REGULATION OF THE RESPIRATORY MOVEMENTS 1145 



inadequate for the proper oxygenation of the blood. They become gradually 

 less and less frequent, and in about half an hour the animal dies of asphyxia. 

 We must conclude therefore that the medullary respiratory centre with 

 the help of the vagi is able to carry out normal respiratory movements. If 

 both vagi are cut, impulses arrive at the centre from the higher parts of the 

 brain, regulating its activity and enabling it to carry out modified but 

 sufficient respiratory movements. Removed from both these sources of 

 afferent impulses, the centre discharges only a series of spasms which are 

 totally inadequate for the renewal of the blood gases, so that the animal 

 dies. 



We may summarise these results as follows : 



Respiratory centre with vagi normal respiration. 

 Respiratory centre with brain modified respiration. 

 Respiratory centre alone inadequate spasmodic contractions of 

 respiratory muscles, and death of animal. 



The nature of the supplemental action of the mid-brain on the medullary respiratory 

 centre has not yet been made out. It is apparently not dependent on afferent impulses 

 arriving at the brain, since section of no cranial nerve affects in any way the activity of 

 the centres. Certain observers have described ' accessory respiratory centres ' in the 

 mid-brain, in the region of the posterior corpora quadrigemina. Stimulation of this 

 part causes increase in the rate of inspiratory movements and finally tonic spasm of the 

 diaphragm. Expiratory effects have been ^produced by stimulation of the anterior 

 corpora quadrigemina, and it would seem that a section has to pass through or behind 

 these bodies in order to produce the results, already described, of cutting off the higher 

 centres from the medulla oblongata after division of the vagi. Other localised spots in 

 the brain from which effects on respiration have been obtained are the inner wall of the 

 optic thalamus and the root of the olfactory tract. Further experiments are necessary 

 before we can regard any of these centres as normally involved in the maintenance or 

 regulation of the respiratory movements. 



APNCEA. If artificial respiration be maintained so as to produce a 

 somewhat greater ventilation than is effected by the normal respiratory 

 movements of the animal, a standstill of respiration is brought about. This 

 condition is called apncea. The first explanation of this standstill was that 

 it was due to over-oxygenation of the blood. The fact that it could be 

 produced by artificial ventilation with inert gases, such as hydrogen and 

 nitrogen, as well as the discovery of the inhibitory influence of distension 

 of the lungs on the respiratory centre, led Head to ascribe it to the summation 

 of a series of inhibitory stimuli. In these experiments however the fact 

 was forgotten that forced ventilation of the lungs with air or any inert gases 

 will reduce the carbon dioxide tension in the blood circulating round the 

 pulmonary alveoli and therefore round the respiratory centre. A respiratory 

 pause will thus ensue and last until the increasing accumulation of carbon 

 dioxide in the blood raises its tension to the normal height, at which the 

 respiratory centre is ' set,' so to speak, to respond by a respiratory dis- 

 charge. If the carbon dioxide content of inspired air be increased to about 

 4-5 per cent., it is impossible to produce an apnceic pause, however rapidly 

 the respiratory movements be carried out. It would seem therefore that 



