572 RESPIRATORY CURVES. [BOOK n. 



As we shall see in detail hereafter, the several parts of the whole 

 act vary much, under various circumstances, in relation to each 

 other. Sometimes expiration, sometimes inspiration is prolonged ; 

 and either inspiration or expiration may be slow or rapid in its 

 development. At times the chest may remain for a while at the 

 height of inspiration, thus making a pause between inspiration and 

 expiration. 



In what may be considered as normal breathing, the respiratory 

 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 influence it. We shall 

 have to consider in detail hereafter the manner 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. 



330. 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 inspira- 

 tion an enlargement takes place in the antero-posterior 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 

 walls 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 very conspicuous, the breast rising and falling with every 

 respiration ; in the male, however, the movements are much more 

 confined to the lower part of the chest. 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. 



