THE MECHANICS OF PULMONARY RESPIRATION. 



339 



tion. In normal breathing hardly any such pause exists, but in cases where 

 the respiration becomes infrequent, pauses of considerable length may be 

 observed. 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 inspi- 



[Fio. 91. 



Tracing of Thoracic Respiratory Movements obtained by means of Marey's Pneumograph. 

 A whole respiratory phase is comprised between a anda; inspiration, during which the lever 

 descends, extending from a to b, and expiration from 6 to a. The undulations at c are caused by 

 the heart's beat.] 



ration 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 re- 

 peated about seventeen 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 twenty to thirteen a minute. It varies 

 according to age arid sex. It is influenced by the position of the body, 

 being quicker in standing than in lying, and in lying than in sitting. Mus- 

 cular 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 labored 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. 



273. 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 antero-posterior diameter, the sternum being thrown forward, and at 

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

 creased. The vertical increase of the cavity is not so obvious from the out- 

 side, 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 en- 



