Compositors and pressmen stand low the former are the lowest in their respiratory power ; the pressmen are much higher. The order in which some of these classes come is as follows : — the most powerful are the Thames police ; next, the sailors ; the pau pers and the gentlemen are nearly equal ; and lastly, diseased cases. The two last lines is the mean of the four healthiest classes, the seamen, firemen, Thames police, and pugilists : their maximum is 3 inches; • the mean of the whole classes together is little more than 2/ inches. The measure of this power when expressed by inches of mercury appears small, yet, when hydrostatically con sidered, it is very great. Men have wondered that they could not elevate more mercury in the tube; but all surprise vanishes when it is recollected, that, by the law of hydrostatics, when a column of 3 inches height of mer cure 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 diffiTent dimensions, for the area of the thorax must also be considered. For instance, we examined a man, 4 feet 7,1 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 lbs., and the former only about 39 lbs. Suppose the base of his chest to be 57 superficial square incises ; 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 lbs. 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 lbs. ; and, allowing the elastic resistance of the ribs as 11 inch of mercury, this will bring the weight resisted by the inspiratory muscles of the thorax as follows: — Or, in round numbers, we may say, that the inspiratory muscles of such a man of ordinary dimensions resisted 1000 lbs. 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, yet it must not he 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 museu 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 be easily proved upon one's own person : — partially empty the chest of air ; then forcibly test your expiration upon the hminadynamorneter: pro bably you can only elevate the mercury 11 inch; then inspire deeply, completely filling the lungs, and now test your expiratory power, —instead of 11 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 hmmadynamometer, 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 lbs. ; 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 lbs. 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 tons ! He exhibited in no other respect any remarkable feature of strength.
A dynamic instrument like the hmmady 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 mernbrana 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 (fig. 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.