of the same agent to each separate organ. Upon the whole then we may conclude, that in the ordinary act of respiration the blood is transmitted through the lungs at all times with. nearly equal facility, and that it is only in extreme cases that the retardation described by Haller and his contemporaries can be supposed to take place,

Besides the pulmonary system of blood vessels, there are other parts of the sanguiferous system which it was supposed would be affected by the mechanical actions of the thorax, and this it was conceived would be especially the case with the aorta and the vena cava, by the pressure of the diaphragm upon them during its contraction. Yet on inspecting the anatomy of the parts, it would scarcely appear that this effect should be expected to take place, as the passages through which these vessels are transmitted are guarded, as if for the express purpose of preventing the stagnation of their contents. Haller indeed informs us that, in some of his experiments, he observed the vena cava to be compressed by the diaphragm, so as perceptibly to impede the passage of the blood through it. Yet we may here, as on so many former occasions, attribute the effect to the state to which the animals were reduced, with all the functions deranged, the circulation probably much exhausted, and the diaphragm convulsed by the near approach of death.

3 Winslow's Anat. v. i. § 570. . 572; Haller, El. Phys. viii. 1.35; and Icon Anat. fasc. 1; Bell's Anat v. i. p. 326, 7. El. Phys. vi. 4. 10; viii. 1. 36; viii. 5. 23.

Certain nerves, which perform very important func tions in the animal economy, as, for example, the par vagum and the great sympathetics, pass through the diaphragm, and are so situated, that it was supposed they must be compressed by the contraction of this organ; it was therefore supposed that very important effects would hence result in the parts to which the nerves are sent, that there would be an alternate transmission and obstruction of the nervous influence, which would explain the motions of the chest, the pulsation of the heart, and the vermicular actions of stomach and intestines. But more accurate observa tions seem to prove that there is no foundation for this opinion, that we have no evidence for this pres sure upon the nerves, and that we are able to explain the phenomena more satisfactorily upon other principles.

As the alternate motions of the chest, produced by the diaphragm, must cause a corresponding motion of the whole of the abdominal viscera; it has been thought that the agitation and pressure which they would hence experience will be instrumental in propelling the blood through the lungs, and in this way indirectly contribute to the formation of the various secreted fluids." But we may presume that this effect has at least been much over-rated; for it appeared in the experiments of Menzies, that the increase of bulk which the body experiences during

5 Martine, in Ed. Med. Ess. v. i. p. 156. et seq.

6 Haller, E!. Phys. viii. 5. 23.

inspiration is precisely equal to the volume of air taken into the lungs, the change of capacity which the abdomen would experience from the contraction of the diaphragm being exactly balanced by the relaxation of the abdominal muscles, and vice versă, so that although the viscera may be supposed to be at all times under a certain degree of compression, the measure of it will be at all times nearly the same. No accelerations of the contents of the veins can therefore be produced by this cause, and indeed, as they are not furnished with valves, we may presume that any unusual pressure upon them would tend to diminish rather than to increase the flow of blood through them. Perhaps, however, we may, toda certain extent, admit of the mechanical effect of the respiratory organs upon the liver, in consequence of its situation with respect to the diaphragm and the ribs, and there may be supposed to be a peculiar necessity for some mechanical force of this kind with respect to the gall bladder, which is not furnished with muscular fibres, and would therefore appear to have no means of evacuating its contents, except what is derived from external pressure."

7 Essay, p. 24. et seq.

I

8 Haller, El. Phys. iii. 2. 3; viii. 5. 23; C. Bell's Dissect. v. i. p. 48.

9 Haller, El. Phys. xxiii. 3. 29. Senac, in an elaborate essay on the diaphragm, in the Memoirs of the Acad. Scienc. for 1729, p. 118. et seq., supposes that the posterior muscles, or pillars, as they have been termed, when they contract, must obstruct the passage of the esophagus, but this does not appear to be confirmed by subsequent observations.

If It may seem also not improbable, that the state of compression in which the abdominal viscera are retained, and the alternate motion to which they are subjected, may have an effect in propelling the chyle along the lacteals and the thoracic duct. These vessels differ from the abdominal veins in the essential circumstance of being plentifully furnished with valves, and thus any increase of pressure, or perhaps. even any change of position in the parts, which must be always attended with some partial compression, will have the effect of pushing forwards their contents. In the forced or extraordinary actions of the thorax, these effects may be even considerable, although we cannot suppose that they will be very powerful in ordinary respiration, and upon the whole we may conclude, that they are much less than was supposed to be the case by the physiologists of the last century.

§3. Changes produced upon the Air by Respiration.

From an inspection of the anatomy of the pulmonary organs, and particularly of the complicated apparatus by which the air and the blood are brought within the sphere of their mutual action, it was natural to conclude, that the principal use of the lungs is to produce some change in the blood through

Senac, Mem. Acad. pour 1724, p. 173; Haller, El. Phys. xxv. 2. 6. Cruickshank supposes that the thoracic duct may be affected by the motion of the diaphragm in certain states of unusual contraction, but not by its ordinary action. On the Absorbent Vessels, p. 168, 9.

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Changes produced upon the Air by Respiration.

the medium of the air. Intimations of this kind are frequently met with, even in the writings of the ancients; but it was not until comparatively of late years, that we obtained any clear conception of the nature or effect of this action. Perhaps the most generally received opinion among the earlier physiologists was, that the air, by being taken into the lungs, removes from the blood a part of its heat and water, for even the most superficial observer could not overlook the fact, that air which is expired from the lungs, differs from that which is taken into them, by the addition of warmth and moisture. During the reign of the mathematical sect, there' was a great controversy respecting the question, whether respiration had the effect of rarefying or of condensing the blood. Those who supposed that a principal use of this function is the exhalation of water from the lungs, concluded that the blood must be condensed; while others, who conceived, that in the process of respiration a quantity of air is absorbed by the lungs, were equally strenuous in maintaining that the blood must be rarefied.

Although every one was aware of the necessity of the uninterrupted continuance of respiration, yet this was attributed more to some mechanical effect, which it produced upon the motion of the blood, than upon a change in its qualities: and it was not

It has been remarked that Cicero, who borrowed his philo sophy, both natural and moral, from the Greeks, speaks of the air possessing a vital spirit, but nothing is said about its nature or mode of operation; De Nat. Deor. lib. ii. § 47.