326 MANUAL OF PHYSIOLOGY. 



THORACIC MOVEMENTS. 



Physiologically, the motions are divided into two sets (1) 

 those which enlarge the thoracic cavity, and cause the air to rush 

 into the lungs, called inspiration; and (2) those which diminish 

 the size of the thorax and force out the air, called expiration. 



No action of life is more familiar than the rhythmical move- 

 ments of respiration. The slow quiet rise and fall of the chest 

 and abdomen are the signs most commonly sought as indicative 

 of life ; for every one knows that constant ventilation must go 

 on in order that the blood may readily obtain the necessary 

 amount of oxygen, and get rid of the carbonic acid gas, the or- 

 dinary diffusion that takes place in the motionless chest being 

 quite insufficient to remove the heavy carbonic acid gas from the 

 lungs. 



The rhythm of the respiratory movements may be represented 

 graphically in many ways, by recording either the changes in the 

 diameter or circumference of the thorax, or by the variations of 

 the pressure in the air-passages. These methods more or less 

 correspond, and give curves of somewhat the same character. 



The respiratory movements are up to a certain point under vol- 

 untary control, and may be varied by the will, or stopped as when 

 one holds one's breath. 



The voluntary control of the respiratory movements is, how- 

 ever, limited ; for, if we hold our breath for any length of time, 

 a moment soon arrives when the " necessity of respiration " over- 

 comes the strongest will. The usual respiratory movements are 

 carried on without our being conscious of them, and are, there- 

 fore, properly involuntary. 



The rate of the respiratory movements varies according to cir- 

 cumstances, being in an adult man about 18 per minute; in most 

 of the lower animals it is much more rapid. It varies with age, 

 being very rapid at birth, decreasing slowly to about 30, and 

 slightly rising towards old age. The following table (Quetelet) 

 illustrates this : 



