MOVEMENTS OF RESPIRATION. 277 



insufflation through a tube inserted into the trachea, and then relieving 

 them from pressure. They at once react with sufficient power to expel 

 the larger portion of the air which had been forced into their cavities. 

 The same elasticity being constantly present during life, the air is sub- 

 jected to its pressure, and is consequently expelled as soon as the mus- 

 cles of inspiration cease to act. Other organs, however, aid in the 

 same process. The costal cartilages, which are also elastic, having 

 been twisted slightly out of position by the elevation of the ribs, resume 

 their original form, and, drawing the ribs down again, thus serve to com- 

 press the sides of the chest. Lastly, the abdominal organs, which have 

 been displaced by the descent of the diaphragm, are forced backward 

 by the elasticity of the abdominal walls and of their own fibrous attach- 

 ments, carrying the relaxed diaphragm before them. By the constant 

 recurrence of these alternating movements of inspiration and expiration, 

 fresh portions of air are incessantly introduced into and expelled from 

 the chest. 



All the air, however, contained in the lungs, is not changed at each 

 movement of respiration. On the contrary, a considerable quantity 

 remains in the pulmonary cavity after the most complete expiration ; 

 and even when the lungs have been removed from the chest, they still 

 contain a certain amount of air, which cannot be entirely displaced by 

 any violence short of disintegrating the pulmonary tissue. It is evi- 

 dent, therefore, that only a comparatively small portion of the air in 

 the lungs passes in and out with each respiratory movement ; and it 

 will require several successive respirations before it can be entirely 

 changed. The proportion existing between the air which is changed at 

 each respiration and the entire quantity contained in the chest varies 

 considerably with the different conditions of the respiratory function ; 

 but the average results obtained by different observers show that, in 

 general, the volume of the inspired and expired air is from 10 to 13 per 

 cent, of that contained in the whole of the pulmonary cavity. Thus it 

 will require from eight to ten respirations to renovate completely the 

 air in the lungs. 



Respiratory Movements of the Glottis. Beside the movements of 

 expansion and collapse already described, belonging to the chest, there 

 are similar movements of respiration which take place in the larynx. 

 If the respiratory passages be examined in the state of collapse in which 

 they are usually found after death, it will be observed that the opening 

 of the glottis is smaller in calibre than the cavity of the trachea below. 

 The glottis presents the appearance of a narrow chink, while the passage 

 for the inspired air widens in the lower part of the larynx, and in the 

 trachea constitutes a spacious tube, nearly cylindrical in shape, and over 

 12 millimetres in diameter. We have found that in the human subject 

 the space included between the vocal chords has an area, on the aver- 

 age, of only one square centimetre ; while the calibre of the trachea in 

 the middle of its length is 2.81 square centimetres. This disproportion, 

 which is so evident after death, does not exist during life. While 



