As the lungs are every where in contact with the cavity containing them, they are necessarily expanded in an equal degree with the thorax. In consequence of this expansion, their capacity is increased, and as there is a free communication with the atmosphere by means of the trachea, a portion of air enters them equal to their in creased capacity. After some time the muscles of the diaphragm and the intercostals relax, and the elasticity of the cartilages of the thorax brings back the parts to their former bulk, and the capacity of the lungs being thus diminished, the additional portion of air which they . had received is expelled. in a short time, however, the muscular contraction is renewed, and is again succeeded by relaxation ;' and this, alternation, which continues to the end of life, constitutes the mechanical process of re spiration. Hence it appears that the state of expiration is what may be termed the natural condition of the respi ratory organs, or that in which they are found, when the position of the parts is not affected by muscular contrac lion. We also perceive, that the air enters the lungs solely in consequence of the increase of the capacity of the thorax, which is affected by muscular contraction ; that this is the only step in the process which can proper ly be regarded as a vital action ; and that the rest of the mechanism of respiration depends upon the elasticity, or other physical properties of the parts concerned.
The above account of the mechanism of respiration is the one generally adopted by the modern physiologists, and appears to explain all the phenomena, without having recourse to any occult agents or gratuitous suppositions. It was not, however, until after many premature attempts, and without numerous and violent controversies, that the correct theory was established; for while the physical pro perties of air were not understood, it is not surprising that errors should prevail respecting the action of the at mosphere on the human body. Boyle, who was so suc cessful in his investigations on the subject of pneumatics, first explained upon correct principles why the air enters the lungs; but his simple doctrine was not suited to that age, where refined hypotheses were so much in vogue, and where every thing was to be explained by sonic ab struse mathematical problem. 'We find, therefore, that for a century or more after he wrote, a variety of learned speculations continued to prevail upon the subject, which were derived either from false principles of philosophy, or from incorrect opinions respecting the anatomy of the organs. Some of these notions were even attempted to be proved by experiment; such, for example, as that a quantity of air is contained in the cavity of the chest, be tween the pleurze, which by its spring compresses the lungs, and produces expiration ; or that the lungs them selves are furnished with pores, through which the air is alternately discharged and re-absorbed in the different states of respiration. Most of the older anatomists sup posed, that the lungs possess an innate power of motion, by which they draw in and expel the air; but all these opinions are now entirely discarded.
Few points in anatomy have been the subject of more warm and protracted discussion than the action of the in tercostals. Between each of the ribs are two sets of mus
cular fibres, one external and the other internal ; both of which are situated obliquely, but in opposite directions. The older anatomists conceived, that the contraction of the external layer would tend to raise the ribs, and thus, by increasing the angle which they form with the spine, enlarge the capacity of the thorax, while the internal layer had the contrary effect of depressing the ribs, and consequently diminishing the chest. Mayow appears to have been the first who decidedly adopted the opinion, that the contraction of both the external and the internal intercostals raises the ribs,—the opinion which is now ge nerally embraced ; but the contrary idea continued to pre vail until the middle of the last century, when it was suc cessfully refuted by Haller, and has not been since revived.
The nature of the diaphragm, its muscular action, and its importance in the mechanism of respiration, were very imperfectly understood by the ancients, and even by the earlier modern anatomists. By some, it was supposed to be the immediate seat of the soul ; by others it was thought to have a kind of independent life; and it was generally regarded as being possessed of some mysterious power, until Fabricius explained its operation upon ration al principles, referring it solely to the combined action of muscular contractility and the elasticity of membrane,— the first producing a change in its form, and the latter bringing it back to its original position after the contrac tion had ceased. It is now universally regarded as the great agent by which the size of the cavity of the thorax is regulated. In its natural or relaxed state it is arched upwards, so as to protrude into the chest and diminish its size ; and when, by the contraction of its muscles, it is flattened, the capacity of the thorax is proportionably in creased.
Many attempts have been made by physiologists to as certain the quantity of air that is taken into the lungs at one time, or the bulk of a single inspiration. Respiration is, however, so much under the control of the will, that all we can obtain by our experiments is the average quan tity; for, although we cannot suspend the action of the lungs for more than a very limited period, we can at plea sure receive into them a greater or less quantity of air. There is also a considerable difference in the form of dif ferent individuals with respect to the size of the chest; and it is probable that peculiar states of the constitution, and perhaps of the air itself, may affect the quantity re ceived into the lungs. And besides the question respect ing the bulk of a single inspiration, there are others which deserve our attention. We may inquire, not only what is the average bulk of an ordinary inspiration, but what is the amount of the greatest quantity of air that we are able to take into the lungs by the most powerful voluntary ef fort, what is the quantity of air left in the lungs after an ordinary expiration, what farther quantity we are able to expel by the most poWerful effort, and what quantity still remains in the lungs.