400 COMPARATIVE PHYSIOLOGY. 



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It is during the first two stages that the blood-pressure rises, 

 and during the third that it sinks, due in the first instance 

 chiefly to excessive activity of the vaso-motor center, and in 

 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-bom 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 ; 



