Supposing inspiration to be the result of muscular force equally distributed over the whole thorax, the inspiratory power is easily calculated. Taking the walls of the chest at 206 superficial inches, and the area of the diaphragm as 51 superficial inches, and placing them separately, it would appear as follows:— At the same time it must be remembered that this result is produced by insuftiation which would excite an undue elastic tension in the diaphragm and abdominal muscles. Yet, on the other hand, in life a greater expansion would be produced by the ribs, and thence a greater resistance.
It may be questioned how far we are entitled to add a resistance as due to the dia phragm. But let us suppose a thoracic cavity collapsed to rest, with a fleshy floor or dia phragm also quiescent ; let us ;suppose the ribs expanded by some power from without ; the air within the chest would be attenuated, and the diaphragm would be forced upwards, by atmospheric pressure, with a force exactly commensurate to the rarefaction of the air within the chest (presupposing that no air is allowed to enter the chest when we expand the ribs). In this case the diaphragm is resisting the same force per inch as the walls of the chest. Or on the other hand, fill the chest with air to perfect distension, allow the ribs to collapse, the diaphragm would have to resist this collapse with a power exactly commensurate with the recoiling elasticity of the ribs ; therefore the diaphragm participates, in common, in such resistances in the tho racic cavity. But, as the question may be open to objection, we shall chiefly notice that power in reference to the parietes of the chest, given in the 3d and 4th columns in the last table. This table shows that the man, when alive, exerted a muscular power with the walls of the chest, when he inspired 200 cubic inches of air, equal to a total weight of 451•9 lbs. avordupots. Independently of the collapsing elasticity of the lungs, which would be not less than 128 lbs. more (see TABLE his inspiratory muscles lifted a weight of 32 stones, — a weight which he could not have lifted with his arms : and yet the animal economy is not conscious of this exertion. We have supposed a uniform mus cular traction, which is not the case, because the distribution of muscular fibre and tho racic mobility, is not equally applied in ex panding the chest. In diagram fig. 670, D represents a section of the thorax : the portion shaded is the range of thoracic mobility be tween extreme inspiration and expiration. The
mobility is unequal ; more is on the anterior than on the posterior part; therefore we may presume muscular traction to be more at one part than at another. This man could exhale 300 cubic inches of air ; and there is every reason to think, from this extensive mobility, according to our last estimate, that the elastic collapse of his ribs, at the termina tion of deep inspiration, would be not less than 1000 lbs.
In the superficial measurements of the thorax we have included every part, even that covering the vertebra. Now certainly we cannot think the elastic collapse over the vertebra equal to that from the sternum ; therefore, if we allow one-third of the chest to be inactive, and the remainder elevated by the respiratory muscles (which we think is within the mark, because every part of the chest is mobile), then in the case of N. C. in deep inspiration the muscles would have to overcome 301 lbs., or 23 ounces on the super ficial square inch of the thorax.
The last act of life is a deep expiration. During life the ribs are always kept under a certain degree of distension, which is ready to send out from 70 to 100 cubic inches of air at any moment (Reserve air, p. 1067). Our inspiratory muscles, in fact, are always antagonising an elastic thoracic collapse ; and this is always increasing or decreasing, ac cording to the stage of respiration, as quiescent or forced, &c. There are cases, as in hang ing, where a man may die, at the moment of full inspiration, from fear, making an effort to resist the dreaded shock which he is about to receive. N. C. died thus in a state of inspira tion. Making allowance for unequal elasticity of the boundaries of the thorax, of + as above stated, it may be safely said that the difa ferent stages of respiration or breathing re quire the following muscular power to antago nise the elastic power of the ribs throughout life.
Thus we see that in the mere act of or dinary breathing there is an elastic resistance, independently of the elastic force of the lungs, equal to more than 100 lbs. This is to be lifted 18 or 20 times every minute of our life. OF THE ELASTIC POWER OF THE LUNGS. — Independently of the powerful collapse of the ribs and their cartilages, expiration is greatly aided by the elasticity of the lungs themselves, which at all times, and in all the stages of respiration, are tending to collapse upon themselves ; and hence, immediately upon puncturing the thorax, the lungs col lapse to half their dimensions.