5

The celebrated experiment of Vesalius, but of which Hooke appears to have been the first to show

phraseology which he employs. He clearly points out the cause of the difficulty which Legallois experienced in explaining the phenomena, and reconciling them to his system. According to Dr. Philip's view of the subject the lungs are supplied with perception, or as he terms it, sensorial power, by means of the par vagum, so that it is through their intervention that the uneasy feeling is conveyed to the sensorium, which ensues from the defect of fresh air in the vesicles; Inquiry, p. 207, 268; also Quart. Journ. v. xiv. p. 98. et seq. The anatomical structure of the parts, and the experiments of Legallois, to which he refers, render this opinion probable; yet there is still considerable difficulty in conceiving how the state of the air in the pulmonary cavities can act upon the nerves, and we have yet to inquire into the nature of the connexion between the 8th pair and the phrenic nerves.

The student may peruse with advantage Bichat's remarks; "Sur la Vie &c." p. 2. Art. 10. § 2. p. 228. et seq., the object of which is to show, that it is by an indirect effect that the lungs cease to act in consequence of a cessation of the functions of the brain; also Art. 11. § 2, where he endeavours to prove that the lungs are the intermediate organs, which cause the death of the heart to succeed to that of the brain. With respect to the effect of dividing the par vagum, it is remarkable that he draws a conclusion from his experiments which is directly the reverse of what appears to be the natural deduction from them, ubi supra, p. 224. There is frequently an obscurity in this Author, from his peculiar phraseology, and from his hypothesis of the two lives, but the valuable information which he affords us is well worth the trouble of developing his meaning from the metaphysical language with which it is embarrassed. Mr. Brodie adopts a conclusion essentially the same with the one stated above, that when the function of the brain is destroyed respiration ceases, although the circulation is still maintained; Phil. Trans. for 1811. p. 36.

See note in p. 55.

the importance, and to deduce from it the just inference, where, by artificially introducing air into the lungs of an animal that had been apparently destroyed by interrupting the respiration, the blood is enabled to undergo its appropriate changes from the venous to the arterial state, while, in proportion as this change is effected, the contractility of the heart is restored, seems to be decisive, in pointing out the connexion between the heart and the lungs, but it does not give us any insight into the nature of the connexion between the lungs and the nervous system. In the case of the heart the connexion is of an indirect kind, and takes place by means of a series of intervening operations, which may be clearly referred to distinct sources, although they become sooner or later necessarily connected together and inseparably united. But when, on the contrary, life is extinguished by a blow on the head, by an injury of the spine, or by any of those causes which do not act upon the blood in the first instance, the immediate effect, as far as the respiration is concerned, appears to be the destruction of the contractile power of the diaphragm, because the nerves which give this organ its power of mechanically contributing to the reception of the air into the lungs, are no longer able to transmit to it their specific influence.

The conclusion, that one of the remote effects of respiration is to produce that change in the blood, which may enable it to preserve the muscles in their contractile state, affords an easy explanation of a circumstance connected with the action of the heart, which has given rise to much discussion; in the

ordinary course of the circulation the right ventricle is always filled with venous blood; this is then trans mitted through the lungs, and is brought to the left ventricle in the arterialized state. As the blood is supposed to be equally the cause of contraction in both the ventricles, it has been asked, how the two sides of the heart can be acted upon in the same manner by blood of such different qualities, or more particularly, how the venous blood can enable the right ventricle to contract, when, in other cases, arterial blood appears to be necessary for this purpose. The speculative physiologists have given us many answers to this inquiry. Some of them have been satisfied with referring it to the action of the vital principle, others have assumed certain specific properties, in the right side of the heart, by which it was enabled to be acted upon by venous blood, an hypothesis which was adopted even by Goodwyn; others have supposed that there was a kind of sympathy between the ventricles, and others again have imagined there to be something in the mechanical structure of the heart, by which the contraction of one of

6 Connexion of Life &c. p. 82, 4. This would appear to be the opinion of Sommering, who says, "Sanguis nempe venosus, aeri non admissus, licet ventriculum cordis pulmonalem stimulare queat, tamen ventriculo cordis aortici incitando impar est." Corp. Hum. fab. t. vi. p. 76. and upon the same principle Dr. Philip observes that the two sides of the heart are "fitted to obey different stimuli;" Phil. Trans. for 1815, p. 81. See also Nicholls's Pathology, p, 71, where the author supposes that the contraction of the ventricle may be affected by the state of the blood which is poured into it.

its parts is necessarily succeeded by that of the remaining part.7

To a certain extent the latter opinion must be considered as not without some foundation. Although there is a good deal of intricacy in the mechanism of the structure of the heart, and in the form and disposition of its muscular fibres, yet, so far as the present question is concerned, it may probably be regarded as composing one muscle, and will therefore possess that simultaneous action in its various parts, which belongs to the muscles generally, in consequence of which, when any one part of them is stimulated, the whole is thrown into contraction. But although this principle may operate to a certain extent, it cannot be regarded as the correct or proper solution of the proposed inquiry. The blood which is poured into the ventricles of the heart, and causes their contraction, acts, not by any specific properties which it possesses, but merely by its mechanical bulk, while the blood which imparts contractility to the heart is contained in the coronary arteries, which, like the other muscular arteries, proceed from the great trunks in their vicinity, and are distributed through the whole substance of the muscles. Provided the blood in these vessels be properly arterialized, it

7 Haller formed a singular opinion on this subject, that the venous blood was the proper stimulus to the heart. He found that when the blood was made to pass through the right side of the heart, after it had ceased to pass through the left side, the contraction of the right side remained longer than that of the left; Sur les Part. sens. et irrit. mem. 2. ex. 515..523. t. i. p. 362..7.

is immaterial what kind of blood enters the interior cavities of the heart; we may venture to assert, that if it were possible to carry on the circulation through the other parts of the body, it would be sufficient for the action of the heart, if its ventricles were filled with any fluid of the proper temperature and consistence, and in the requisite proportion.R

These remarks upon the effect of respiration in promoting the contractility of the muscular fibre, will enable us to understand the immediate cause of death from drowning, suffocation, or any other of those accidents which prove fatal by preventing the access of fresh air to the lungs. When the action of the pulmonary organs was conceived to be altogether of a mechanical nature, the cessation of the motion of

This point is well treated by Bichat; Sur la Vie &c. Art. 6. §2; but, I believe, this manner of viewing the subject is not original in him, as has been intimated by some of the French writers; See Cuvier, Leç. Comp. t. iv. p. 300. I recollect this doctrine being explicitly taught by Mr. Allen, in his admirable lectures on the animal œconomy, which I had the good fortune to attend in the year 1796 and 1797. Mr. Coleman controverts Goodwyn's opinion respecting the state of the two sides of the heart very satisfactorily; he supports his argument by the peculiarities of the fœtal circulation; Dissert. p. 40. et seq. This may also be regarded as the legitimate deduction from Mr. Brodie's experiments, in which the action of the heart continued after the destruction of the nervous system, until, in consequence of the respiration being suspended, the circulation also ceased; Phil. Trans. for 1811, p. 36. Here the immediate effect depended upon the want of the appropriate change in the blood by the air rendering the muscular fibres of the heart no longer contractile.