VITAL CAPACITY.— There are two ways of measuring the permeability of the lungs, or the volumes of air which they can displace, viz. by measuring the actual movement or mobility of the thoracic boundaries, or by directly measuring the absolute cubic inches of the volume of air expired. The former is open to an error, but the latter is not.
If we take the movement as an index to the permeability of the lungs, we obtain an evidence only of movement, and not of the permeability of 'the lungs for air, for we may move the thoracic boundaries, and yet not breathe. But when we measure the volume of air, it is self-evident that this must be the measure of both the permeability of the lungs and of mobility of the thoracic bound aries, because we cannot breathe without moving. The classes of persons we examined were as follow :— To determine these points, we constructed an air receiver, denominated " Spirometer." We used a bent tube (hmmadynamometer) for ascertaining the respiratory power, scales and stand for the height and weight, and a com mon tape-measure for measuring the mobility of the chest.
We rarely exceeded three consecutive ob servations with the spirometer, because after this the volume of respired air diminishes from mere fatigue.
To measure the vital capacity volume. The SPIROMETER (fig. 704.1, consists of a vessel containing water, out of which a re ceiver is raised by breathing into it through a tube ; the height to which the receiver is raised, indicates the volume ofthe vital capacity.
To Prepare the Spironieter for Use :— 1st. Place the spirometer about three feet from the ground, upon a firm, level table. 2nd. Turn off the water-tap 4, and open the air-tap 1.
sufficient to bring the water down to the edge of the index.
5th. Pour a little coloured spirit into the bent tube 5, until it stands about 31 inches, as at 6.
6th. Turn off the air-tap 1, then suspend the counterbalance weights, I I, 11, from the cord over the gullies.
The spirometer is now ready for an observa tion. The flexible tube, terminated with a glass mouth-piece, is held by the person about to be examined, and the tap 1 is to be kept open by the operator while the deep expira tion is being made.
To discharge the air out of the receiver. It will be seen that if the tap 1 be opened, the receiver will rise out of the reservoir by the power of the counterbalance weights, until it touches the cross-head 9. To return
the receiver into its original position, the con tained air must be discharged ; this can be done by slowly depressing the receiver down into the reservoir, and so pressing the air out by the way it entered,— through the air-tube. But, in order to do so more rapidly, a large valve at 14 admits of an instantaneous escape of the air. Therefore, to discharge the air, remove the plug 15 out of the socket 14 with one hand, while the other returns the receiver into its original position.
Let the person to be examined loose his vest, and any other tight garment — for the least pressure from dress affects the mobility stand perfectly erect (fig. 705.), with the head 3rd. Pour into the spout, behind, clear cold water, until it is seen to rise behind the slip of glass 3 (above the air-tube).
4th. Slide the moveable index 2, opposite 0 on the graduated scale 13, and add more water until it is exactly on a level with the straight edge of this index ; if too much water be poured in, draw off, by the tap 4, thrown well back ; then slowly and effectually fill his chest with air, or inspire as deeply as possible, and then he must lift the mouth piece of the spirometer 12 to his lips, still standing in the same erect position, and place the glass mouth-piece between the lips, holding it there sufficiently tight so as not to allow any breath to escape, he then slowly makes the deepest expiration, displacing all the air he can out of his lungs through the mouth-piece into the spirometer, where it is measured to cubic inches, and confined there by a stop-cock, until examined. This ob servation should be taken three times. The operator, while the experiment is going on should place his left hand upon the shoulder of the person being examined ; in this way be can determine as to the perfect inflation of the lungs and expulsion of air from them as well as the character of the thoracic expansion. The thumb of the operator should cross the clavicle, while the fingers rest on the upper edge of the scapula, then he feels the expand ing effect of inspiration, the swelling up of the apex of the thorax.