PHYSIOLOGY KOR BEGINNERS 



in inspiration cannot be maintained without muscular effort, so 

 that when this ceases the chest returns to its original size, and 

 the air is driven out of the lungs again. Moreover, the lungs 

 themselves are very elastic and when distended will, as we have 

 seen, return to their original size when the distending force 

 ceases to act. Thus inspiration is brought about by a muscular 

 effort, while expiration is in the main the result of the cessation 

 of that effort, and the passive return to the natural condition 

 is due to the elasticity of the chest walls and of the lungs 

 themselves. 



Quiet Respiration. Quiet respiration is effected by 

 movements of both the diaphragm and of the ribs, but the 

 relative part they play varies much in different animals and in 

 ourselves in the two sexes. In men the diaphragm takes a 

 greater share, that is, diaphragmatic respiration is more 

 evident, while in women the movement of the ribs, that is, 

 costal respiration, is more conspicuous. When a person is 

 quiet and at rest the rate of respiration is about seventeen 

 every minute, but is very variable. An inspiration takes place 

 which is followed at once by an expiration, then there is nearly 

 always a pause, and then another inspiration and expiration 

 occur, and so on. 



Laboured Respiration. When deep inspiration or 

 laboured inspiration, as it is called, occurs, other muscles 

 which can raise the ribs and the sternum contract. These are 

 chiefly the large muscles passing from the upper part of the 

 spine to the upper ribs behind, and those passing from the 

 neck to the sternum in front. Moreover, unlike what happens 

 in quiet expiration, when laboured expiration occurs certain 

 muscles contract to help to drive out the air. The most 

 important of these are the muscular sheets which form the 

 anterior wall of the abdomen, the abdominal muscles. 

 When these contract they cause pressure on the abdominal 

 organs, and these press up the arch of the diaphragm 

 still farther into the thorax. The second deeper layer of 

 intercostal muscles, the internal intercostal muscles, 

 which pass from each rib obliquely downwards and back- 

 wards to the rib below, depress the lower ribs when they 

 contract. These, with other muscles which pass from the 

 lower part of the spine upwards to the lower ribs and can 



