THE RESPIRATORY SYSTEM. 379 



and reserve air, or about 3,500 cc. (320 cubic inches). Consider- 

 ing the creeping slowness of the capUlary circulation, it would 

 not be supposed that the respiratory process in its essential 

 parts should be the rapid one that a greater movement of the 

 air would imply. 



THE RESPIRATORY RHYTHM. 



In man, and most of our domestic mammals, a definite 

 though somewhat different relation between the cardiac and 

 respiratory movements obtains, there being about three to five 

 heart-beats to one respiration, which would make the rate of 

 breathing in man about sixteen to eighteen per minute. Usual- 

 ly, of course, the largest animals have the slower pulse and res- 

 piration ; and this is an invariable nale for the varieties of a 

 species, as obser viable in the canine race, to mention a well- 

 known instance. The horse breathes 9 to 13 times in a minute; 

 the ox 15 to 20 ; the sheep 13 to 17 ; and the dog 15 to 30. 



The rate of the respiratory movements is to some extent a 

 measure of the rapidity of the oxidative processes in the body, 

 as witness the slow and intermittent breathing of cold-blooded 

 animals as compared with the more rapid respiration of birds 

 and mammals (Fig. 305). 



Pathological. — Any condition that lessens the amount of re- 

 spiratory surface, or diminishes the mobility of the chest-walls, 

 is usually accompanied by accelerated movements, but beneath 

 this is the 'demand for oxygen, part of the avenues by which 

 this gas usually enters having been closed or obstructed by the 

 disease. So that it is not surprising that, in consequence of 

 the effusion of fluid into the thoracic cavity, leading to the 

 compression of the lung, the opposite one should be called into 

 more frequent use, and even enlarge to meet the demand. 

 These facts show how urgent is the need for constant ventila- 

 tion of the blood, and at the same time how great is the power 

 of adaptation to meet the emergency. 



The difference between the inspiratory and the expiratory 

 rhythm may be gathered by watching the movements of the 

 bared chest, or more accurately from a graphic record. It is 

 usually considered that expiration is only slightly longer than 

 inspiration, and that any marked deviation from this relation 

 should arouse suspicion of disease. Normally the respiratory 

 pause is very slight, so that inspiration seems to follow directly 



