great cause of this expulsion is the restoration of the chest to its former dimensions; and the elasticity of the yellow tissue composing the bronchial ramifications, which have been put upon the stretch by the air rushing into them, during inspiration. The restoration of the chest to its dimensions may be effected simply by the cessation of the contraction of the muscles, that have raised it; and the elasticity of the cartilages, which connect the bony portion of the ribs with the sternum or breast-bone. In active expiration, however, the ribs are depressed, by the action of appropriate muscles, and the chest is thus still farther contracted. The chief expiratory muscles are the triangularis sterni, the broad museles of the abdomen, rectus abdominis, sacro-lumbalis, longissimus dorsi, serratus posticus inferior, &c. Haller conceived, that the ribs, in expiration, are successively depressed towards the last rib; which is first fixed by the abdominal muscles and quadratus lumborum. The intercostal muscles then act and draw the ribs successively downwards. Magendie contests the explanation of Haller; and the truth would seem to be, that the muscles, just mentioned, participate with the intercostals in every expiratory movement. By this action, the capacity of the chest is diminished; the lungs are correspondently pressed upon, and the air issues by the glottis. It has been already remarked, that, during expiration, the arytenoidei muscles contract, and the glottis appears to close. Still space sufficient is left to permit the exit of the air. It has been asked—is the air expired precisely that which has been taken in by the previous inspiration? It is impossible to empty the lungs wholly by the most forced expiration. A portion still remains; and hence it has been assumed, that the use of inspiration is to constantly renew the air remaining in the air-cells. On this subject we are not well informed; but it is probable, that the lighter and more rarefied air gives way to the newly-arrived and denser medium; and that, thus, fresh air is continually exposed to the blood of the pulmonary vessels. A multitude of experiments have been made to determine the change of bulk which air experiences by being respired. According to Sir Humphry Davy, it is diminished, by a single inspiration and expiration, from th toth part of its bulk. Cuvier makes it about th; Allen and Pepys a little more than a half per cent. Berthollet from 0.69 to 3.70 per cent.; and BOSTOCKth, as the average diminution. Assuming this last estimate to be correct, and forty cubic inches to be the quantity of air drawn into the lungs at each inspiration, it will follow, that half a cubic inch disappears each time we respire. This, in a day, would amount to 14,400 cubic inches, or to rather more than eight cubic feet. The experiments of MM. Dulong and Despretz make the diminution considerable. The latter gentleman placed six small rabbits in forty-nine quarts of air for two hours, at the expiration of which time the air had diminished one quart. A portion of the inspired air must consequently have been absorbed. Attempts have been made to estimate the quantity of air remaining in the lungs after respiration; but the sources of discrepancy are here as numerous as in the cases of inspiration or expiration. Goodwyn estimated it at 109 cubic inches; Menzies at 179; Jurine at 220; Fontana at 40; and Cuvier, after a forced inspiration, at from 100 to 60. Davy concluded, that his lungs, after a forced expiration, still retained 41 cubic inches of air. After a "natural expiration they contained 118 cubic inches. After a natural inspiration, After a forced inspiration, 135 254 By a full forced expiration after a forced inspiration, he threw It is impossible, from such variable data, to deduce any thing like a satisfactory conclusion; but if we assume with Bostock, (and Dr. Thomson is disposed to adopt the estimate,) 170 cubic inches as the quantity, that may be forcibly expelled, and that 120 cubic inches will be still left in the lungs, we shall have 290 cubic inches as the measure of the lungs in their natural or quiescent state; to this quantity, 40 cubic inches are added by each ordinary inspiration, giving 330 cubic inches as the measure of the lungs in their distended state. Hence it would seem, that about one-eighth of the whole contents of the lungs is changed by each respiration; and that rather more than two-thirds can be expelled by a forcible expiration. Supposing, that each act of respiration occupies three seconds, or that we respire twenty times in a minute, a quantity of air, rather more than 2 times the whole contents of the lungs, will be expelled in a minute, or about four thousand times their bulk in twenty four hours. The quantity of air, respired during this period, will be 1,152,000 cubic inches, about 666 cubic feet. Such is Bostock's estimate. It is the residuary air, that gives to the lungs the property of floating on the surface of water, after they have once received the breath of life, and no pressure that can be employed, will force out the air, so as to make them sink. Hence, the chief proofs, whether a child has been born alive or dead, are deduced from the lungs. These proofs constitut ethe docimasia pulmonum, or Lungenprobe of the Germans. Expiration, like inspiration, has been divided into three grades: ordinary, free, and forced; but it must necessarily admit of multitudinous shades of difference. In ordinary passive respiration, expiration is effected solely by the relaxation of the diaphragm. In free active expiration, the muscles, that raise the ribs, are likewise relaxed, and there is a slight action of the direct expiratory muscles. In forced expiration, all the respiratory muscles are thrown into action. In this manner, the air makes its way along the airpassages through the mouth or nostrils or both; carrying with it a fresh portion of the halitus from the mucous membrane. This it deposits, when the atmosphere is colder than the temperature acquired by the respired air, and if the atmosphere be sufficiently cold, as in winter, the vapour becomes condensed as it passes out, and renders expiration visible. The number of respirations, in a given time, differs considerably in different individuals. Dr. Hales reckons them at twenty. A man, on whom Menzies made experiments, breathed only fourteen times in a minute. Sir Humphry Davy made between twenty-six and twenty-seven in a minute. Dr. Thomson about nineteen, and Magendie fifteen. Our own average is sixteen. The average deduced from the few observers, that have recorded their statements, -or twenty per minute,—has generally been taken; but we are satisfied it is above the truth; eighteen would be nearer the general average; and it has, accordingly, been admitted by many. Eighteen in a minute give twenty-five thousand nine hundred and twenty in the twenty-four hours. The number of respirations is influenced by various circumstances. The child and the female breathe more rapidly than the adult male. We find as much variety, too, in him as we do in the horse; whilst some men are short-winded, others are long-winded; and this last condition may be improved by appropriate training; to which the pedestrian, and the prize-fighter, equally with the horse, are submitted for some time before they exhibit their powers. In sleep, the respiration is generally deeper, less frequent, and appears to be effected greatly by the intercostals and diaphragm. Motion has also a sensible effect in hurrying the respiration, as well as the distention of the stomach by food; certain mental emotions, &c. and its condition during disease becomes a subject of interesting study to the physician, and one that has been much facilitated by the acoustic method, introduced by Laennec. To his instrument—the stethoscope—allusion has already been made. By it or by the ear applied to the chest, we are able to hear distinctly the character of the respiration; and thus to judge of the nature of the existing pulmonary disease. But this is a topic that appertains to pathology. There are certain respiratory movements, concerned in effecting other functions, which require consideration. Some of these have already been topics of discussion. Adelon has classed them into:First. Those employed in the sense of smell, either for the purpose of conveying the odorous molecules into the nasal fossæ; or to repel them, and prevent their ingress. Secondly. The inspiratory action, employed in the digestive function, as in sucking. Thirdly. Those connected with muscular motion, when forcibly exerted; and particularly in straining or the employment of violent effort. Fourthly. Those concerned in the various excretions, either voluntary, as in defecation and spitting, or involuntary, as in coughing, sneezing, vomiting, accouchement, &c.; and lastly, such as constitute phenomena of expression, as, sighing, yawning, laughing, crying, sobbing, &c. Some of these, that have already engaged our attention, do not demand comment, others are topics of considerable interest and require investigation. Straining. The state of respiration is much affected during the more active voluntary movements. Muscular exertion, of whatever kind, when considerable, is preceded by a long and deep inspiration; the glottis is then closed; the diaphragm and respiratory muscles of the chest are contracted, as well as the abdominal muscles which press upon the contents of the abdomen in all directions. At the same time that the proper respiratory muscles are exerted, those of the face participate, owing to their association through the medium of the respiratory nerves. By this series of actions, the chest is rendered capacious; and the force, that can be developed, is augmented, in consequence of the trunk being rendered immovable as regards its individual parts; and thus serving as a fixed point for the muscles that arise from it, so that they are enabled to employ their full effect. The physiological state of muscular action, as connected with the mechanical function of respiration, is happily described by SHAKESPEARE, when he makes the 5th Harry encourage his soldiers at the siege of Harfleur:— "Stiffen the sinews, summon up the blood; "Now set the teeth, and stretch the nostrils wide: In the effort required for effecting the various excretions, a similar action of the respiratory muscles takes place. The organs, from which these excretions have to be removed, exist either in the thorax or abdomen; and, in all cases, the organs have to be compressed by the parietes of those cavities. See Fig. 107. A full inspiration is first made; the expiratory muscles, with those that close the glottis, are then forcibly and simultaneously contracted, and by this means the thoracic and abdominal viscera are compressed. Some difference, however, exists, according as the viscus, to be emptied, is seated in the abdomen or thorax. In the evacuation of the fæces, the lungs are first filled with air; and whilst the muscles of the larynx contract to close the glottis, those of the abdomen contract also; and as the lung, in consequence of the included air, resists the ascent of the diaphragm, the compression bears upon the large intestine. The same happens in the excretion of the urine and in accouchement. Fig. 107. When the organ, that has to be cleared, is in the thorax,—as in coughing to remove mucus from the air-passages, the same action of the muscles of the abdomen is invoked; but the glottis is open to allow of the exit of the mucus. In this case the expiratory muscles contract convulsively and forcibly, so that the air is driven violently from the lungs, and in its passage sweeps off the irritating matter and conveys it out of the body. To aid this, the muscular fibres, at the posterior part of the trachea and larger bronchial tubes, contract, so as to diminish the calibre of these canals; and, in this way, as Dr. Physick has suggested, the expectoration is facilitated. The action differs, however, according as the expired air is sent through the nose or mouth; in the former case, Constituting A. Right lung.-B. Left lung.-C. Right ventricle of the heart.-D. Right auricle of the heart.-E. Vena cava superior. sneezing; in the latter, -F, F. Subclavian veins.-G, G. Internal jugular veins.-H. Ascending aorta.-I. Pulmonary artery.-K. Diaphragm.coughing. The former is L, L. Right and left lobes of liver.-M. Ligamentum rotunmore violent than the lat-dum-N. Fundus of gall-bladder.-O. Stomach.-P. Spleen. -Q, Q. Situation of the kidneys, behind the intestines.-R, R. ter, and is involuntary; Small intestines. whilst the latter is not necessarily so. In both cases, the movement is excited by some external irritant, applied directly to the mucous membrane of the windpipe or nose; or by some modified action in the very tissue of the part, which acts as an irritating cause. In both cases the air is driven forcibly forward, and both are accompanied by sounds that cannot be mistaken. In these actions we have striking exemplifications of the extensive association of muscles, through the system of respiratory nerves, to which we have so often alluded. The pathologist, too, has repeated opportunities for observing the extensive sympathy between distant parts of the frame, as indicated by the actions of sneezing and coughing, especially of the former. If a person be exposed for a short period to the partial and irregular application of cold, so that VOL. II. 12 |