1080 PHYSIOLOGY 



There is often also secretion of saliva, inhibition or increase of intestinal 



movements, constriction of the pupil, and so on. 



(3) At the end of the second minute after the stoppage of the aeration of 

 the blood, the expiratory convulsions cease almost suddenly, and give way t6 

 slow deep inspirations. With each inspiratory spasm the animal stretches 

 itself out and opens its mouth widely as if gasping for breath. The whole 

 stage is one of exhaustion : the pupils dilate widely, and all reflexes are 

 abolished. The pauses between the inspirations become longer and longer, 

 until at the end of four or five minutes the animal takes its last breath. 



If we increase the activity of the centre, and therefore its gaseous inter- 

 changes, by warming the blood in the carotid arteries, there may be a con- 

 siderable quickening of respiration unaccompanied by any deepening, a 

 condition which is spoken of as tachypncea. On the other hand, we may slow 

 the respiratory movements by placing a small piece of ice on the floor of the 

 fourth ventricle. 



In the production of the phenomena of asphyxia two factors must be at 

 work. In the first place, there is an accumulation of carbon dioxide in the 

 blood bathing the centre or an increased tension of this gas in the centres 

 themselves, either as a result of deficient excretion or increased production. 

 On the other hand, the centre is deprived of oxygen, either by failure of 

 renewal of the oxygen supply, or by increased using up of this gas in the meta- 

 bolism of the centre. The question arises, which of these two changes is 

 responsible for the different physiological events which characterise 

 asphyxia ? At various times these phenomena have been ascribed either to 

 the increased tension of carbon dioxide or to the diminished tension of 

 oxygen in the centre. The view that the normal stimulus to the respiratory 

 centre in asphyxia was the lack of sufficient oxygen and that the normal 

 activity of this centre was determined by the tension of oxygen in the 

 blood circulating through the brain was first put forward by Rosenthal. 

 "When sufficient oxygen was present, the centre, according to this observer, 

 would cease to act, so that a condition of apnoea would be produced. Ac- 

 cording to Traube, on the other hand, the special respiratory stimulus was 

 the excess of carbon dioxide in the blood, and this view was supported 

 strongly by Miescher. The tendency of recent work, especially by Haldane 

 and his pupils, has been to show that there is an element of truth in both 

 views that indeed the respiratory centre can be excited either by excess 

 of carbon dioxide or by lack of oxygen, but that its sensitivity to carbon 

 dioxide is by far the most important factor in the determination of the 

 increased respiratory movements in asphyxia, and is the only chemical 

 factor which can be regarded as playing any part in the regulation of the 

 respiratory movements under normal conditions. This factor is well brought 

 out if we investigate the effect on the respiratory movements of altering the 

 tensions of the two gases in the air breathed. If by this means we succeed 

 in altering the tension of the two gases in the alveolar air we may assume that 

 the tensions of the gases in the arterial blood leaving the lungs are altered in 

 the same ratio. The results of such experiments are very striking. Even 



