HANDBOOK OF PHYSIOLOGY. 



lated to action by the absence of sufficient oxygen in the blood circulat- 

 ing in it, and not by the presence of an excess of carbonic acid. 



But this is not all, since it has been proved by Marckwald that the 

 medullary centres are capable of acting for some time in the absence 

 of any circulation, and after excessive bleeding. The view taken by 

 this author with regard to the action of the centres is as follows : the 

 respiratory centres are set to act by the condition of their metabolism, 

 much in the same way as the heart is set to beat rhythmically. When 

 anabolism is completed, catabolism or discharge occurs, and this alter- 

 nate but crude and spasmodic action will occur without a definite blood- 

 supply, as long as the centres are properly nourished and stimulated by 

 their own intercellular fluid. The afferent impulses brought by the vagi, 

 in consequence of the stimulation of their terminal fibres in the lungs, 

 have a tendency to bring about catabolism, and to convert crude respi- 

 ratory spasms into regular and rhythmic discharges. In the absence of 

 the vagus stimulation, the impulses from the cerebrum may be effectual 

 for the same purpose. 



It is unreasonable to think, however, that the respiratory centres are 

 independent of the character of the blood-supply either as regards quan- 

 tity or quality. This must have a great influence upon their irritability ; 

 it is certain, for example, that venous blood greatly increases the respi- 

 ratory movements, first of all both of inspiration and of expiration, and 

 then of the latter to a greater degree. It may be that the diminution 

 of oxygen in the blood acts as a stimulator of catabolism, in both in- 

 spiratory and expiratory centres, but particularly in the latter, in a 

 manner similar to but not identical with, that of the vagus. It has also 

 been shown that the presence of the products of great muscular metabo- 

 lism in the blood will greatly increase the irritability of the respiratory 

 centres, even if the blood itself be not particularly venous in character. 



It appears that the inspiratory and expiratory respiratory centres are 

 bilateral, and that each pair may act independently, since the bulb may 

 be divided longitudinally, and then if one vagus be divided, the respi- 

 ratory rhythm on the two sides of the body becomes unequal, the move- 

 ments of the side upon which the vagus is divided being slower than on 

 the other side, while stimulation of the divided nerve acts only upon 

 the movements of its own side. 



Apnoea. When we take several deep inspirations in rapid succes- 

 sion by voluntary effort, we find that we can do without breathing for a 

 much longer time than usual; in other words, several rapid respirations 

 seem to inhibit for a time normal respiratory movements. It was 

 thought that the reason for this partial cessation of respiration, which 

 was called apncea, is that by taking several deep breaths we overcharge 

 our blood with oxygen, and that as the respiratory centre can only be 

 stimulated by blood in which the standard of oxygen is below a certain 



