264: HANDBOOK OF EHYSIOLOG1. 



The expansion of the chest in inspiration presents some peculiarities 

 in different persons. In young children, it is effected chiefly by the 

 diaphragm, which being highly arched in expiration, becomes flatter as 

 it contracts, and, descending, presses on the abdominal viscera, and 

 pushes forward the front walls of the abdomen. The movement of the 

 abdominal walls being here more manifest than that of any other part, 

 it is usual to call this the abdominal type of respiration. In men, to- 

 gether with the descent of the diaphragm, and the pushing forward of 

 the front wall of the abdomen, the chest and the sternum are subject to 

 a wide movement in inspiration (inferior costal type). In women, the 

 movement appears less extensive in the lower, and more so in the upper, 

 part of the chest (superior costal type). 



Expiration. From the enlargement produced in inspiration, the 

 chest and lungs return in ordinary tranquil expiration, by their elastic- 

 ity; the force employed by the inspiratory muscles in distending the 

 chest and overcoming the elastic resistance of the lungs and chest-walls, 

 being returned as an expiratory effort when the muscles are relaxed. 

 This elastic recoil of the chest and lungs is sufficient, in ordinary quiet 

 breathing, to expel air from the lungs in the intervals of inspiration, 

 and no muscular power is required. In all voluntary expiratory efforts, 

 however, as in speaking, singing, blowing, and the like, and in many in- 

 voluntary actions also, as sneezing, coughing, etc., something more than 

 merely passive elastic power is necessary, and the proper expiratory 

 muscles are brought into action. By far the chief of these are the ab- 

 dominal muscles, which, by pressing on the viscera of the abdomen, push 

 up the floor of the chest formed by the diaphragm, and by thus making 

 pressure on the lungs, expel air from them through the trachea and 

 larynx. All muscles, however, which depress the ribs, must act also as 

 muscles of expiration, and therefore we must conclude that the abdom- 

 inal muscles are assisted in their action by the greater part of the inter- 

 nal intercostals, the triangularis sterni,ihe serratus posticus inferior 

 and quadratus lumborum. When by the efforts of the expiratory mus- 

 cles, the chest has been squeezed to less than its average diameter, it 

 again, on relaxation of the muscles, returns to the normal dimensions 

 by virtue of its elasticity. The construction of the chest-walls, there- 

 fore, admirably adapts them for recoiling against and resisting as well 

 undue contraction as undue dilatation. 



In the natural condition of the parts the lungs can never contract 

 to the utmost, but are always more or less " on the stretch," being kept 

 closely in contact with the inner surface of the walls of the chest by 

 cohesion as well as by atmospheric pressure, and can contract away from 

 these only when, by some means or other, as by making an opening into 

 the pleural cavity, or by the effusion of fluid there, the pressure on the 

 exterior and interior of the lungs becomes equal. Thus, under ordinary 



