374 PHYSIOLOGY. 



I 



Second stage, convulsive stage. Here the inspiratory muscles lose 

 their force, whilst the expiratory movements become more active ; next 

 all the muscles of the body, including the expiratory ones, become con- 

 vulsed, due to the carbon dioxide stimulating the central nervous 

 system. This stage lasts about a minute. 



Third stage, or stage of exhaustion. This usually comes on 

 suddenly, the carbon dioxide paralyzing the center of respiration ; the 

 pupils dilate, the eyelids do not close when the cornea is touched; a 

 state of general calm ensues, which is in marked contrast with the 

 previous agitation ; consciousness is abolished ; the animal lies motion- 

 less and seems dead; occasional inspiratory acts take place, then they 

 become feebler and of a gasping character ; finally, the nostrils dilate, 

 the limbs of the animal are extended, opisthotonos ensues, the pulse 

 disappears and death closes the scene. This state lasts about three 

 minutes. This pulseless condition is properly denominated asphyxia. 

 The phenomena of slow asphyxiation follows the same course, but less 

 rapidly. 



CIRCULATORY EFFECT OF ASPHYXIA. The blood-pressure ascends 

 during the first and second stages to about 160 millimeters of mercury 

 and falls during the third stage to 20 millimeters and to zero. This 

 rise of pressure is due to increase of carbon dioxide stimulating the 

 vasoconstrictor centers, which causes the arterioles to contract, espe- 

 cially those in the splanchnic area. The immediate result is a filling of 

 the venous system, which is assisted by the contraction of the inspira- 

 tory muscles of the trunk. The great veins are so distended that they 

 spurt like arteries when cut. The blood-pressure falls mainly because 

 the heart-muscles lose their irritability due to carbon dioxide and the 

 slowing of the heart caused by the carbonic acid stimulating the cardio- 

 inhibitory center. After death the right side of the heart is filled with 

 blood and the left side is empty, or comparatively so. The heart is 

 enlarged to nearly double its former size. The engorged veins fill the 

 right side of the heart with blood, which dilates the heart because the 

 internal pressure and the weakened tonus of the muscle by the carbon 

 dioxide co-operate. The enfeebled right side of the heart is unable to 

 expel its blood. At the same time the left side of the heart, having 

 been also dilated by high arterial pressure, loses its power by the 

 carbonic acid to contract, and the blood pools in the pulmonary vessels 

 and the right side of the heart is filled with blood. 



At first the rate of the heart is accelerated, but it soon becomes 

 slower and more vigorous ; the vagus center becoming excited by the 

 carbon dioxide makes the beat of the heart slower. 



