On certain occasions, however, there is a necessity for an extraordinary quantity of air to be received into the lungs, either For the performance of some mechanical operation. or to produce a greater degree of effect than ordinary upon the blood. In these cases, it would seem that the organs of respiration are influenced either by an instinctive feeling,or by the direct efforts of volition. The act of sneezing commences by a powerful inspiration. For the accomplishment of which all the muscles of the thorax and diaphragm are brought into a state of strong contrac tion, and this is done in the most complete manner by the newly born infant, who must necessarily be quite uncon scious of the end in view, and ignorant of the means by which it is brought about. There are, on the contrary, many actions of the respiratory organs that are completely voluntary, such as sighing, straining, &c. in which differ ent muscles are called into action to produce a specific ef fect; in consequence of previous experience, in the same manner with the motions of the arms and legs. According, therefore, to the degree in which the lungs are called into action, the muscles connected with the thorax appear to be influenced by different powers ; in ordinary cases by some impression upon them, depending on the state of the air and the blood ; while in extraordinary inspira tions, the principles of instinct and of volition, as in other parts of the body, produce muscular contractions, which tend directly to produce some useful purpose in the ani mal economy.
Most of the older physiologists, and even several of the moderns, have supposed that the varying bulk of the chest, in the different states of expiration and inspiration, Must have a considerable. effect upon the circulation This opinion was maintained by Hales and Haller, and many experiments were adduced by which it appeared to be countenanced. But it is probable that in these cases the effects of ordinary and of extraordinary respiration have been confounded together. When experiments are made on living animals, we may presume that the respiration must be rendered laborious, that the air will be taken in at longer intervals, and consequently in a larger bulk at i once, at the same time that the circulation, being in a lan guid state, is more liable to be affected by slight causes acting upon it. That the circulation is not affected by the state of the lungs in ordinary cases may be inferred from the fact, that although the heart contracts four or five times as frequently as the diaphragm, and that con sequently the blood must pass through the lungs in all their different states of distention, yet the pulse remains the same, without any alteration corresponding to the changes in the lungs Upon the whole, therefore, we may conclude, that when the organs are in their natural condition, the different states of the lungs do not mate.
Flatly affect the circulation of the blood.
We now proceed to the second division of our subject, the direct effects of respiration These are to be con sidered in two points of view ; 1st. The effects upon the air ; and, 2d. Those upon the blood. the inspec tion of the anatomy of the pulmonary organs, and par ticularly of the complicated apparatus by which the air and the blood are brought within the sphere of their mutual action, it was nattnal to conclude that the use of the lungs is to effect a change in the blood through the operation of the air. The ancients had some rude conception of the existence of this action, although their idea of its nature was very imperfect. Perhaps the most generally received opinion among the eat lier physiologists was, that the im mediate effect of respiration is to remove from the blood a quantity ol heat and water, for it was a fact too obvious to be overlooked, even by the most superficial observers, that air which is expired from the lungs, differs from that which is taken into them, by the addnion of warmth and moisture. During the prevalence of the mathematical sect, there was a great contro%etsy respecting the ques tion, whether respiration has the etiect or rarefying or of condensing the blood. One party conceived that the blood must necessarily be condensed, because a quantity of water is evaporated from it, while their opponents, who supposed that a portion of air is absorbed by the blood in passing through the lungs, were equally strenuous in maintaining that it must be rarefied.
Although nothing could be more obvious than the ne cessity 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 to any change in its qualities, and it was not until the time of Boyle that we became fully sensible of the fact, that a constant supply of successive portions of fresh unrespired air is essential to life ; a fact which he discovered in the course of his experiments upon the air pump. His in vestigations, however, were principally directed to the knowledge of the mechanical properties of the air; and it was in this point of view that he almost exclusively re garded its action upon the lungs. Yet he did not en tirely overlook the other physical changes which it expe riences ; he noticed the moisture which is exhaled along with it, and farther observed, that it carries off what he calls recrementitious steams; but he does not enter into any explanation of their nature. He also observed, that, under certain circumstances, the air in which an animal has been confined is diminished in bulk ; and this he ac counts for by saying that it has lost its spring.