120 



RESPIRATION 



ration a passive one. It is a complex movement of recoil in tissues 

 variously stretched, twisted, displaced, or bent, and of the fall of more or 

 less heavy tissues raised, by the active muscular movements of inspira- 

 tion. All of these strains together force the tidal air, 500 c.c. at a breath, 

 upward and outward. The expiratory movement follows the com- 

 pletion of the inspiratory movement normally without appreciable pause. 

 Expiration requires normally one-fifth more time than does inspiration. 

 The pressure of the air in the trachea during expiration, according to 

 Bonders, is about 2 mm. of mercury, which may be increased in forced 

 expiration to 100 mm. As the air passes upward the larynx rises slightly, 

 raised partly by the expanding lungs and partly as the effect of contrac- 

 tion of the thyrohyoid muscles. 



FIG. 67 



The Anolis stethograph: the padded arms of the levers at A and B partly enclose the cha- 

 meleon, mouse, or other vertebrate of proper size (fixed by its legs to the frog-board), just 

 behind its forelegs; C is the thread by which the respiratory movements are conveyed to a light 

 aluminum lever for record on the smoked kymograph-drum. 



The Respiratory Rhythm. One of the most conspicuous elements 

 in both the bodily movements and the sensation-mass of man is the 

 rhythmic rise and fall of the chest walls in respiration. We have already 

 considered the probable cause of this complicated system of movements, 

 finding that it is regulated by a compound center in the medulla, 

 which is in turn actuated by certain lacks or excesses in the constituents 

 of the capillary blood flowing through it. There are certain elements 

 in the rhythm itself which are of scientific interest and practical impor- 

 tance. Among these are its time-relations and its rate under various 

 organic conditions. Many researches have been carried out on men, 

 as well as on animals, in the study of the respiratory rhythm and its 

 derangements. One method of registering these movements is by means 

 of the pneumograph devised by Marey and improved by Fitz. By the 



