THORAX. 



1057 



so tightly knitted together, that at its mini- 

 mum contraction there still remains an elastic 

 force in operation. In fact we might expect 

 this, because the respiratory movements are 

 so small that it is necessary that an extensive 

 elastic power should be ready at all the re- 

 spiratory stages ; and therefore the parts are 

 upon the stretch before we begin to inspire in 

 order to increase to the geometrical degree re- 

 quired, of two, three, or four pounds to the 

 inch, by a very limited movement, which 

 would not be the case did we begin to in- 

 spire when the thoracic boundaries were at 

 zero, at least, if it did, the walls, &c., would 

 have to be much stronger. 



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 5 1 superficial inches, and placing 

 them separately, it would appear as follows: 



TABLE M. Inspiratory Muscular Power re- 

 quired to overcome Costal Elasticity, mea- 

 sured by the Insufflation of 200 Cubic Inches 

 of Air. (Case, N. C.) 



At the same time it must be remembered 

 that this result is produced by insufflation 

 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 

 VOL. iv. 



the ribs ; therefore the diaphragm participates, 

 in common, in such resistances in the tho- 

 racic cavity. But, as the question ma}' 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 45P9 Ibs. avordupois. Independently of 

 the collapsing elasticity of the lungs, which 

 would be not less than 128 Ibs. more (see 

 TABLE O), 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 Ibs. 



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 

 vertebras 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. iu 

 deep inspiration the muscles would have to 

 overcome 30 1 Ibs., 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 anv 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 dif- 

 ferent stages of respiration or breathing re- 

 quire the following muscular power to antago- 

 nise the elastic power of the ribs throughout 

 life. 



3 Y 



