Apparatus for renewing the air in the lungs in the human species. —In man, as in the other Mammalia, this consists of three distinct parts : — 1st, of a movable framework composed of articulated bones and cartilages, but chiefly of the former, termed the thorax ; 2dly, of muscles for enlarging and diminishing the capacity of the thorax ; 3dly, of nerves through which the movements of these muscles are excited and regulated. The uses of this apparatus are not, however, restricted to respiration. The bones of the thorax furnish a certain degree of protection to the lungs, heart, and other important parts enclosed by them ; and during certain violent efforts of the voluntary muscles, as in lifting a weight, they are no longer mobile as in the respiratory move ments, but are rendered fixed, and afford a firm and steady point d'appui to the powerful muscles passing between the external surface of the thorax and the thoracic extremities, during their contraction. The same muscles which act involuntarily in dilating and contracting the chest in respiration, are frequently engaged in the performance of voluntary muscular movements, as in articulate speech, straining, Szc. They also, in connexion with other muscles, or even alone, perform various in voluntary muscular movements which are not respiratory, as in the excito-motory movements of coughing, sneezing, defecation, and urin ation, and in the sensational and emotional involuntary muscular movements of laughter, sighing, yawning, vomiting, &c.
The thorax can be enlarged in all its dia meters by the action of its muscles, — in the vertical or atlanto-sacral, in the antero-pos terior or vertebro-sternal, and in the trans verse. Its enlargement in the antero-posterior and transverse directions is effected by the elevation of the ribs, and its enlargement in the vertical direction by the descent of the diaphragm, and by the elevation of the upper part of the thorax, but chiefly by the former. As the ribs in the human species differ in length, in the degree of their inclination to the spine,!in the form and extent of their curva ture, in the manner in which the anterior extremities of their cartilages of prolongation terminate, and in some other anatomical points which must influence their mode of action, the phenomena attending the elevation of the ribs are not the same over all parts of the chest, but it will be sufficient for our present purpose to state tbe general effects of these movements.* As the osseous arches formed by the ribs are so inclined upon the vertebral column that their lower edges form acute angles with that column, and their anterior or sternal are placed lower than their vertebral ends, and as their vertebral or posterior ends have a very limited extent of motion t, their elevation brings them to or near the horizontal plane, and carries forward their sternal extremities ; and as the greater num ber of the ribs are attached to the sternum through their cartilages of prolongation, this bone must by this movement be pushed for wards, and the antero-posterior diameter of the thorax be enlarged.
The transverse diameter of the thorax is increased by the circumstance that the ribs during their elevation do not simply ascend, but perform a slight rotation round an axis passing between their anterior and posterior extremities, by which two effects are pro duced ; 1st, their lower, which form a segment of a somewhat larger circle than their upper edges, are turned somewhat outwards, and the upper slightly inwards, so that the con cavities of the arches formed by the ribs are now perpendicular, or nearly so, to the median plane of the body, instead of being oblique as before their elevation ; 2dly, the middle portion of the greater number of ribs, which before was placed below a straight line passing through their two extremities, in consequence of the shaft of the rib bending upwards near the sternal end at what has been termed the anterior angle, is now placed on the same plane with the two extremities, and the whole rib rendered horizontal. This rotatory motion is greater at the middle of certain of the ribs as they rotate upon their two extremities, so that each rib in the performance of this move ment may be considered as forming two levers, the two extremities being the pivots, and the middle of the ribs the ends of the levers most remote from the pivots.t The forward move ment of the sternum is greater at its lower than at its upper part, in consequence of the greater length and inclination of the lower vertebro-sternal or true ribs, and the greater length of their cartilages, and the more acute angles formed by their articulation with the sternum ; and this difference in the extent of movement in the two portions of the sternum must be still greater before the manubrium and the body of the bone are united by ossific matter.* Though this description of the movements of the sternum in respiration, which is that given by Haller +, has been called in question by some modern anatomists, there can be no doubt of its correctness, for it can be proved, by an appeal to the mechanism of the thorax, that, in the upward and for ward movement of this bone, its upper and lower ends will pass through paths which differ considerably in their curvature and direction4 Though Haller was wrong in maintaining that the first rib is almost im movable in these actions of the thoracic walls, yet there can be as little doubt that Magendie is in error in asserting that this is the most movable of all the ribs ; for however favour able the nature of its vertebral articulation may be for motion, this is counteracted by the mode in which its cartilage of prolongation is united to the sternum. §
The position and form of the diaphragm is well adapted for enlarging, by its contraction, the vertical diameter of the thorax ; and being placed in the most capacious part of the thorax, even a slight elongation of the vertical diameter there will add considerably to the area of its inner surface. The convex or upper surface of the diaphragm, in its relaxed state, projects upwards on each side of its central or cordiform tendon into the thorax, and is higher anteriorly than posteriorly, and on the right side than on the left. This cordiform tendon is made a fixed point for the arched fibres that run from it to the ribs during their contraction, since it is pulled upon from below and behind by the two erura of the diaphragm, and in front by the short muscular fibres which pass to it from the point of the sternum and the lower edges of the cartilages of the ribs. If the lower ribs have been previously rendered steady by the action of the quadrati lumborum and serrati postici inferiores muscles, the arched muscular fibres of the diaphragm have another fixed point during their contraction. As the heart rests on the upper surface of the cordi form tendon, and the base of the lungs on the upper portion of the arched part of the diaphragm, the descent of the arched muscular fibres and their change to the horizontal posi tion, causes a considerable enlargement of that part of the chest occupied by the lungs, while the position of the heart is compara tively little affected when the respiratory movements are moderate; but during forcible inspiration the heart recedes deeper into the chest, and during expiration it again collies forward.* The vertical diameter of the chest may be increased in inspiration by the pulling up of its superior portion, by the strong muscles of the neck attached to it, at a time when the lower portion is prevented from ascending, but an increase in the vertical dia meter by an elongation of its upper part must have a much less effect in enlarging its capacity than an elongation of its lower part seeing that the thorax is at least four times as large at its lower as at its upper end. In ordinary respiration, and when the body is at rest, the ribs move little in the male, and the muscular movements of inspiration are chiefly carried on by the diaphragm.± The ribs are elevated, in ordinary respira tion, by the levatores costarum, external and internal intercostal muscles I, and also, niore especially in the female, by the scaleni muscles. When the respiration becomes hurried or more laboured, the diaphragm and the muscles that elevate the ribs not only act more vigorously in inspiration, but numerous other muscles, which may be termed auxiliary muscles of inspiration, act in unison with these.* In cases of great dyspncea, as in a fit of asthma, the shoulders are fixed, the head is thrown back, and all the auxiliary muscles of inspi ration are brought into violent action. When the shoulders are fixed by the action of the levatores anguli scapulm, the rhombodei ma jores et minores, and the humeri also fixed by the scapulo-humeral muscles, or by the person grasping some fixed object by the hands, then the muscles, or portions of them which pass between the thoracic extremities and the an terior and lateral walls of the chest, as the serrati magni, the pectorales minores et ma jores, the subclavi, and perhaps the costal portion of the latissirni dorsi, act as muscles of inspiration, by pulling the ribs upwards and outwards t ; and when the head, cervical vertebrm, hyoid bone, and larynx are fixed by the numerous muscles capable of performing this action, then the sterno-cleido-mastoidei, the sterno-hyoid, and sterno-thyroid muscles, may aid the scaleni muscles in drawing the superior part of the thorax upwards.$ The serrati postici superiores, and the cervicales descendentes, are also accessory muscles of inspiration, if the former be not, at times, in fact a muscle of ordinary inspiration. The su perior aperture of the larynx is dilated during inspiration by the crico-arytenoidei postici muscles when the breathing is in the least hurried ; and in laboured breathing the nos trils are expanded by the contraction of the muscles, which draw the alm of the nostrils outward. The greater or less demand for fresh air in the lungs regulates the number of these accessory respiratory niuscles brought into play, and the energy of their contraction.