474 THE RESPIRATORY CENTRE. [BOOK 11. 



destinations. On the contrary we have reason to think that 

 the respiratory motor nerves, like other motor nerves, are con- 

 nected, just as they are about to issue from the spinal cord, 

 with a nervous machinery, in 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 im- 

 pulses 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 ani- 

 mals, 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 breath- 

 ing may occasionally be observed. These are the exceptional 

 instances mentioned above. 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 spinal bulb, and secondary centres elsewhere, and 

 in continuing to speak of the centre in the spinal bulb 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. 



294. Admitting then the existence of this bulbar 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 affect 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 pro- 

 ceeding 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 the skin, or in a hundred other ways ; but the fact 

 that the action of the centre may be thus modified from with- 

 out, is no proof that the continuance of its activity is dependent 

 on extrinsic causes. 



In attempting to decide this question we naturally turn to 

 the pneumogastric 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 con- 

 tinues, though in a modified form. This proves distinctly that 

 afferent impulses ascending those nerves are not the efficient 



