THE REGULATING MECHANISM 195 



least eight nuclei of grey matter in the 

 medulla, all of which are important. The 

 neurones in this grey matter are all intimately 

 related to each other. The bulb is also 

 richly supplied with capillary blood vessels, 

 and it lies practically in a lymphatic space 

 while perivascular lymphatic channels sur- 

 round its vessels. Waste matters are thus 

 quickly removed. All the blood vessels in 

 the bulb are of remarkably small diameter, 

 as one would expect. To and from this 

 centre, or rather group of centres, there run 

 numerous nerve fibres connected with the 

 vagi or pneumogastric nerves, and some of 

 the cranial nerves. Fibres also pass into the 

 sympathetic chain of ganglia. 



111. As already mentioned, the bulb has 

 to do with the innervation of the heart and of 

 the respiratory mechanism. Both of these 

 movements are rhythmic in their character. 

 We feel this in the beat of the heart and in 

 the regular periodic movements of inspiration 

 and of expiration. If these movements depend 

 on the bulb, the question arises as to whether 

 or not the bulb acts automatically. Is there 



