1 34 THE INVOL UNTAR Y NER VO US S YS TEM 



THE CONNECTOR NEURONS OF THE BULBAR OUTFLOW. 



In the region of the medulla oblongata the nerves to be con- 

 sidered are the vagus, glossopharyngeal and facial. 



The sensory neurons of the vagus and glossopharyngeal 

 nerves send root fibres into the medulla oblongata, which ter- 

 minate round cells of connector neurons, and as in the spinal 

 cord, many of these root fibres travel some distance before they 

 reach their connector cells ; these latter fibres form a bundle 

 known as the fasciculus solitarius or the descending root of the 

 vagus and glossopharyngeal, the fibres of which connect with 

 nerve cells along the whole of its course. 



Certain of the sensory root fibres of the vagus and glosso- 

 pharyngeal end in some of the cells in the large group on the 

 floor of the fourth ventricle, known as the dorsal nucleus of the 

 vagus. From cells of this nucleus fibres pass into the vagus 

 nerve, which cause on stimulation contraction of the bronchial 

 muscles, inhibition of the heart and other phenomena, character- 

 istic of the stimulation of motor fibres belonging to the involuntary 

 nervous system (Fig. 3, .). For this reason Edinger divides this 

 dorsal nucleus into a sensory and motor part ; this means 

 nothing more than that sensory root fibres connect with its cells, 

 and some of those cells send into the vagus fibres which connect 

 near the periphery with motor neurons of the involuntary nervous 

 system ; in other words, this mass of dorsally situated cells are 

 the cells of connector neurons, some of which connect receptor 

 neurons of the vagus group with the excitor neurons of the 

 same group belonging to the involuntary nervous system. The 

 axons of others of these connector neurons in all probability 

 pass to the cells of the nucleus ambiguus, and so connect receptor 

 neurons of the vagus group with excitor neurons of the same 

 group belonging to the voluntary nervous system (Fig. 3, #.), they 

 thus enable primary reflexes in the voluntary nervous system 

 to take place in these segments. In this respect these cells in 

 the dorsal nucleus of the vagus resemble the posterior horn cells 

 in the spinal segments. Now the shifting which takes place, 

 owing to the opening out of the central canal in the medullary 

 region, brings the cells of the posterior horn, as far as the splanch- 

 nic segmentation is concerned, in among the cells of this dorsal 

 nucleus, as Edinger has shown ; while at the same time, the 



