Chap, ii.] RESPIRATION". 433 



or expiration may be slow or rapid in its development. At 

 times the chest may remain for a while at the height of inspira- 

 tion, thus making a pause between inspiration and expiration. 



In what may be considered as normal breathing, the respir- 

 atory act is repeated about 17 times a minute, the duration of 

 the inspiration as compared with that of the expiration (and 

 such pause as may exist) being about as ten to twelve ; but the 

 rate varies very largely ; and in this as in the volume of each 

 breath it is very difficult to fix a satisfactory average, the figures 

 given varying from 20 to 13 a minute. It varies according to 

 age and sex. It is influenced by the position of the body, being 

 quicker in standing than in lying, and in lying than in sitting. 

 Muscular exertion and emotional conditions affect it deeply. 

 In fact, almost every event which occurs in the body may influ- 

 ence it. We shall have to consider in detail hereafter the man- 

 ner in which these influences are brought to bear. 



When the ordinary respiratory movements prove insufficient 

 to effect the necessary changes in the blood, their rhythm and 

 character become changed. Normal respiration gives place to 

 laboured respiration, and this in turn to dyspnoea, which, unless 

 some restorative event occurs, terminates in asphyxia. These 

 abnormal conditions we shall study more fully hereafter. 



The Respiratory Movements. 



§ 261. When the movements of the chest during normal 

 breathing are watched, or when a graphic record is taken by one 

 or other of the methods just described, it is seen that during 

 inspiration an enlargement takes place in the anteroposterior 

 diameter, the sternum being thrown forwards, and at the same 

 time moving upward. The lateral width of the chest is also 

 increased. The vertical increase of the cavity is not so obvious 

 from the outside, though when the movements of the diaphragm 

 are watched by means of an inserted needle or otherwise, it is 

 clear that the upper surface of that organ descends at each 

 inspiration, the anterior Avails of the abdomen bulging out at 

 the same time. In the female human subject, the movement of 

 the upper part of the chest is usually conspicuous, the breast 

 rising and falling with every respiration ; while in the male, the 

 movements of the lower part of the chest are more marked. 

 In laboured respiration all parts of the chest are alternately 

 expanded and contracted, the breast rising and falling as well 

 in the male as in the female. We have now to consider these 

 several movements in greater detail, and to study the means by 

 which they are carried out. 



§ 262. Inspiration. There are two chief means by which the 

 chest is enlarged in normal inspiration, viz. the descent of the 

 diaphragm and the elevation of the ribs. The former causes 



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