146 RESPIRATION 



one to four. This makes the average number of respirations 

 about eighteen per minute for adults. In a general way this 

 rate is subject to variations from the same causes as that of 

 the pulse. Any appreciable .fall in the amount of oxygen in 

 the inspired air will increase the number of respirations for 

 obvious reasons. The frequency and depth usually bear an 

 inverse ratio to each other. 



Types of Respiration. (i) Costal respiration is that car- 

 ried on by the chest walls; (2) diaphragmatic, that effected 

 by the diaphragm. In the former type movements of the 

 thorax are concerned ; in the latter, movements of the abdo- 

 men. According as the movements in costal respiration are 

 more pronounced in the upper or lower segment of the chest, 

 that type is subdivided into (a) superior costal, and (b) in- 

 ferior costal. 



In young children the diaphragmatic, or abdominal, type 

 prevails ; in adult males a combination of the inferior costal 

 and abdominal; in adult females the superior costal. The 

 last circumstance is probably due in part to the mode of 

 dress in civilized countries, and in part to the provision 

 against encroachment of the uterus upon the abdominal 

 cavity during pregnancy. 



Intrapulmonary and Intrathoracic Pressure. It is evi- 

 dent that during inspiration the pressure inside the lungs 

 (intrapulmonary) is less than the ordinary atmospheric 

 pressure ; this, in fact, is the immediate cause of the entrance 

 of air. It is also evident that during expiration the intrapul- 

 monary pressure, owing to the compressing effect of the 

 lung tissue and the thoracic walls, is greater than the outside 

 atmospheric pressure ; this is the immediate cause of the exit 

 of air. In both acts the air rushes in or out, as the case may 

 be, in an effort to maintain the same pressure inside the 

 lungs as exists in the surrounding atmosphere. It is con- 

 venient to call the pressure which is less than atmospheric 

 negative, and that which is greater positive pressure. 



The intrapulmonary pressure is negative during inspira- 



