RESPIRATION 383 



a bell jar. Pass a glass tube through a rubber stopper. Fasten a small 

 toy balloon to the lower end of the tube. Close the small end of the jar 

 with the stopper. Adjust the tube so that the balloon shall hang free in 

 the jar. If now the rubber sheet is pulled down by means of the string, the 

 air pressure in the jar is reduced and the toy balloon within expands, owing 

 to the air pressure down the tube. When the rubber is allowed to go 

 back to its former position, the balloon collapses. 



Coughing, Sighing, and Sneezing. — Coughing is a sudden strong ex- 

 piration, with the glottis (or top of the windpipe) closed. A sigh is a 

 quick inspiration followed by a quiet expiration. A sneeze is a sharp 

 expiration, the air passing through the nose because the passage to the 

 mouth is closed by the descent of the soft palate. 



Rate of Breathing and Amount of Air Breathed. — During quiet 

 breathing, the rate of inspiration is from fifteen to eighteen times per minute; 

 this rate largely depends on the amount of physical work performed. About 

 thirty cubic inches of air are taken in and expelled during the ordinary 

 quiet respiration. The air so breathed is called tidal air. In a "long 

 breath," we take in about 100 cubic inches in addition to the tidal air. 

 This is called complemental air. By means of a forced expiration, it is 

 possible to expel from 75 to 100 cubic inches more than tidal air; this air 

 is called reserve air. What remains in the lungs, amounting to about 

 100 cubic inches, is called the residual air. (See diagram, page 384.) The 

 value of deep breathing is seen by a glance at the diagram. It is only by 

 this means that we clear the lungs of the reserve air with its accompanying 

 load of carbon dioxide. 



The actual amount of oxygen used in the body during the course of a 

 day is nearly 25 ounces; this being almost entirely used in oxidizing the 

 food materials taken into the body during the 24 hours. 



Respiration under Nervous Control. — The muscular movements 

 which cause an inspiration are partly under the control of the will, but in 

 part the movement is beyond our control. The nerve centers which govern 

 inspiration are part of the sympathetic nervous system of which we shall 

 learn later. That the sympathetic nervous system controls respiratory 

 movements is seen, for example, in the involuntary short breath taken 

 by the bather who plunges into cold water. Anything of an irritating nature 

 in the trachea or larynx will cause a sudden expiration or cough. When a 

 boy runs, the quickened respiration is due to the fact that oxygen is used 

 up rapidly and a larger quantity of carbon dioxide is formed. These facts, 

 together with the presence of certain other poisonous materials in the lung 

 cells, stimulate the nervous center which has control of respiration to greater 

 activity, and quickened inspiration follows. 



Experiments to determine Changes undergone by Air in the 

 Lungs. — 1. Breathe on the bulb of a thermometer and record any changes 

 2. Breathe gently on any polished glass or metal surface. Note what 

 happens. 



