EXTERNAL RESPIRATION AND RESPIRATORY MOVEMENTS. 647 



A similar amount is breathed in, of course, in the previous inspira- 

 tion, antl the term tidal air designates the amount of air that flows 

 in and out of the lungs with each quiet respiratory movement. 

 Here, again, there are individual variations. The average figure 

 for the adult man is 500 c.c. The complemental air. This term 

 designates the amount of air that can be breathed in over and above 

 the tidal air by the deepest possible inspiration. It is estimated 

 at 1600 c.c. The supplemental air. By this term is meant the 

 amount of air that can be breathed out, after a quiet expiration, by 

 the most forcible expiration. It is equal also to 1600 c.c. It is 

 evident that the complemental air plus the supplemental air plus 

 the tidal air constitute the vital capacity. The residual air. After 

 the most forcible expiration the lungs are far from being entirely 

 collapsed. The volume of air that remains behind, after the sup- 

 plemental air has been driven out, is known as the residual air. 

 The amount of this air has been estimated directly on the cadaver 

 (Hermann). The thorax was first pressed into a position of forced 

 expiration; the trachea was then ligated, the chest opened, the lungs 

 removed and their volume estimated by the amount of water dis- 

 placed when they were immersed. The average result from such 

 estimations was, in round numbers, 1000 c.c. Under conditions of 

 normal breathing the reserve supply of air in the lungs is equal to 

 the residual air plus the supplemental air, — that is, 2600 c.c. Mini- 

 mal air. When the thorax is opened the lungs collapse, driving out 

 the supplemental and residual air, but not quite completely. Before 

 the air cells are entirely emptied the small bronchi leading to them 

 collapse and their walls adhere with sufficient force to entrap a little 

 air in the alveoU. It is on this account that the excised lungs float 

 in water and are designated as lights by the butcher. The small 

 amount of air caught in this way is designated as the minimal air. 



In the fetus before birth the lungs are entirely solid, but after 

 birth, if respirations are made, the lungs do not collapse completely 

 on account of the capture of the minimal air. Whether or not the 

 lungs will float has constituted, therefore, one of the facts used in 

 medicolegal cases to determine if a child was stillborn. The lungs 

 during life may, under certain conditions, again become in parts 

 entirely solid. If any of the alveoli become completely shut off 

 from the trachea, by an accident or by pathological conditions, the 

 air caught in them may be completely absorbed, after a certain 

 interval, by the circulating blood. 



The Size of the Bronchial Tree and the Ventilation of the 

 Lungs. — Since the reserve supply of air in the lungs may amount to 

 2600 c.c, while the new air breathed in at each inspiration amounts 

 to only 500 c.c, it would seem at first that the alveolar air is not 

 very efficiently renewed by a quiet inspiration. The actual amount 

 of ventilation effected depends on the capacity of the broncliial 



