270 ESSENTIALS OF PHYSIOLOGY. 



expiratory movements become more and more exaggerated, and the first 

 stage passes into 



(2) The Stage of Expiratory Convulsions. During this period 

 every muscle which can assist expiration is called into action, and at the 

 same time convulsive movements of the limbs take place. This period 

 lasts about a minute, and is succeeded by 



(3) The Stage of Exhaustion, during which the animal lies passive, 

 the muscles are flaccid except for an occasional deep inspiration, the 

 pupils are widely dilated, and all reflexes are absent. The respiratory 

 movements become less frequent, and at the end of 1 \ to 2 minutes death 

 takes place. The blood after death is almost free from oxygen. 



If the blood pressure is observed in an animal during asphyxia, the 

 vagi being cut, it may be seen that towards the end of the first stage 

 the blood pressure rises rapidly, and soon reaches a very high level, which 

 is maintained for a short time ; towards the end of the second stage it 

 begins to fall, and continues to fall steadily until the animal dies. 



This sequence of events is brought about in the following manner. 

 At the beginning of asphyxia, the accumulation of carbonic acid in the 

 blood excites the respiratory centre, producing hyperpnosa. Towards 

 the end of the first stage the increasing deficiency of oxygen begins to 

 make itself felt, leading to the further stimulation of the respiratory 

 centre and to loss of consciousness. In the second stage, the lack of 

 oxygen stimulates the whole of the central nervous system, giving rise 

 to the general convulsions which are observed ; this effect is soon 

 succeeded by paralysis, resulting from the prolonged deficiency of 

 oxygen, and ending in death. 



The vascular changes are also due partly to excess of carbonic acid, 

 1 partly to lack of oxygen, as is shown in figs. 104 and 105. These figures 

 make it clear that a rise of blood pressure similar to that seen in 

 asphyxia (fig. 104, A) is produced either when the animal is allowed to 

 breathe air containing no oxygen and no excess of carbonic acid 

 (fig. 104, B), or when it breathes air containing an adequate supply of 

 oxygen and a large excess of carbonic acid (fig. 105, A), or, finally, when 

 lactic acid is injected into the circulation (fig. 105, B). 



The rise of blood pressure is due to stimulation of the vaso-motor 

 centre, which produces general constriction of the arterioles ; adrenalin 

 also is set free into the blood stream, and contributes to the rise of 

 pressure. By enclosing a loop of intestine in a plethysmograph it may 

 .be shown that, with the onset of asphyxia, the volume of the loop 

 diminishes owing to constriction of its blood-vessels, and that this con- 

 striction persists until the death of the animal. The final fall of blood 

 pressure must be due, therefore, to failure of the heart ; owing partly 



