6. The remote effects of Respiration on the
Living System.

Having now given an account of the direct effects of respiration, I shall next proceed to consider its remote effects upon the living system.

This inquiry must be considered as in fact identical with an investigation into the uses of respiration, for according to the conception which we are led to entertain of the structure and powers of the living body, we conceive that every action which it performs must produce some useful purpose in its economy,' and be essential to the existence and well-being of the whole. But although no object to which the human mind can be directed is so interesting and delightful as tracing out the final causes of the phenomena which we observe, it has been found by experience that it is not the most appropriate method of arriving at a correct knowledge of the facts themselves. On this account I have thought it more desirable, in a work like the present, to ascertain, in the first instance, as far as the present state of our knowledge will admit of it, the exact nature of the operations of the animal economy, and either altogether to leave the application to be made by the reader, or at least to consider this only as a secondary object of our attention.

The remote effects of respiration will naturally arrange themselves under two heads, those which more immediately affect the vital functions, and those the operation of which is more of a mechanical

nature,

Among the remote effects which have been ascribed by the older writers to the function of respiration, there are some, which are so obviously founded upon incorrect principles, that it will not be necessary to enter upon the examination of them, or to do more than merely to allude to them, as forming a part of the history of science. The following, however, are more deserving of our attention, either as being supported by direct experiment, or as having received the sanction of some of the most eminent modern physiologists. The effect of respiration in producing heat, in preserving the contractibility of the muscles, in preventing the decomposition of the body, in promoting the process of sanguification, in the formation of the voice and the various sounds emitted from the larynx, and in the mechanical operations depending upon the motion of the thorax, or upon its connexion with the contiguous viscera."

7 Sommering enumerates the following uses of respiration; Corp. Hum. Fab. t. vi. § 72. 1. To promote the circulation by a mechanical action, 2. to mix together the components of the blood, 3. to condense the blood by discharging a portion of aqueous vapour, 4. to promote the secretions and excretions by pressing upon the viscera, 5. to assist in chylification, 6. to enable us to exercise the sense of smell, 7. to enable the infant to suck, 8. to enable the lungs to inhale, 9. it is doubtful whether the body acquires electricity by respiration, 10. by respiration the blood is purified and prevented from putrifying, 11. it maintains the temperature of the body, 12. assists in sanguification, 13. is necessary for the voice and speech. Probably some of the above uses may be thought very problematical; but in addition to them we have the various indirect mechanical purposes which is served by the respiration, which will be enumerated hereafter.

160 Maintains the contractility of the Muscles.

The first of these effects, the production of animal heat, on account of the great extent of the inquiry into which it must necessarily lead us, and still more, from the peculiar, and as it were, specific nature of the operation, I shall consider in the following chapter, as a distinct function. In all warm-blooded animals, where an uninterrupted continuance of the respiration is essential to life, we find that the first effect which ensues from a deficiency of the supply of unrespired air, is the cessation of the contraction of the heart. If we examine the heart when it is in this state, we shall find that its capillary arteries are filled with blood which exhibits the purple venous aspect, and by observing the coincidence between the contractility of the muscular fibres and the nature of the blood which is sent to their minute vessels, we seem to be warranted in the conclusion, that a regular supply of arterialized blood is essential to the support of their contractility, and that the deficiency of this species of blood is the immediate cause of the change which they experience when the respiration is impeded. Goodwyn's observations and experiments were directed to this object, and they fully substantiate his hypothesis, so far as the general question is concerned, respecting the nature of " the connexion of life with respiration." But the mode of account

8

8 See the 5th Sect. of his Essay; also Young's Lectures, v. i. p. 739; where the author remarks, that "the muscles are furnished by the blood with a store of that unknown principle, by which they are rendered capable of contracting." Spallanzani goes farther and states that the oxygen which the blood absorbs,

ing for this change will necessarily depend upon the opinion which we adopt respecting the direct effect of respiration upon the blood. If we suppose that this function acts merely in abstracting a portion of carbon from it during its passage through the lungs, we must conclude that the presence of this superabundant carbon in the blood prevents it from preserving the muscles in their contractile state, or if we think it more probable that the oxygen is absorbed in the lungs, we may conceive that the want of contractility is owing to the combined influence of the absence of the ordinary proportion of oxygen and the excess of that of carbon. And whichever supposition we may adopt, it is very possible that some other change may have taken place in the blood, either in its chemical or its mechanical constitution, which may render it no longer fit for the continuance of its appropriate functions, although of the nature of such change we are entirely ignorant. The production of animal heat, should it appear that this is one of the effects of respiration, is essentially connected with the discharge of a portion of carbon, and it will thus follow that these two processes ultimately depend upon the same operation, and that this is to be resolved into a change in the chemical, and possibly also a consequent change in the mechanical nature of the blood.

The share which the nerves have in respiration, or

unites with the muscular fibres of the heart and endows it with its contractility; Memoires, p. 327; but his hypothesis is deduced from very insufficient premises.

the connexion which subsists between the nervous system and the lungs, has long been a subject of controversy, and has given rise to many elaborate dissertations, as well as to numerous experiments. This question being intimately connected with the effects that result from dividing the par vagum, it may be proper to introduce into this place an account of the facts that have been ascertained, and of the opinions which have been formed respecting this operation.

9

These nerves, from the peculiarity in their ana tomical relations, have been, at all times, an object of great interest to physiologists. The other cranial nerves are primarily destined to the organs of sense, or to some of their appendages, whereas these nerves are carried to a considerable distance from the head,

There has been some difference among anatomists in the nomenclature which they have employed with respect to these nerves. The term "8th pair" is considered by many as synonimous with "par vagum;" Boyer, Anat. t. iii. p. 350, 1; while others, as it would appear with more accuracy, regard the par vagum, (or as they have been named by the French physiologists, the pneumo-gastric nerves) as only the principal branch of the 8th pair. See the synoptical table in J. Bell's Anat. v. iii. p. 113, 4. For a description of the part it may be sufficient to refer to Winslow's Anat. Sect. 6. 104.. 142 (he terms them nervi sympathetici medii); J. Bell's Anat. v. iii. p. 153..9; Sæmmering, Corp. Hum. fab. t. iv. § 259; et De bas. enceph. in Ludwig, t. ii. § 84..6; Bichat, Anat. des. t. iii. art. 3. § 2. p. 209.. 222; and for a delineation of it to Vicq. d'Azyr, pl. 17, 18; to Semmering's plate of the base of the brain, for its origin; C. Bell's Dissect. pt. 1. pl. 8, and to his engravings of the nerves, pl. 2 and 3.