1204 PHYSIOLOGY 



THE CHEMICAL REGULATION OF THE RESPIRATORY 



MOVEMENTS 



If, the nervous centres being intact, the proper aeration of the 

 respiratory centre be interfered with in any way, the respiratory 

 movements increase in strength and frequency, and if the disturbing 

 factor be not removed the animal dies, presenting a train of phenomena 

 which are classified together under the term ' asphyxia.' 



The phenomena of asphyxia may be divided into three stages : 



(1) In the first stage, that of hyperpnoea, the respiratory move- 

 ments are increased in amplitude and in rhythm. This increase 

 affects at first both inspiratory and expiratory muscles. Gradually 

 the force of the expiratory movements becomes increased out of all 

 proportion to the inspiratory, and the first stage merges into : 



(2) The second, which consists of expiratory convulsions, in which 

 almost every muscle of the body may be involved. Just at the end 

 of the first stage consciousness is lost, and almost immediately after 

 the loss of consciousness we may observe a number of phenomena 

 extending to almost all the functions of the body, some of which have 

 been already studied. Thus the vaso-motor centre is excited, causing 

 universal vascular constriction. 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 to 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 

 interchanges, by warming the blood in the carotid arteries, there may 

 be a considerable quickening of respiration unascompanied by any 

 deepening, a condition which is spoken of as tachypnoea. 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 metabolism of the centre. The 



