35 RESPIRATION. [ CH - XXIV - 



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

 the upper, part of the chest (superior costal type). 



There are also differences in different animals. In the frog, 

 for example, the air is forced into the lungs by the raising of the 

 floor of the mouth, the mouth and nostrils being closed. 



Expiration. From the enlargement produced in inspiration, 

 the chest and lungs return, in ordinary tranquil expiration, by 

 their elasticity ; the force employed by the inspiratory muscles 

 in distending the chest and overcoming the elastic resistance of 

 the lungs and chest-walls, is 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 involuntary 

 actions also, -as sneezing, coughing, fec., .something more than 

 merely passive elastic power is necessary, and the proper expira- 

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

 are the abdominal 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 abdominal muscles are 

 assisted in their action by the interosseous part of the internal 

 intercostals, the triangularis sterni, the serratm posticus inferior, 

 and quadratus lumborum. When by the efforts of the expiratory 

 muscles, 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, therefore, 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 

 pressxire, 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. 



Methods of recording: Respiratory Movements. 



The movements of respiration may be recorded graphically in several 

 ways. One method is to introduce a tube into the trachea of an animal, 



