Imatges de pÓgina
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of the femoral or axillary arteries, between my finger and thumb, whilst a ligature has been placed upon them; and I do not, therefore, believe, that the blood is propelled in these arteries with a force that is not readily overcome by moderate pressure; or that, in a healthy man, any circulation goes on in an artery, when the pulse of that artery has ceased in consequence of pressure. It is not, indeed, con. sistent that it should, for, if the circulation can go on so as to cause a dangerous hæmorrhage, without any pulsation of the artery, the continuance of it in the smaller arteries in a state of health, would almost appear unnecessary. If it be said that it is not circulation, but mere. ly a little blood that passes between the sides of the vessel that are not exactly in contact; I would reply, that if such an occurrence did take place, the quantity must be so small as to be unworthy the attention of the surgeon ; for, if it were in greater quantity, it would be attended by pulsation of the artery.

" I do not mean, in the slightest degree, to doubt the correctness of Mr Bell's statements of his inability to suppress the circulation in the cases of aneurism he has adduced. I mean to assert only, that the passage of the blood through a healthy artery, can be effectually prevented by moderate pressure ; that when the pulse has ceased in a large artery in consequence of this pressure, the circulation is suppressed for every purpose in surgery ; and that the surgeon may, therefore, divest himself of all fcar of hæmorrhage. It is, indeed, a fact so notorious in the medical department of the army, that I need not have noticed it thus particularly, if I did not think the great authority of Mr Bell's opinion might prevail when the practice of the peninsular war shall be forgotten.”

The various doctrinal and practical discussions in which Mr Guthrie engages, are illustrated and enlivened by a variety of cases, and observations of the highest interest.

Such a book, in short, as this, was wanted; and we now have one which ought to be in the hands of every military surgeon.

III.

An Inquiry into the Causes of the Motion of the Blood, &c. By

James Carson, M. D. &c. Liverpool, 1815. It is now so long since the discovery of the circulation

of the blood first attracted the attention of physicians, and so much has been written on it, that we did not expect to find any thing new on this subject in Dr Carson's Treatise, for he confines his attention to the mechanical part of the question. It iş not his object to inquire into the principle on which the action of the heart and blood vessels depends, but simply by what means the' motion of the blood is supported. His views of this subject, which seems to admit of so little difference of opinion, have at least the merit of novelty.

In the following observations we shall, in the first place, lay before the reader the opinions of Dr Carson; then endeavour to ascertain how far they are correct; and lastly, make some general observations on the manner in which his work is executed.

We shall give an account of his opinions in our own words, because we think it may be given in a much more concise form, and in much simpler language than it is given by him.

Dr Carson has said a great deal of the cause of the motion of the blood in the arteries, although his opinion on this head does not differ from that generally received. Some of his trains of reasoning in this part

of his work do not appear to us to be correct, particularly his demonstration of the direction in which the blood is propelled by the sides of the contracting vessels. He regards the arterial system as a cone, the trunk forming the apex, the extreme branches the base. In an extended sense of the word we may call it a cone, but every one must see the inaccuracy of applying to it a demonstration founded on the properties of the cone, properly so called. Thus, as might be expected, Dr Carson's demonstration certainly does not apply to the arterial system. Others of his trains of reasoning appear to us superfluous, particularly that by which he demonstrates that the quantity of blood which moves through every part of the aortic system in any given time, must be precisely the same with the quantity of blood thrown by the heart into the aorta in that time. This is self-evident, when it is known that the heart always receives from the veins precisely the same quantity which it throws into the arteries, which of course must be the case. But without farther observations on this part of the work, let us proceed to that in which Dr Carson brings forward the opinions peculiar to himself.

These relate to the motion of the blood in the veins, and its reception into the cavities of the heart. Dr Carson has shewn, by several experiments, that, on admitting the air into the cavity of the chest, the lungs collapse with considerable force; it appears, from these experiments, with a force equal to the weight of a column of water seven inches in beight. It therefore follows, that while the external surface of the chest sustains the whole weight of the atmosphere, its internal surface is pressed with this weight, minus the weight of a column of water of seven inches, the measure of the force with which the lungs are constantly tending to collapse.

Now it is evident, as Dr Carson observes, that the internal surface of the heart must be regarded as part of the external surface of the chest, as far as relates to the lungs; and the external surface of the heart, as part of the internal surface of the chest ; the former pressed by the whole weight of the atmosphere communicated through the venous blood, the latter with that weight, minus a weight equal to a coluinn of water of seven inches. It appears from this statement, that the weight of the atmosphere tends constantly to distend the cavities of the heart with a force equal to the weight of this columu of water ; the consequence of which is, that the blood passes from that part of the great veins lying nearest to the heart into its cavities with this force. The blood in this part of the great veins being thus displaced, according to the most simple law of hydrostatics, namely, that a fluid, contained in cavities communicating with each other in all preserves the same level, the blood from the other veins flows towards the heart, and thus a current of blood is produced in this direction throughout the whole venous system,

But Dr Carson does not maintain that the motion of the blood in the veins arises from this cause alone, but shews, by a very simple train of reasoning, that the force with which the blood enters the capillary veins is precisely equal to that with which the heart propels it into the great arteries. This is evidently a consequence of the quantity of blood which moves through every part of the vascular system in a given time being necessarily the same, for a reason we have just had occasion to point out.

Such then are the means by which, according to Dr Carson, the blood is moved in the veins. He maintains that the veins themselves possess no power of propelling the blood. *

The theory which we have now laid before our readers is evidently the result of considerable reflection, and no small share of ingenuity. Were these, however, the only things which recommend it, we should neither have troubled our readers with so particular a statement of it, nor with the observations which we mean to make on it. We have no hesitation in saying, that Dr Carson has pointed out a principle which, to a certain degree, influences the circulation, which, as far as we know, has escaped

* We have not noticed, because we cannot admit what Dr Cargon says of the heart possessing a peculiar power of dilating itself.

the attention of other writers. Every contribution to our knowledge on so important a subject ought to be received with gratitude, and examined with care, in order to see to what consequences it may lead.

lead. We are now to inquire what contribution Dr Carson has really made to our knowledge. We fear he will not be pleased with our observations, because they reduce this contribution to a sum so very much smaller than his estimate makes it.

Dr Carson, as we have just seen, admits, as indeed is most evident, that the tendency of the lungs to collapse, or, as he terms it, their resilience, cannot bestow on the heart such a power of suction as to support the motion of the blood in the veins independently of the vis a tergo which is derived from the arteries. Of the force with which the blood enters the capillary veins Dr Carson makes a just estimate. In proportion as the blood moves slower in the capillaries than when it leaves the heart, it moves in greater quantity; and the force of a moving fluid is directly as its velocity multiplied by its quantity of matter. But it is evident in the present question, that it is of no use to ascertain the momentum with which the blood enters the capillary veins, if we cannot ascertain what part of this momentum it retains after it is in the veins. If there is a cause extinguishing its momentum almost as soon as it enters the veins, it signifies not with what force it enters them. We apprehend such a cause does exist. When we compare the motion of the blood in the aorta, for example, with its motion in the capillary system, we find not only that its velocity is greatly diminished in the latter, of which Ďr Carson is aware, but that with this diminished velocity, it has a degree of friction to overcome in the capillaries, equal to that opposed to it in the aorta multiplied we know not how many times. Of this Dr Carson makes no account whatever. Now, it would not be difficult to shew, that the momentum with which the blood enters the capillary veins would not so far overcome this cause of retardation, as to support the motion of the blood above a very short way in them. But as this calculation would be tedious, it is fortunate that we have direct experiment to appeal to. From the experiments made by Dr Wilson Philip with a view to ascertain the cause of inflammation, an account of which is given in the introduction to the second part of his treatise on Febrile Diseases, it appears, that when the capillaries of even a very small part lose their power, the vis a tergo can no longer propel the blood through it. When the power of the capillaries was lost, the blood remained stationary in them. Dr Carson, therefore, must wholly abandon the vis a tergo as a means of the blood's motion in the veins. Having done so, he will confess that his other powers are inadequate to the task.

But we must go farther ; even these powers in the extent in which he claims them we cannot allow him. We admit that the tendency of the lungs to collapse is a power tending to dilate the heart; but, when we compare this power (admitting the accuracy of Dr Carson's experiments, on wbich its supposed amount evidently depends) with the velocity with which we know the blood moves in the veins which pour it into the heart, we must, we conceive, at once reject the idea of the latter being a consequence of the former. If Dr Carson's experiments are correct, we have sufficient data to ascertain the question by calculation ; but we believe that our readers will agree with us, that this task is superfluous. If we admit that the resiliency of the lungs has but comparatively little effect in distending the auricles, a similar mode of reasoning applies to the ventricles, into which the blood enters with the momentum acquired by the contraction of the auricles, the power of which Dr Carson surely greatly undervalues.

It may be farther observed, that the hydrostatical law, above referred to, will, even according to Dr Carson's theory, only influence the distribution of the blood in the veins above the level of the heart, anastomosing branches being everywhere numerous In the erect position, consequently, it can influence but a very small portion of the venous system. If we admit the common opinion, that the blood is thrown into the auricle by the vis a tergo, combined with the power of the vessel itself, it is evident, that the hydrostatic law in question has no place here whatever.

Dr Carson admits, that the arteries possess a muscular power, without giving any reason for denying it to the veins, in which the proofs of its existence rest upon precisely the same grounds. We shall not here inquire into the proofs of the blood-vessels possessing a truly muscular power ;--an opinion very generally admitted, notwithstanding it is opposed by some writers, and particularly by Bichât, but we think on very insufficient grounds. In the experiments of a writer we have already had occasion to mention, we find an additional proof of the muscularity of the vessels. It appears, from Dr W. Philip's second paper on the relation between the sanguiferous and nervous symptoms, published in the Phil. Trans. of London for the last year, that the relation which subsists between the nervous system and the blood. vessels is, in all respects, the same with that which subsists between that system and the heart, whose muscularity cannot be questioned.

But even those who deny the muscularity of the vessels, ada

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