THE NERVOUS MECHANISM OF RESPIRATION. 371 



Since in such cases the rhythmically repeated movements of the respiratory 

 muscles are sometimes accompanied by rhythmic movements of the fore and 

 hind limbs not respiratory in nature, it may be doubted whether these 

 experiments really prove the existence of distinct respiratory centres in the 

 spinal cord ; and at most they merely show that the respiratory nervous 

 mechanism is not entirely confined, as was once thought, to the centre in the 

 medulla, but also embraces other subsidiary mechanisms, which may perhaps 

 be spoken of as centres, in the spinal cord below. It has, indeed, been main- 

 tained by some that these lower spinal centres are the chief centres, and that 

 the medullary centre acts merely in the way of regulating these ; but it is diffi- 

 cult to reconcile this view with the experience that interference with the 

 medulla, limited entirely to the medulla, so often leads to the entire aboli- 

 tion of the respiratory movements. This matter is not at present thoroughly 

 worked out, but we shall probably not greatly err in regarding the respira- 

 tory nervous system as in many ways analogous to the vasomotor nervous 

 system, with its head centre in the medulla and secondary centres elsewhere, 

 and in continuing to speak of the centre in the medulla as being " the respira- 

 tion centre," while admitting that it works through other nervous machinery 

 placed lower down in the spinal cord, and that this subordinate machinery 

 may in exceptional cases carry out, though inadequately, the work of the 

 chief centre. 



306. Admitting then the existence of this medullary respiratory centre 

 the question naturally arises, Are we to regard its rhythmic action as due 

 essentially to changes taking place in itself, or as due to afferent nervous 

 impulses or other stimuli which affect it in a rhythmic manner from with- 

 out? In other words, Is the action of the centre automatic or purely 

 reflex? We know that the centre may be influenced by impulses proceed- 

 ing from without, and that the breathing may be affected by the action of 

 the will, or by an emotion, or by a dash of cold water on thp skin, or in a 

 hundred other ways ; but the fact that the action of the centre may be thus 

 modified from without, is no proof that the continuance of its activity is de- 

 pendent on extrinsic causes. 



In attempting to decide this question we naturally turn to the pneumo- 

 gastric as being the nerve most likely to serve as the channel of afferent 

 impulses setting in action the respiratory centre. If both vagus nerves be 

 divided, respiration still continues, though in a modified form. This proves 

 distinctly that afferent impulses 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 sen- 

 sory impulses, save those which may pass into it from the cranial nerves of 

 sensory function ; and that these sensory cranial nerves are not specially 

 concerned in developing the activity of the respiratory centre is shown by 

 the fact that the division of these cranial nerves by themselves, when the 

 medulla and spinal cord are left intact, does not do away with the continu- 

 ance of respiration. One cranial nerve, as we shall see, is especially con- 

 cerned in respiration, viz., the vagus nerve ; but if, after removal of the 

 brain above the medulla, both vagus nerves be divided, respiration still 

 goes on ; indeed, the respiratory impulses proceeding from the centre are, 

 though in a peculiar way, exaggerated. Hence, though we cannot put the 

 matter to an experimental test by dividing every sensory nerve in the body, 

 while leaving the motor nerves of respiration intact, such an operation being 

 practically impossible, we may infer that the respiratory impulses proceed- 



