THE RESPIRATORY SYSTEM. 379 
THe RESPIRATORY RHYTHM. 
In man, and most of our domestic mammals, a definite rela- 
tion between the cardiac and respiratory movements obtains, 
there being about four to five heart-beats to one respiration, 
which would make the rate of breathing in man about sixteen 
to eighteen per minute. Usually, of course, the largest animals 
have the slower pulse and respiration; and this is an invariable 
rule for the varieties of a species, as observable in the canine 
race, to mention a well-known instance. 
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. 318). 
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 di- 
rectly on expiration; though the latter act reminds us of the 
prolongation of the ventricular systole after the blood is ex- 
pelled. 
If, in the tracing, the small waves on the upper part of the 
expiratory curve really represent the effect of the heart-beat, it 
makes it easier to understand how such might assist in venti- 
lating the blood when the respirations occur only once in a 
considerable interval and very feebly then, as in hibernating 
animals or individuals that have fainted; though it must be 
