664 STRUCTURE OF THE BULB, PONS, AND MID-BRAIN [CH. XL7L 



10. Vagus or pneumogastric. This is a nerve with varied efferent 

 and afferent functions; its branches pass to pharynx, larynx, 

 oesophagus, stomach, lungs, heart, intestines, liver and spleen. 

 These functions we have already studied in connection with those 

 organs. 



11. Spinal accessory. The internal branch of this nerve blends 

 with the vagus, and its larger external division supplies the trapezius 

 and the sterno-mastoid muscles. 



12. Hypoglossal. This is the motor nerve to the tongue muscles. 

 A mere enumeration of the nerves connected to the bulb shows 



how supremely important this small area of the brain is for carrying 

 on the organic functions of life. It contains centres which regulate 

 deglutition, vomiting, the secretion of saliva, etc., respiration, the 

 heart's movements, and the vaso-motor nerves. 



When we further consider that the various centres are connected 

 by groups of association fibres, we at once realise the reason for 

 the complexity of the structures where all this busy traffic takes 

 place. 



In the enumeration of the cranial nerves, it will be noticed that 

 many of them are either wholly motor or wholly sensory, and that 

 some of them, like the spinal nerves, have a double function. The 

 motor nerve fibres start as axons from the groups of nerve-cells in 

 the grey matter of this region, just as the motor fibres in the spinal 

 nerves originate from the cells of the spinal grey matter. There is 

 a corresponding resemblance in the origin of the sensory fibres of 

 the cranial and spinal nerves. In the latter, it will be remembered, 

 they originate as outgrowths from the cells of the spinal ganglia, one 

 branch growing to the periphery, and the other to the spinal cord, 

 where it terminates after a more or less extended course by forming 

 synapses with the cells of the grey matter. In the sensory cranial 

 nerves the fibres have a corresponding origin in peripheral ganglia, 

 and those branches which grow towards the bulb terminate by arboris- 

 ing around special groups of cells spoken of as the sensory nuclei. 



The following diagram (fig. 408) roughly indicates the position 

 of these nuclei ; the motor nuclei are coloured blue, and the sensory 

 red. It must, however, be clearly recognised that while the motor 

 nuclei are true centres of origin, the so-called sensory nuclei are 

 groups of cells around which the entering sensory fibres arborise ; these 

 cells do not give origin to the axons of the sensory nerves. After we 

 have studied the internal structure of the bulb we shall be able to 

 return once more to the cranial nerves, in order that we consider their 

 origin and function in greater detail. 



But this diagram will give a general idea of the positions of the 

 nuclei. "We see that the so-called sensory nuclei (coloured red) 

 are in the minority; they comprise the sensory nucleus of the 



