CHAP, n.j RESPIRATION. 355 



several efferent nerve-fibres to their several muscular, destinations. 

 On the contrary we have reason to think that the respiratory motor 

 nerves, like other special nerves as they are about to issue from the 

 spinal cord, are connected with a nervous ganglionic machinery, 

 a point which we shall consider more fully in treating of the spinal 

 cord ; and that the respiratory impulses pass into and are modified 

 by such spinal nervous machinery immediately before they issue 

 along the motor nerve-roots. Indeed recent observations shew 

 that under particular conditions, and especially in young animals, 

 respiratory movements may be carried out in the entire absence of 

 the medulla oblongata. Thus in a kitten, after removal of the 

 medulla, if the excitability of the spinal cord be heightened by 

 small doses of strychnia, not only may respiratory movements of 

 the chest be induced, in a reflex manner, by pinching or by 

 blowing on the skin, but even transient spontaneous efforts of 

 breathing may with care be observed. These are the excep- 

 tional instances mentioned above ; and they shew that the respi- 

 ratory nervous mechanism is not confined, as was once thought, 

 to the centre in the medulla, but also embraces other subsidiary 

 centres in the spinal cord below. The respiratory nervous sytem 

 seems in fact in many ways analogous to the vaso-motor nervous 

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

 elsewhere, and to the cardiac nervous system with its potent 

 ganglia in the sinus, and its secondary ganglia in the auricles, and 

 auriculo-ventricular groove. The matter is not at present thorough- 

 ly worked out, but we shall probably not greatly err 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 t>ut, though 

 inadequately, the work of the chief centre. 



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 cr 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 proceeding 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 without, 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 vagi be divided, respiration still continues though in a 

 modified form. This proves distinctly that afferent impulses 



2S-2 



