356 NERVOUS MECHANISM. [BOOK n; 



ascending those nerves are not the efficient cause of the respiratory 

 movements. We have seen that when the spinal cord is divided 

 below the medulla, the facial and laryngeal movements still 

 continue. This proves that the respiratory centre is still in 

 action, though its activity is unable to manifest itself in any 

 thoracic movement. But when the cord is thus divided, the 

 respiratory centre is cut off from all sensory impulses, save those 

 which may pass into it from the cranial nerves ; and the division 

 of these cranial nerves by themselves, when the medulla and 

 spinal cord are left intact, does not destroy respiration. Hence 

 we may infer that the respiratory impulses proceeding from the 

 respiratory centre are not simply afferent impulses reaching the 

 centre along afferent nerves and transformed by reflex action in 

 that centre. They evidently start de now from the centre itself, 

 however much their characters may be affected by afferent impulses 

 reaching that centre at the time of their being generated. 'The 

 action of the centre is automatic, not simply reflex. 



Among the afferent impulses which affect the automatic action 

 of the centre, the most important are those which ascend along 

 the vagi. If one vagus be divided, the respiration becomes slower; 

 if both be divided, it becomes very slow, the pauses between 

 expiration and inspiration being excessively prolonged. The 

 character of the respiratory movement too is markedly changed ; 

 each respiration is fuller and deeper, so much so indeed that, 

 according to some observers, what is lost in rate is gained in extent, 

 the amount of carbonic acid produced and oxygen consumed in a 

 given period remaining after division of the nerves about the 

 same as when these were intact. Without insisting too much on 

 the exactness of this compensation we may at least conclude from 

 the effects of section of the vagi, in the first place, that during life 

 afferent impulses are continually ascending the vagi and modifying 

 the action of the respiratory centre, and in the second place, that 

 the modification bears chiefly on the distribution in time of the 

 efferent respiratory impulses, and not so much on the amount to 

 which they are generated. 



These afferent impulses are probably started in the lungs by 

 the condition of the blood in the pulmonary capillaries acting as a 

 stimulus to the peripheral endings of the nerves, though possibly 

 the altered air in the air-cells may also act as a stimulus to the 

 nerve-endings. It has further been suggested that the mere 

 movements of expansion and contraction may also serve as a 

 stimulus. Thus when air is mechanically driven into the chest, an 

 expiratory movement follows, and when air is drawn out, an in- 

 spiratory; and this not only with atmospheric air but with in- 

 different gases, such as nitrogen; when both vagi are cut, these 

 effects do not appear. So also, when in an animal, after division 

 of the spinal cord below the medulla, artificial respiration is kept 

 up, the respiratory movements of the nostrils follow the rhythm 



