REGULATION OF RESPIRATORY MOVEMENTS 1079 



apart from the incidence of afferent impressions. The respiratory centre 

 tends to respond to all stimuli, continuous or rhythmic, by means of rhythmic 

 discharges, and there can be no doubt that if we take the medulla in connec- 

 tion with the rest of the hind- and mid -brain we are justified in regarding its 

 activity as automatic. 



The automatic activity of the heart is intimately dependent on the 

 saline constituents of the blood. It may be abolished or diminished by 

 modifying these constituents, and can be maintained for a considerable length 

 of time by perfusing the heart with solutions containing inorganic salts in 

 the concentration in which they exist in the blood-plasma. When we 

 speak of the automatic activity of the respiratory centre we imply in the 

 same way that its activity is dependent on the normal composition of the 

 blood circulating through its vessels. In this case, however, it is the 

 gaseous contents of the blood which are of supreme importance. If the 

 normal ventilation of the lungs be prevented, as by ligature of the trachea 

 or opening both pleural cavities, the blood becomes more and more venous. 

 As this venous blood circulates through the medulla the activity of the centre 

 is continually increased, until finally the impulses discharged from the 

 centre may set into activity practically every muscle of the body, producing 

 asphyxial convulsions. On the other hand, the activity of the respiratory 

 centre can be diminished or even abolished if, by an artificial ventilation of 

 the alveoli, we maintain an over-arterialisation of the blood, so that the 

 fluid passing to the brain contains more oxygen and less carbon dioxide 

 than is the case under normal circumstances. What are the factors involved 

 in this chemical regulation of respiration ? 



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 movements 

 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 con- 

 sciousness 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. 



