646 



PHYSIOLOGY OF RESPIRATION. 



the diaphragm and liver, or by hooks attached directly to muscu- 

 lar slips of the diaphragm. Registration of the movements in man 

 during quiet breathing give us such a 

 record as is seen in Fig. 267. It will 

 be seen that the inspiration (descend- 

 ing limb) is followed at once by an 

 expiration, as we should expect, 

 since, as soon as the inspiratory 

 muscles cease to act, the physical 

 factors mentioned above at once tend 

 to bring the chest back to its normal 

 position. The expiration (ascending 

 limb) is at first rapid and toward the 

 end very gradual, so that there is al- 

 most a condition of rest, — an expira*- 

 tory pause. 



The Volumes of Air Respired 

 and the Capacity of the Lungs. — 

 The volume of air respired varies, of 

 course, with the extent of the move- 

 ments and the size of the individual. 

 This volume may be determined 

 readily in any given case by means 

 of a spirometer, — a form of gasometer 

 adapted to this purpose. The con- 

 struction of this apparatus is repre- 

 sented in Fig. 268. It consists of a 

 graduated cylinder (A) and a receiver 

 (B) filled with water. The cylinder 

 ^4 is counterbalanced by a weight (g) 

 so as to move up and down in the 

 water of B with the least possible re- 

 sistance. The tube C passes throusb 

 the wall of B and ends in the interior of A above the level of the 

 water. The free end of this tube is connected with the mouth 

 or nose. When one breathes through this tube the expired air 

 passes into A, which rises from the water to receive it. If A is 

 graduated the amount of air breathed out may be measured 

 directly. The following terms are used: Vital capacity. By 

 vital capacity is meant the quantity of air that can be breathed 

 out by the deepest possible expiration after making the deepest 

 possible inspiration. It gives a rough measure of lung capacity, 

 and is used in gymnasiums and physical examinations for this j^ur- 

 pose. The actual amount varies with the individual; an average 

 figure for the adult man is 3700 c.c. Tidal air. By this term is 

 meant the amount of air breathed out in a normal quiet expiration. 



_ rig. 268.— Wint rich's modifi- 

 cation of Hutchinson's spirometer. 

 — (.Reichert.) 



