THORAX. 



1003 



cury is sustained, the force exerted by the 

 diaphragm alone is equal to the weight of as 

 much mercury as it would take to cover a 

 space of the same area as the diaphragm, 

 three inches deep. The column of mercury 

 raised, therefore, will not safely serve to 

 compare the respiratory power of men of 

 different dimensions, for the area of the thorax 

 must also be considered. For instance, we 

 examined a man, 4 feet 7^ inches high (circum- 

 ference of the chest 29 inches), who raised 

 3*15 inches; and another man, 7 feet high 

 (chest 50 inches in circumference), who could 

 only elevate 3 inches of mercury : but the 

 dissimilarity between the area of the dia- 

 phragm in the dwarf and giant was such, 

 that the latter in reality lifted about 500 Ibs., 

 and the former only about 39 Ibs. Suppose 

 the base of his chest to be 57 superficial 

 square inches ; had this man raised 3 inches 

 of mercury by his inspiratory muscles, his 

 diaphragm alone must have opposed a resist- 

 ance equal to more than 23 oz. on every inch 

 of that muscle, and a total weight of more 

 than 83 Ibs. Moreover, the sides of the 

 chest, by attenuating the air within, resist 

 an atmospheric pressure equal to the weight 

 of a covering of mercury 3 inches in thick- 

 ness, or more than 23 oz. upon every inch 

 surface, which, if we take at 318 square 

 inches, the chest would be found to resist 

 a pressure of 713 Ibs. ; and, allowing the 

 elastic resistance of the ribs as 1-| inch of 

 mercury, this will bring the weight resisted 

 by the inspiratory muscles of the thorax as 

 follows : 



Diaphragm - 

 Walls of the chest 

 Costal resistance (elastic) 

 Lung ... 



83 Ibs. 



- 731 



- 232 



* 100 



1146 Ibs. 



Or, in round numbers, we may say, that the 

 inspiratory muscles of such a man "of ordinary 

 dimensions resisted 1000 Ibs. This is a re- 

 sistance not counterbalanced ; for were it 

 counterbalanced, it would only be mere dis- 

 placement. We have made a safe addition 

 for the elasticity of the lungs. We think it 

 may be confidently stated that nine-tenths 

 of the thoracic surface conspire to this act, 

 allowing the remainder to lie dormant. 



Although the difference between the in- 

 spiratory and expiratory powers, when tested 

 to their utmost, is so great, jet it must not be 

 thought that these two powers are in their 

 ordinary action so dissimilar; and indeed, 

 when all things are considered, the question 

 may still be asked, is the inspiratory or ex- 

 piratory act the strongest ? In the last table 

 (TABLE S.) there is uniformly a difference, 

 because the two powers are unequally taxed 

 with resistance. All elastic force is co-ope- 

 rating with the expiratory power, whilst it 

 antagonises the inspiratory power; therefore all 

 the power manifested in inspiration is muscu- 



lar ; but in expiration it is partly muscular and 

 partly elastic power. This probably causes 

 the great apparent difference between inspira- 

 tion and expiration ; at least, if we separate 

 the resistance we assign to the elasticity of the 

 ribs and lungs from the expiratory power, we 

 shall nearly equalise the two. This can beeasily 

 proved upon one's own person : partially 

 empty the chest of air ; then forcibly test your 

 expiration upon the haemadynamometer: pro- 

 bably you can only elevate the mercury 1 

 inch; then inspire deeply, completely filling 

 the lungs, and now test your expiratory power, 

 instead of 1 inch, it will probably be 

 5 inches. This difference appears due to 

 two causes. 1st. In the deep inspiration the 

 ribs are put more upon the stretch than in 

 the moderate inspiration. 2d. The chest, when 

 distended with air, presents points of attach- 

 ment for muscular traction, to a greater me- 

 chanical advantage. 



The most remarkable respiratory power, 

 as tested by the haemadynamometer, was in 

 the case of a Chatham recruit, who was fre- 

 quently examined by Dr. Andrew Smith, on 

 whose accuracy we place implicit confidence. 

 The man's age was 18; height, 5 feet 6 inches, 

 weight, 10 stones 5 Ibs. ; circumference of his 

 chest, 35 inches ; vital capacity, 230 ; his 

 inspiratory power was equal to 7 inches of 

 mercury, and his expiratory power to 9 inches ! 

 The thoracic power of this man, according 

 to our last calculation, was equal to a gross 

 weight of 2200 Ibs. This was the amount 

 manifested, and we may safely consider 50 per 

 cent, of muscular power to be lost by the 

 obliquity of the respiratory muscles ; so that 

 this man possessed a vital power equal to 

 nearly 2 tonsT He exhibited in no other 

 respect any remarkable feature of strength. 



A dynamic instrument like the haemady- 

 namometer would be useful to those whose 

 duty it is to examine men for certain public 

 services, as for the army, navy, police, fire- 

 brigade, &c. With care, *it would often detect 

 disease. The efforts required to move the 

 mercury test the whole trunk of the body. 

 The inspiratory test produces a rarefaction of 

 the air within the thorax, causing an extra 

 (unbalanced) atmospheric pressure upon the 

 body from without. In this way we have de- 

 tected rupture of the membrana tympani ; for 

 the air rushing in by this opening equalised 

 the difference otherwise produced. The ex- 

 piratory test is of a contrary order, increasing 

 the pressure from within ; in this way we 

 have detected hernia. 



The difference between the healthy and dis- 

 eased respiratory powers is broadly marked. 

 It is shown in the annexed diagram (Jig. 702.); 

 the lower curve is the power manifested by 

 diseased, and the upper curve that of healthy 

 persons. The difference is about 50 per cent., 

 because weakness is the most prominent 

 symptom of disease. We do not compare 

 the expiratory power for the reason already 

 assigned. We affix at the bottom of the 

 diagram the relative powers in figures. 



3 Y 4 



