384 



RESPIRATION AND EXCRETION 



dlaphragi 



Diagram showing portion of diaphragm and ribs 

 in (a) inspiration ; (b) expiration. 



the chest cavity larger. The lungs, which lie within this cavity, are filled 

 by the air rushing into the larger space thus made. An expiration is 



simpler than an inspiration, 

 for it requires no muscular 

 effort ; the muscles relax, 

 the breastbone and ribs sink 

 into place, while the dia- 

 phragm returns to its orig- 

 inal position. 



A piece of apparatus 

 which illustrates to a de- 

 gree the mechanics of 

 breathing may be made as 

 follows : Attach a string to 

 the middle of a piece of 

 sheet rubber. Tie the rub- 

 ber over the large end of a 

 bell jar. Pass a glass Y 

 tube through a rubber stop- 

 per. Fasten two small toy 

 baUoons to the branches of the tube. Close the small end of the jar with 

 the stopper. Adjust the tube so that the balloons 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 balloons within expand, 

 owing to the air pressure down the tube. When the 

 rubber is allowed to go back to its former position, 

 the balloons collapse. 



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 per- 

 formed. About 30 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 ad- 

 dition 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. 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. 

 Respiration under Nervous Control. — The muscular movements 

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



febrn-^-Q lS 



Apparatus showing 

 mechanicsof breath- 

 ing. 



