REGULATION OF RESPIRATORY MOVEMENTS 1095 



meet the increased needs thrown on the respiratory mechanism by aug- 

 mented metabolism, such as occurs in violent muscular exercise. 



The important part played by the vagi in the regulation of normal 

 respiration is shown still more strikingly if the respiratory centre in the 

 medulla be separated from the higher parts of the brain before the section 

 of the vagi is carried out. Separation of the medulla from tke higher parts 

 of the brain, as by section just behind the corpora quadrigemina, has 

 practically no influence on the respiratory rhythm. If now both vagi be 

 divided the normal respiratory movements cease entirely, being replaced 

 by a series of inspiratory spasms, each of which lasts several seconds and 

 is followed by a pause of half to one minute's duration. These spasms are 

 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 

 respiratorj T 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 respiraotry 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 or 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. 



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

 somewhat greater ventilation than occurs by the normal respiratory move- 

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

 condition is called apnoea. 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 



