CHAP. IL] RESPIRATION. 543 



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 o'bvious 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 very 

 conspicuous, the breast rising and falling with every respiration ; 

 in the male, however, the movements are almost entirely 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. 



331. 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 that 

 movement in the lower part of the chest and abdomen so 

 characteristic of male breathing, which is hence called diaphragm- 

 atic ; the latter causes the movement of the upper chest character- 

 istic of female breathing, which is called costal. These two main 

 factors are assisted by less important and subsidiary events. 



Even in the female human subject, the share taken in respira- 

 tion by the diaphragm is an important one, in the male the 

 diaphragm must be regarded as the chief respiratory agent, and in 

 some animals its use, for this purpose, is so prominent that the 



