356 PHYSIOLOGY. 



Professor Gad, of Prague, has constructed an instrument to 

 measure the volume of the air expired and inspired. It is called 

 an aeroplethysmograph. It consists of two boxes, one inside the 

 other; the space between is filled with water. The inside mica box 

 receives the air expired. At its posterior surface there is an axis 

 which allows the anterior surface to elevate and depress itself. The 

 movements of the mica box are recorded by a pen attached to it. 

 The box itself is counterpoised by a weight. The instrument must 

 be graduated, in order that one may determine the volume of 

 inspired and expired air. 



NUMBER OF RESPIRATIONS. 



In an adult, the number of respirations per minute may vary 

 from 16 to 24. It is usually stated that 4 pulse-beats occur during 

 each respiration. The number is varied by the position of the body; 



Fig. 124. Gad's Aeroplethysmograph. (KRUSICH.) 



thus, there may be counted 13 while recumbent, 19 in the sitting 

 posture, and 22 respirations per minute while standing. 



During infancy and childhood the number of respirations is 

 always greater than in the adult. Exercise temporarily increases 

 respiration both as to number and to depth. 



Every athlete knows of that condition popularly termed "second 

 wind/' At the beginning of severe exercise there is a marked 

 dyspnoea which passes away after a short time, even though the exer- 

 cise be uninterrupted. It is believed to be in a very great measure 

 cardiac, due to a want of oxygen. 



Pathological. Eespirations may be increased by reason of fever, 

 pleurisy, pneumonia, some heart diseases, and anaemia. Diminution ii 

 occasioned by pressure upon the respiratory center in the medulla; 

 this occurs in coma. 



