on the important subject of the heart's action, to be open to some objections, permit me to make the following observations on it, by giving publicity to which you will much oblige, Sir, your obedient servant,

John Snow M.R.C.S.

54, Frith-street Soho,

Jan. 5, 1839.

After some preliminary observations, Mr. Goodman, speaking of the thorax, says,-- "The outer walls, or bony arch, possess such strength of material, and peculiar convex form, that the pressure of the atmosphere generally understood to be 15 lbs. on every square inch of surface, is firmly and perfectly sustained by this mechanism, and the contained viscera are protected from its influence."

Now, the most delicate structures on the earth bear the pressure of the atmosphere without detriment, so long as it is equal in all directions; a distended bladder, and bubbles blown in soap and water, bear it, because it is equal inside and out; but this is not what Mr. G. means with respect to the thorax, for he says, "the contained viscera are protected from its influence," which they could not be unless the pressure were entirely on the outside. A thorax, 10 inches deep and 30 inches in circumference (not a very large one), has 300 square inches of surface, and would, in this case, have to resist a force of 4500 lbs., or more than two tons; and, in addition, the diaphragm and the parts closing the top of the thorax, would have to resist half as much; this would require thick walls of cast iron instead of mere flesh and bone. The truth is, that with very slight variations the pressure on every part of the thoracic viscera is exactly the same as on the exterior of the chest. The walls of the thorax are moveable and elastic, and are exactly applied to the surface of the lungs in all their changes in size, and the atmosphere that presses on these walls presses them on the lungs; the atmospheric pressure also, on the walls of the abdomen, is communicated by means of the abdominal viscera and diaphragm, without the slightest diminution to the viscera of the thorax; and the atmosphere communicating with the interior of the lungs, balances, between the respirations, exactly the pressure outside ; it makes no difference that the mouth and nostrils be closed, for the pressure of any given volume of air is precisely the same as that by which it has been compressed. The slight variations of which I have spoken are, that during inspiration the pressure in the interior of the chest is a little less, and during expiration a little more, than on the exterior; that this difference is very minute I will prove at the end of this paper.

Immediately after the sentence I have quoted, Mr. Goodman continues:--"Not only does this protecting arch resist the force of external pressure, but it is also capable, to a certain extent, of expanding itself, and producing a partial vacuum within its parietes." The latter part of this sentence is not at variance with fact. Mr. G. continues,--"The diaphragm, or floor of this cavity, is also able to resist the power of atmospheric pressure, although of muscular construction." I need not comment on this, after what I have said above.

The experiment of removing the heart through an opening in the diaphragm, and then inflating the lungs, does not prove that the pericardium is capable of resisting any [653/654] degree of pressure, either from the lungs or the atmosphere, since, when the abdomen was opened, the diaphragm would descend to allow of the distention of the lungs with the greatest ease.

Mr. G. says there is, apparently, no increase in the volume of a muscle to account for its diminution in length; and that he means thickness, or circumference, by volume, and not the entire bulk, is evident from the application he makes of it. Now, he has only to place his hand over the biceps muscle, and contract it, to perceive the opposite of this; and on turning to page 211 of "Baly's Translation of Müller's Physiology," and seeing it there stated that "the primitive fibrils of the muscles of man are five or six times smaller than the red particles of his blood," Mr. G. will, I am sure, at once relinquish his "speculative conclusion, that the primitive muscular fibre is composed of an extremely minute cellular structure of, probably, globular cells, arranged longitudinally in its substance, and for the purpose of receiving and being distended by arterial blood," [etc.]

Further on we find it stated, that "either the anterior walls of the pericardium must give way, and follow the contraction of the ventricles, or a vacuum must take place betwixt the apex of the heart, and the sternum it has now relinquished." Mr. Goodman must have derived this opinion from the speculative conclusion alluded to above, for all previous authors say that the apex of the heart is projected forward against the sternum during the contraction of the ventricles, and that this projection commences with the very beginning of the contraction is evident from the impulse being exactly synchronous with the first sound of the heart, which is caused by the contraction of the ventricles.

With respect to the ingenious experiments with the glass pericardium, it is necessary to observe, firstly, that if the thoracic viscera were defended from atmospheric pressure, as Mr. G. states, then the phenomena he exhibits could not take place, whatever were the structure of the pericardium, since it is by this pressure these phenomena are produced; and secondly, that with those viscera, situated as they are, no vacuum, partial or complete, could exist in the pericardium, unless it were as firm and unyielding as the glass vessel by which Mr. G. has represented it; whereas the pericardium, though a strong fibrous membrane, and capable of resisting solution of its continuity, is not able to retain any particular shape in opposition to any part of the pressure of the atmosphere, which is freely communicated through the diaphragm below and the lungs on each side, and must retain it as accurately applied to the surface of the heart during all its motions, as it keeps the sides of the thorax applied to the surface of the lungs; and the serous coat of the pericardium, reflected over the heart, must glide over the serous lining of the fibrous coat during these motions as freely as the pleura pulmonalis glides over the pleura costalis. Mr. Goodman says, that the impulse of the heart is caused by the pressure of the atmosphere forcing the blood into the auricles, and "causing its apex to strike the posterior surface of the sixth rib, in a similar manner that the piston of a condensing engine is, by the power of the steam, carried with terrific violence through the vacuitous portion of the cylinders."

If the theory of the pericardium given in the paper under consideration were correct, it would be almost impossible for the circulation to continue a moment after the pericardium became adherent all over the surface of the heart; yet Dr. Hope tells us that persons may live for some years after such is the case, and I have examined a heart that was firmly adherent everywhere to the pericardium, whilst the inflammation which caused this adhesion took place some months before the patient's death.

There is one tendency to a vacuum which does assist the circulation of the blood, and to which Mr. G. has not alluded. Dr. Williams tells us that the hand, grasping the heart of a donkey, cannot prevent the expansion of the ventricles when they are contracted; this active expansion, then, must remove, in part, the pressure of the atmosphere and cause the blood to be driven into the ventricles.

Mr. Goodman states that the ventricles propel the sanguineous fluid against the whole pressure of the atmosphere throughout the body, [etc.] Now, the ventricles could do no such thing; and that they have to do no such thing, is evident from what I have said about the thorax; except the deviation that respiration causes, the atmospheric pressure does not interfere, and during expiration the flow of arterial blood from the thorax is assisted and during inspiration retarded, and the extent of this deviation has been measured by M. Magendie, and is given in his lectures in a recent number of the Lancet, where it is also seen that the force of the arterial current in dogs is generally equal to a column of from 70 to 100 millimetres of mercury, which is from three to four inches, or from one-tenth to one-eighth of the pressure of the atmosphere.

One of the conclusions at which Mr. G. arrives from his experiments is, "that there is no necessity at all for muscular fibre in the parietes of the auricles for facilitating or producing the action of the heart." But since anatomists tell us that muscular fibres exist there, we may fairly conclude that they perform some function; and most persons who have experimented on animals [654/655] have seen the auricles contract, and generally before the contractions of the ventricles.

As the remainder of the paper is principally occupied with conclusions from the parts on which I have already commented, I need offer no further remark on it.

It is strange that Müller, in his Physiology, attributes to Sir David Barry this very idea of a vacuum in the pericardium; and it is more strange that he seems to receive this idea and to concur in it. I give the passage from p. 234 of "Baly's Translation:"-- "Sir David Barrv has recently given a new turn to these inquiries, respecting the circulation in the veins. The heart, he says, when distended with blood, completely fills the pericardium; but when it contracts, it no longer occupies the same space, and a partial vacuum ensues. To enable the auricles to fill this vacuum, the blood rushes into them from the great venous trunks. But, Sir D. Barry attributes more importance to the effect of inspiration" [etc.] Now, Sir David Barry's experiments on the pericardium were instituted to show that the partial vacuum which exists in the thorax during inspiration, extends also to the pericardium, and not that there is any special vacuum in the pericardium caused by the contraction of the heart, and he found that when a tube was inserted into the pericardium of an animal, fluid was drawn up into that cavity during inspiration; and the only passage in his book that might lead to an idea that the contraction of the heart at all influences the result, is the following, at page 20:-- "Although the fluid invariably halted, or descended, during expiration, there was an oscillation of the fluid upwards, which seemed independent of respiration, and could not be perceived during inspiration, because then it was confounded with the general motion of the liquid upwards. This third movement was acknowledged by my friend Mr. Bennett."

The real amount of variation that respiration causes in the atmospheric pressure, exerted on the thoracic viscera, I have ascertained by the following simple experiments:--By fitting a glass tube into one of the nostrils, leaving the other nostril open, and dipping the lower end of the tube into mercury, it is found that the mercury rises in the tube during inspiration a very little above its level on the outside, perhaps about a line, and sinks the same depth below its level during expiration; if the tube is dipped in water instead of mercury, the water rises and falls about an inch above and below the surface. If the other nostril be closed, and a powerful act of inspiration made, the tube being dipped in mercury, the mercury will be raised nearly two inches; if dipped in water, that fluid will be raised about two feet; during a powerful expiration the fluids are depressed an equal degree below their level. It is thus evident, that during natural respiration the variation in atmospheric pressure is about one 400th part of the whole pressure; and, in violent respiration, about one-15th of the whole pressure, or one pound on the square inch, can be alternately added to and removed from, the equable pressure. On comparing MM. Magendie and Poiseuille's experiments on the large veins near the thorax in dogs, detailed in the Lancet of Dec. 15th, with these, and turning the millimetres into inches, I find there is reason to believe that there is not much difference between the respiration of dogs and human respiration as regards variation in pressure. I find that in spasmodic expiration, the chest being first expanded, as sneezing, mercury may be depressed for a part of a second, four or five inches below its level; this is best seen by having the mercury in a bent tube, when the rise of the mercury is seen in the other limb. The tube must not be put in the mouth during these experiments, as the act of suction performed by the mouth can raise mercury eight or nine inches, in addition to the act of respiration.

It is evident from these experiments that Sir David Barry much overrated the assistance which respiration gives to the venous circulation; but since M. Magendie has recently put the question in its true light, by experiments on the veins themselves, I need not dilate upon it. I will merely observe, in conclusion, that Sir D. Barry, in explaining the modus operandi of his important apparatus for preventing absorption in poisoned wounds, ought to have said, that in removing the atmospheric pressure from a part, he engages that pressure on the surrounding parts to oppose the absorption; instead of saying, as he does, that atmospheric pressure is the great agent of absorption, which is, therefore, prevented by removing that pressure.