: for the consequent derangement which must necessarily ensue in every part of their economy. And we have, in support of the opposite opinion, an experiment of Goodwyn's, which appears to have been carefully performed, and which bears directly upon this question. He introduced a quantity of water between the pleuræ of a dog, and when even as much as one-third of the cavity of the thorax was filled, he could not perceive that the passage of the blood through the lungs was retarded, although the respiration was rendered laborious. The same view of the subject was also taken by Bichat, who has detailed various experiments and observations, made for the express purpose of ascertaining how far the circulation is affected by the state of distention of the lungs, the results of which appear to be quite decisive.1

*

3

It has been observed that when a portion of the cranium is accidentally removed, an alternate eleva. tion and depression is visible, corresponding with expiration and inspiration, and Haller extends the

4 Essay, p. 45. Morbid cases of a similar nature, so far as this point is concerned, are not unfrequently met with, where fluids of various kinds are effused between the pleuræ, and where the respiration is much affected, while the circulation proceeds without any impediment. See Morgagni, on the Seats of Diseases, by Alexander, v. i. p. 408.

+ Sur la Vie, &c. part 2. art. 6. § 1.

s Haller, El. Phys. vi. 4. 9; viii. 4. 27. et alibi; Mem. sur les Parties Irrit. et Sens. t. ii. p. 172..192; see also the experiments of Haller's correspondents, Tossetti and Caldani, ibid. t. ii. p. 198. and t. iii. p. 141; with his own observations, t. iv.

operation to some of the other viscera.

This effect has been attributed to the greater resistance which the blood experiences in its passage through the lungs while in the state of expiration; a stagnation is thus brought about in the right side of the heart, and consequently the blood is retained in the veins, which, in a highly vascular part, produces an increase of its bulk. When from the enlargement of the thorax the lungs become more pervious to the passage of the blood, the veins are enabled to empty themselves, and the increase of bulk is removed. It is probable that the phenomenon depends upon the cause that has been assigned; but it must be remarked, that in cases of this kind, where so considerable an injury is received by the cranium, or where of the internal viscera are laid open, the respiration may be supposed to be more laborious than natural, so that the air will be taken in at

any

p. 15; also Opera Minora, t. i. p. 131..7, 141. I had once an opportunity of witnessing this phenomenon, in a very striking manner, in a patient who had lost a portion of the cranium, owing to the growth of a fungous tumour; upon removing the tumour the bone was found to be in a diseased state, and was likewise removed, leaving the brain exposed. It would appear that the motion of the brain was observed by Lamure about the same time that Haller was engaged in his experiments on this subject. See Haller's Letter in Op. Min. t. i. p. 242; Lamure in Mem. Acad. Scienc. pour 1749, p. 541. et seq. His experiments were performed in the years 1751 and 1752, and were read to the Academy in 1752; the volume was published in 1753.

El. Phys. vi. 4. 9; Op. Minor. t. i. p, 137.

longer intervals and consequently in larger quantity, while the system being, at the same time, in an exhausted state, the circulation will probably be more easily affected by the operation of slight causes, which would not be perceived in its healthy and natural condition."

But there is a consideration which is of more weight in the determination of this question, than the result of any experiments can be, in which it

7 The so much celebrated experiment of Hooke, in which, after the motion of the heart had ceased, it was re-produced by inflating the lungs, has been adduced to prove that the lungs are rendered more pervious to the blood by inspiration; Haller, El. Phys. viii. 4. 12. But it must be borne in mind, that in this case, the thorax was laid completely open, and that the lungs would consequently be collapsed into a much smaller bulk than while they were within the cavity of the thorax, at the same time that we account for the renewed motion of the heart upon a different principle, totally unconnected with a mechanical operation. With respect to the experiment itself, it is not a little remarkable, that although it excited so much attention when it was performed by Hooke, and was viewed by his contemporaries with almost childish astonishment, the very same process had been performed long before by Vesalius; see Corp. Hum. Fab. lib. 6. c. 19. in Oper. t. i. p. 571, 2. This great work was published in 1543; Hooke's experiment was performed in Oct. 1667. Hooke drew the correct inference from his experiment, and directly states his opinion that it was the fresh air, and not any alteration in the capacity of the lungs, which caused the renewal of the heart's motion. He farther informs us that the circulation through the lungs went on freely when the lungs were suffered to subside. For the original account of the experiment see Phil. Trans. No. 28. p. 539; see also Lowthorpe's Ab. of Phil. Trans. v. iii. p. 66; and Sprat's Hist. of the R. S. p. 282.

appears impossible to obviate every source of uncertainty. In the healthy state of the system we respire upon the average about twenty times in a minute, while the average velocity of the pulse may be estimated at eighty, so that the heart contracts four times during each act of respiration; and must consequently receive the blood during all the various states of distention to which the lungs are subject, yet we do not perceive that the pulse exhibits any corresponding variations either in its strength or its velocity. And farther, we shall find it extremely difficult to produce any effect upon the pulse by the most powerful voluntary efforts of inspiration or expiration; yet, in such cases, the capacity of the thorax will certainly undergo a much greater change, than it can possibly experience in its ordinary action."

The connexion, which was supposed by the older writers to exist between the heart and the thorax depended upon their idea of the structure and mechanism of these organs, but some of the modern physiologists, who have insisted upon the reality of this connexion, have ascribed it to a cause rather of a metaphysical, than physical nature. Hunter speaks of a sympathy or association existing between the actions of the heart and the lungs; we also

8 This consideration appears to me, in a great measure, to counteract the force of M. Magendie's reasoning in a paper in his Journal, t. i. p. 132. et seq.; most of the experiments which he relates must be regarded as showing what takes place in an unusual action of the respiratory organs.

9 On the Blood, p. 54.

find an opinion of the same tendency advanced by Currie,' and Darwin still more explicitly refers to the effects of association. He says that "innumerable trains or tribes of other motions are associated with these muscular motions that are excited by irritation; as by the stimulus of the blood in the right chamber of the heart the lungs are induced to expand themselves, and the pectoral and intercostal muscles, and the diaphragm, act at the same time by their associations with them."2 But, in opposition to this hypothesis, we may remark, that the lungs of a newly born animal act with full force immediately upon being placed in a situation where they can have access to the air, long before the effect of association can possibly operate. Besides, in after life, the periods of the contraction of the heart and the diaphragm bear no ratio to each other, the one being often much increased or diminished in frequency, while the other is not affected. The heart and the lungs are, however, indirectly connected with each other, inasmuch as they are both of them affected by the quantity and the quality of the blood which is transmitted through them, and, to a certain extent, are both of them under the influence of the nervous system. But this connexion is not to be referred to the effect of habit or association, but to the direct application

Med. Reports, p. 77.

2 Zoonomia, v. i. p. 40. Beddoes, proceeding upon this principle, goes so far as to form a project for rendering animals amphibious; Observ. on Fact. Airs, part i. p. 41. Upon this I shall have occasion to offer some remarks in a subsequent part of this chapter.