EFFECTS OF CHANGES IN COMPOSITION OF AIR 387 



piratory movement of normal respiration and the most violent convulsion of 

 asphyxia. An additional proof that these convulsions are carried out by 

 the agency of the medulla is afforded by the fact that convulsions of a 

 wholly similar character are witnessed when the supply of blood to the 

 medulla is suddenly cut off by ligaturing the bloodvessels of the head. In 

 this case the nervous centres, being no longer furnished with fresh blood, 

 become rapidly asphyxiated through lack of oxygen, and expiratory con- 

 vulsions quite similar to those of ordinary asphyxia, and preceded like them 

 by a passing phase of dyspnoea, make their appearance. Similar " anaemic " 

 convulsions are seen after a sudden and large loss of blood from the body at 

 large, the medulla being similarly stimulated by the lack of arterial blood- 

 In ordinary fainting, which is loss of consciousness due to an insufficient 

 supply of blood to the brain, the diminution of blood-supply is not great 

 enough to produce these convulsions. 



Such violent efforts speedily exhaust the nervous system ; and the convul- 

 sions after being maintained for a brief period suddenly cease and are fol- 

 lowed by a period of calm. The calm is one of exhaustion ; the pupils, 

 dilated to the utmost, are unaffected by light ; touching the cornea calls 

 forth no movement of the eyelids, and, indeed, no reflex actions can any- 

 where be produced by the stimulation of sentient surfaces. All expiratory 

 active movements have ceased ; the muscles of the body are flaccid and 

 quiet ; and though from time to time the respiratory centre gathers sufficient 

 energy to develop respiratory movements, these resemble those of quiet 

 normal breathing, in being, as far as muscular actions are concerned, almost 

 entirely inspiratory They occur at long intervals, like those after section 

 of the vagi ; and like them are deep and slow. The exhausted respiratory 

 centre takes some time to develop an inspiratory explosion ; but the impulse 

 when it is generated is proportionately strong. It seems as if the resistance 

 which had in each case to be overcome was considerable, and the effort in 

 consequence, when successful, productive of a large effect. 



Very soon these inspiratory efforts become less frequent ; their rhythm 

 becomes irregular ; long pauses, each one of which seems a final one, are 

 succeeded by several somewhat rapidly repeated inspirations. The pauses 

 become longer, and the inspiratory movements shallower. Each inspiration 

 is accompanied by the contraction of accessory muscles, especially of the 

 face, so that each breath becomes more and more a prolonged gasp. The 

 inspiratory gasps spread into a convulsive stretching of the whole body ; and 

 with extended limbs, and a straightened trunk, with the head thrown back, 

 the mouth widely open, the face drawn, and the nostrils dilated, the last 

 breath is taken in. 



Thus we are able to distinguish three stages in the phenomena which 

 result from a continued deficiency of air: 1. A stage of dyspnoea, charac- 

 terized by an increase of the respiratory movements both of inspiration and 

 expiration. 2. A convulsive stage, characterized by the dominance of the 

 expiratory efforts, and culminating in general convulsions. 3. A stage of 

 exhaustion, in which lingering and long-drawn inspirations gradually die 

 out. When brought about by sudden occlusion of the trachea these events 

 run through their course in about four or five minutes in the dog and in 

 about three or four minutes in the rabbit. The first stage passes gradually 

 into the second, convulsions appearing at the end of the first minute. The 

 transition from the second stage to the third is somewhat abrupt, the con- 

 vulsions suddenly ceasing early in the second minute. The remaining time 

 is occupied in the third stage. 



The duration of asphyxia varies not only in different animals, but in the 

 same animal under different circumstances. Newly born and young animals 



