A diminution of the capacity of the thorax or an act of expiration, by which part of the air is expelled from the lungs, follows imme diately each inspiratory movement. In ordi nary respiration, after the muscles of inspi ration have ceased to contract, the elasticity of the thoracic walls, especially of the carti laginous portion, causes it to return to the state in which it was before its dilatation ; and when the contracted diaphragm has re laxed, the elasticity of the parts displaced by its descent, is sufficient, without much, if any, aid from the abdominal muscles, to push the diaphragm again upwards. The gas present in greater or less quantity in the digestive tube, being compressed during the descent of the diaphragm, will, from its elasticity, assist in pushing upwards the relaxed diaphragm.* In more forcible expirations, when the walls of' the chest are compressed beyond the state they assume when the muscles of inspiration are relaxed, the compressing muscles expe rience considerable resistance from the elas ticity of the walls of the chest.
When the expirations are performed more forcibly than ordinarily, the diaphragm is pushed up, and the sternum and ribs de pressed by the contractions of the three broad muscles of the abdomen, by the recti abdo minis, and by the triangularis sterni muscles. The levator ani, one of the antagonist muscles of the diaphragm, assists also in pushing the abdominal viscera upwards. In hurried or laboured expirations the diaphragm is pushed more forcibly upwards by the muscles mentioned, and the ribs are pulled downwards, and the chest compressed, by the quadrati lumborum, serrati postici inferiorest, sacro-lumbales and longissimi dorsi muscles.
MM. Beau and Maissiat have described three kinds of ordinary respiratory move ments : I. the abdominal, in which the abdo minal walls chiefly act : 2. the costo-inferior, in which the movements chiefly take place in the lower ribs, from the seventh inclusive, downwards : 3. the costo-superior, in which the superior part of the chest is carried up wards by the elevation of the superior ribs and the sternum. The first kind, or the ab dominal type, is observed in infants up to the end of the third year in both sexes ; but after this period the costo-superior type in girls, and the costo-inferior and abdominal types in boys, generally prevail, and this difference becomes more marked as they advance in years. Almost all men, therefore, breathe by the lower part, and women by the upper part of their chest, and this independently of the effects of particular articles of dress !* This difference in the mode of respiration in the two sexes is, in general, maintained even in dyspncea, unless it be very severe. As the
costo-inferior and abdominal types of respi ration would be impeded in the female when pregnant, the ordinary costo-superior type of respiration in the female has apparently a reference to that condition.t Valentin T, Dr. Hutchinson§, and Men delssohn II, have lately made experiments upon the force of the muscular movements of in spiration and expiration. Those of' Dr. Hutch inson are much the most extensive, and are 1500 in number. He found that the power of expiration is nearly one third stronger than that of inspiration ; and he states that whenever the expiratory are not stronger than the inspiratory muscles, that some dis ease is present. He tested the force of the two classes of respiratory muscles by causing persons to make the most powerful efforts of which they were capable when breathing through the nose into an instrument con structed for the purpose, and the subjects of experiment were taken from individuals of the male sex, following very different occu pations. In examining the results of the whole experiments, and including all the thirteen classes of men subjected to expe riment, the power of the inspiratory muscles is found greatest in men of 5 feet 9 inches in height, their inspiratory power being equal, on an average, to a column of 2-75, and their expiratory power to 3-97 inches of mercury ; while in four of these classes, composed ge nerally of active, efficient, and healthy indi viduals, viz. Firemen, Metropolitan Police, Thames Police, and Royal Horse Guards, the inspiratory power of the men of 5 feet 7 inches was the greatest, being equal to 3.07 inches of mercury, and those of 5 feet 8 inches to 2-96 (nearly 3 inches). The aver age power of the 5 feet 7 inches and 5 feet 8 inches men of all the thirteen classes was only 2-65 inches of mercury. The inspiratory power of twelve six-feet men in the first bat talion of Grenadier Guards was only 1.92 inches, while that of thirty-one of the same height in the 13lues (Life Guards) was 2.71 inches. He infers from these experiments that a healthy man of 5 feet 7 inches or 5 feet 8 inches, should elevate by inspiration 3 inches of mercury. The force of the expi ratory muscles is more liable to be affected by the ordinary occupation of the individual than that of the inspiratory muscles, and therefore the state of the former is less to be relied upon in judging of the health of the individual than that of the latter.% The elas ticity of the walls of the chest is, no doubt, one cause of the greater force of the expi ratory over that of the inspiratory muscles.