REGULATION OF RESPIRATORY MOVEMENTS 1203 



of excitation, and the animal does not survive sufficiently long to 

 allow this condition to pass off. Hering has shown that in the ' spinal 

 cord frog ' (i.e. one in which the brain has been destroyed) section of 

 all the posterior roots absolutely destroys all mobility, the injection 

 of strychnine being without effect. A typical spasm, however, can 

 be at once produced by exposing and stimulating the stump of one 

 of the cut posterior roots. We might suppose that the respiratory 

 centre would be similarly devoid of automatism if absolutely free 

 from afferent stimuli. It must be mentioned, however, that accord- 

 ing to Sherrington it is possible to excite strychnine or asphyxial 

 spasms in a dog or cat with isolated spinal cord, in which all the 

 afferent roots below the transection have been divided six or seven 

 hours previously. He therefore is of opinion that in the mammal 

 the motor nervous mechanism can be set into activity 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 

 connection 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 consider- 

 able 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 ? 



