THE RESPIRATORY SYSTEM. 405 
the second to its exhaustion and the weakening of the heart- 
beat. 
These violent movements are owing, we repeat, to the action 
of blood deficient in oxygen on the respiratory center (or cen- 
ters), leading to inordinate action followed by exhaustion. 
The duration of the stages of asphyxia varies with the ani- 
mal, but rarely exceeds five minutes. In this connection it may 
be noted that newly-born animals (kittens, puppies) bear im- 
mersion in water for as much as from thirty to fifty minutes, 
while an adult dog dies within four or five minutes. This is 
to be explained by the feeble metabolism of new-born mam- 
mals, which so slowly uses up the vital air (oxygen). 
If the chest of an animal be opened, though the respiratory 
muscles contract as usual there is, of course, no ventilation of 
the lungs, which lie collapsed in the chest; and the animal dies 
about as quickly as if its trachea were occluded. It passes 
through all the phases of asphyxia as in the former case; but 
additional information may be gained. The heart is seen to 
beat at first more quickly and forcibly, later vigorously though 
slower, and finally both feebly and irregularly, till the ventri- 
cles, then the left auricle, and finally the right auricle cease to 
beat at all or only at long intervals. The terminations of the 
great veins (representing the sinus venosus) beat last of all. 
At death the heart and great veins are much distended 
with blood, the arteries comparatively empty. Even after 
rigor mortis has set in, the right heart is still much engorged. 
These phenomena are the result of the operation of several 
causes. The increasingly venous blood at first stimulates the 
heart probably directly, in part at least, but later has the con- 
trary effect. The nutrition of the organ suffers from the de- 
graded blood, from which it must needs derive its supplies. 
The cardio-inhibitory center probably has a large share in the 
slowing of the heart, if not also in quickening it. Whether 
the accelerator fibers of the vagus or sympathetic play any 
part is uncertain. The increase of peripheral resistance caused 
by the action of the vaso-motor center makes it more difficult 
for the heart to empty its left side and thus receive the venous 
blood as it pours on. At the same time the deep inspirations 
(when the chest is unopened) favor the onflow of venous blood; 
and in any case the whole venous system, including the right 
heart, tends to become engorged from these several causes act- 
ing together. The heart gives up the struggle, unable to main- 
tain it, but not so long as it can beat in any part. 
