At the commencement of "ordinary breath ing" the collapsing power of the lungs in our experiment was 7.2 in., or nearly of a lb. per superficial square inch. This is a very notable power, not less in the gross, oftentimes, than 100 lbs. of dead uncounter balanced resistance to the respiratory mus cles. In the female, with nearly an arith metical increase of 100 cubic inches per vo lume, the collapsing power increases 5 to 10 and 20. The insulated volume in the male being less regular, the collapsing power ma nifested is also less regular. But, taking the mean resistance of the reserve and breathing volumes in the male combined (160 cub, in.), the power of collapse was 7.7 inches. In the female, the mean of nearly the same quantity of air in the lungs (within 10 cubic inches) allowing for little or no residual volume at the starting point when we inflated, is likewise 7.7 inches. We believe that the elastic power of the lungs in the two sexes is the same ;— indeed why should it not be so? because the office of the elastic tissue is to drive out of the lungs volumes of air no longer required ; and it is probable that the resistance given by the air, against the sides of the air tubes, in both sexes, is the same; and unless the number and calibre of the air tubes are different, the resistance, by friction, to the elastic collapse of the lungs, from the trans mission of similar volumes of air in the two sexes, must be the same.
In three experiments we found the elastic collapse of the lungs cease at different degrees; i. e. different volumes of residual air were displaced when the elastic force had come to its minimum.
There is no order in the numbers 30, 60, and 45, relative to the other measurements. We do not know what quantity of air remained after these volumes were displaced. When we ex hausted the remainder of the residual volume, which is not affected by the elastic collapse of the lungs, the sides of the air tubes themselves collapsed by the atmospheric pressure. They likewise are elastic, but in a contrary direc tion; an expanding elasticity keeping them open. We found that the expanding elasticity acted so as to draw or suck inwards the air with the different powers represented by 7,1.5 and inches of water in the haemadynamo meter, when we attempted to withdraw out of the lungs more air than the lungs themselves naturally displace by their collapsing elasticity. Therefore these figures may represent the ex panding elasticity of the air tubes. In these cases there is no apparent order ; but we learn the fact that such elasticity exists. In the case C there was a tubercular condition of lung, in the milliary form, in one apex ; in there was extensive pleuritic adhesions ; but in the case of M the lungs were remarkably healthy. It is interesting to notice that there are here two elasticities in contrary direc tions,—an elastic collapse which has its limit at a certain point, and an elastic expansion of the air tubes, which likewise have their limit of expansion at the same point, protecting the calibre of the air tubes from any further collapse.
In the case of M the expansion of the air tubes was equal to inches of water • by calculation it appears that the collapse of the elastic tissue upon the vital capacity volume would be about 14 inches. These antago nistic forces are quite independent of the will, or any nervous stimuli : one is for maintain ing an expiratory current of air, and the other for preserving an open channel in the lungs for inspiration.
The lungs are very delicate organs, and can resist but little artificial force ; for, if once inflated to the ordinary state of either the breathing volume or vital capacity volume, they do not appear able to collapse again to their original size ; — probably intra-lobular emphysema is produced.
In our experiments we forced air into the lungs; they were expanded because we inflated them. We now think it would be better to inflate them by expanding them (removing the external atmospheric pressure), and allow the air to drop into the air vesicles by its own gravitation (as in living respiration), when they would in all probability collapse freely to their original position. When they are in flated by expansion (the natural way of life) the delicate cells of the lungs can safely resist a force of from 3 to 9 inches of mercury, or from 40 to 121 inches of water (see next column) ; but when expanded by inflation, their collapsing power was damaged so that it could sustain only 17 or 20 inches of water. This is worthy of notice in resuscitating the apphrently asphyxiated person, at which times we have long been of opinion that bellows and pumps are highly dangerous instruments to use for maintaining artificial respiration.
Of the muscular contractility of the lungs. —In the trachea transverse muscular fibres extend across posteriorly, connecting the tips of the incomplete rings of cartilage. In the smaller bronchi they encircle the whole tubes, and there appears to be little doubt but that these circular fibres are to be found in every part of the air tubes as far as the terminal air vesicles.
Physiologists have disputed whether these fibres are muscular or not ; if muscular, they are important agents in respiration, acting as a series of little expiratory muscles.
Dr. C. J. B. Williams lately read a paper at the meeting of Glasgow (September 1840), upon the subject.* He experimented upon the lungs of some of the lower animals, several dogs, a rabbit, a bullock, a horse, &c., — as soon as possible after death, submitting their lungs to galvanic stimuli, and securing a hmmadynamometer with a stop-cork to the trachea. He found that, upon applying this stimulus, the fluid in the bent tube rose from 1 to 2 inches, and it immediately fell on break. ing contact. This effect could only be pro duced by muscular contraction.
Kiilliker, a very accurate observer, confirms these views.' He found in man, in the larger and finer bronchial tubes, a coat formed of annular fibres, in one or more layers according to the size of the tube, consisting of unstriped muscular fibres ; and over this coat a thin layer of fibro-cellular tissue with nucleus-fibres. He found no mus cular fibres directed longitudinally. He ob serves, " In former observations I thought I had convinced myself of the existence of unstriped muscular fibres in the air-cells; but in my resumed examination of the lungs of man and mammalia, I can with certainty see nothing distinctly characteristic as such mus cular tissue." Our knowledge of these cir cular muscular fibres was first chiefly derived from the researches of Reisseissen. Laennec considered spasmodic asthma to be assignable to a spasm of these circular fibres. We are as yet ignorant of the possible extent of contrac tion of these fibres. The discharge of air is paramount, and that from the most remote vesi cle ; and we know that by no mechanial means can we obtain this, and therefore a molecular power is necessary, which we readily imagine can be obtained by these tissues acting so as to give a peristaltic motion, and thus dis placing from every individual vesicle the de licate stream of air necessary to be dis charged. We do not need these tissues for inspiration, because the atmosphere, by its mere weight, can penetrate into the most remote air-cell, overcoming all the friction against the sides of the air-tubes. The in spiratory volumes of air are but for one pur pose, to arate the blood ; but the expiratory volumes are for the voice and other purposes sometimes requiring great force to aid in cer tain expulsive efforts.