374 RESPIRATION. 



ends of the cords which are attached to them, increasing the area 

 nearly twofold. When these muscles cease their contraction, as 

 they do at the end of the inspiratory act, then the elasticity of the 

 cartilages brings the muscles back to the position they occupied at 

 the beginning of inspiration. These movements of the glottis 

 occur synchronously with the respiratory movements of the thorax. 

 The muscles of the larynx have already been described (p. 355). 



CAPACITY OF THE LUNGS. 



At the beginning of an ordinary inspiration the lungs contain 

 air, which so distends them that the visceral layer of the pleura 

 is in contact with the parietal layer. As the thorax enlarges the 

 air in the lungs distends them still more, so that they are still 

 kept in contact with the thoracic walls. This contact between the 

 visceral and parietal layers of the pleura is constant, irrespective 

 of the amount of distention of the lungs. The expansion of the 

 air in the lungs makes it of less density than the external air 

 with which it is in communication through the air-passages, and 

 immediately there is a flow of external air into these passages 

 to establish an equilibrium : this inflow constitutes inspiration. 

 Immediately following air is expelled from the lungs, and this 

 outflow constitutes expiration. To this volume of air which flows 

 in and out during ordinary respiration the name of tidal air is 

 given, from the resemblance which the process bears to the flow 

 and ebb of the tide. The amount of this tidal air is variously 

 stated by different authorities; some place it as low as 49 c.c., 

 and others as high as 1640 c.c. It varies greatly in different 

 individuals, and in the same individual according to the manner 

 and frequency of his breathing. Hutchinson has made 80 deter- 

 minations on different individuals, and obtained from 114 c.c. to 

 196 c.c. in a condition of rest, and from 262 c.c. to 360 c.c. 

 during exercise. One observation was as high as 1262 c.c. In 

 newborn children it is 35 c.c. 



Each individual has the power, however, of taking into the 

 lungs an additional amount of air over and above the tidal air, by 

 a deep or forced inspiration. To this additional amount the term 

 complemental air is applied, and it may be regarded as averaging 

 about 1500 c.c. 



As more air is taken in by forced inspiration than is usually 

 inhaled during an ordinary inspiration, so by a forced expiration 

 more air is expelled than is ordinarily exhaled during an ordinary 

 expiration. To the air thus expelled during a forced expiration 

 the name of reserve or supplemental air is given. Hutchinson states 

 the amount to vary from 1148 c.c. to 1804 c.c., while by some it 

 has been placed at 2624 c.c. 



But even after all the air has been expelled that can be by 



