286 



HUMAN PHYSIOLOGY 



followed by a forced expiration during which, by contraction 

 oflhe pillars of the fauces and descent of the soft palate, the 

 air is forced through the nose. 



Hiccough consists in a sudden reflex contraction of the 

 diaphragm causing a sudden inspiration which is interrupted 

 by a spasmodic contraction of the glottis. Abdominal irrita- 

 tion is its chief cause. 



Sighing and Yawning are deep involuntary inspirations 

 which serve to accelerate the circulation of the blood when 

 from any cause this becomes less active (see p. 294). They are 

 probably due to cerebral anaemia, which they help to correct 

 by increasing the general arterial pressure. 



II. Amount of air respired. The amount of air respired 

 varies according to whether the respirations are ordinary or 

 forced. 



In ordinary respiration about 300 ccms. of air enter and 

 leave the chest. This is called the tidal air. Its amount 

 varies with the size and muscular develop- 

 ment of the chest. 



By a forced inspiration a much larger 

 Co'^c.f- ( l uant ^ t y f ft i r mav be made to pass into 



/5OO CC 



the lungs a quantity varying with the 

 size and strength of the individual but 

 on an average about 1500 ccms. 

 This is called the complemental air. 

 By forced expiration an amount of air 

 FIG. 134. The amount much larger than the tidal can be ex- 

 of air respired in pelled, an amount usually about the 

 same as the complemental air, and called 



the reserve air. 



The total amount of air which an in- 



dividual can draw into and drive out of his lungs is a fair 

 measure of the size and muscular development of the thorax, 

 and it has been called the vital capacity of the thorax. 

 This vital capacity may be measured by means of a spiro- 

 meter. Its amount, as thus indicated, depends a good deal 

 upon practice ; the instrument, therefore, cannot be con- 

 sidered as of much practical value, and a more reliable 

 conclusion as to the vital capacity may be arrived at by 



tion and expiration. 



