1222 PHYSIOLOGY 



affects in any way the activity of the centres. Certain observers have described 

 ' accessory respiratory centres ' in the raid-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 dblongata 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 

 hi the maintenance or regulation of the respiratory movements. 



APNCEA. If artificial respiration be maintained so as to produce 

 a somewhat greater ventilation than occurs 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 o ver- 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 therefore 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 discharge. 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 ordinary apnoea is entirely due 

 to deficiency of carbon dioxide tension in the respiratory centre, and 

 that although the vagus nerve is inhibitory of respiration, it is 

 impossible to summate a series of vagus inhibitions by artificial respira- 

 tion so as to produce a lasting cessation of respiratory movements. 

 The chief use of the vagi in respiration seems to be for maintaining, 

 by frequent inhibitions, the excitability of the respiratory centre at a 

 maximum. 



Miescher distinguished three types of apnoea, viz. : 



Apnoea vera, due to the washing out of C0 2 from the lungs, and the conse- 

 quent reduction of the tension of this gas in the blood. 



Apnoea vagi, a stoppage of respiration caused by stimulation of the inhibitory 

 fibres of the vagi. This stoppage is limited, as we have seen, to the immediate 

 duration of the stimulus (whether electric or produced by distension of the 

 lungs). 



