CHAP. IL] RESPIRATION. 617 



which nerve cells play a part a point which we shall consider more 

 fully in treating of the spinal cord ; we have reason to think that 

 the respiratory impulses starting from the respiratory centre pass 

 into and are modified by secondary spinal nervous mechanisms 

 .before they issue along the motor nerve-roots. Indeed observations 

 shew that under particular conditions, and especially in young 

 animals, respiratory movements may be carried out in the entire 

 absence of the spinal bulb. Thus if in a kitten or puppy, or 

 young rabbit, after division of the spinal cord below the bulb, 

 artificial respiration be kept up, and then pauses be made in 

 the artificial respiration, during these pauses not only may what 

 appear to be respiratory movements be induced, in a reflex 

 manner, by pinching or by blowing on the skin, but, especially 

 if the excitability of the spinal cord be heightened by small 

 doses of strychnia, even spontaneous efforts of breathing may 

 occasionally be observed. These are the exceptional instances 

 mentioned above. 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 shew that the respiratory nervous 

 mechanism is not entirely confined, as was once thought, to 

 the centre in the bulb, but also embraces other subsidiary 

 mechanisms, which may perhaps be spoken of as centres, in 

 the spinal cord below. It has indeed been maintained by some 

 that these lower spinal centres are the chief centres and that the 

 bulbar centre acts merely in the way of regulating these ; but 

 it is difficult to reconcile this view with the experience that 

 interference with the bulb, limited entirely to the bulb, so often 

 leads to the entire abolition of the respiratory movements. The 

 matter is not at present thoroughly worked out, but we shall 

 probably not greatly err in regarding the respiratory nervous system 

 as in many ways analogous to the vaso-motor nervous system, with 

 its head centre in the bulb, and secondary centres elsewhere, 

 and in continuing to speak of the centre in the medulla as being 

 " the respiratory 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. 



363. Admitting then the existence of this medullary respira- 

 tory 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 atfect 

 it in a rhythmic manner from without ? In other words, Is the 

 action of the centre automatic or purely reflex ? We know that the 

 centre may be influenced by impulses proceeding from without, and 

 that the breathing may be affected by the action of the will, or by 



p. ii. 40 



