General Somatic Afferent System 31 



In man, where such sensibility is not specially highly de- 

 veloped, the root is relatively small as compared with its size 

 in the rat, which has great general sensibility in the snout 

 region, particularly in connection with the vibrissae, or 

 '' whiskers ".1 In transverse sections, it appears as a large, 

 crescentic area covering most of the lateral aspect of the 

 medulla oblongata, in which position it may be followed 

 from the beginning of the spinal cord up to the level of the 

 fifth nerve. Traced up in this way, it will be seen to increase 

 in size, being much smaller in the lower part of its course than 

 in the upper. The obvious explanation of this fact is that all 

 the fibres do not descend to the lower part of its course, a 

 series of them leaving it all the way along to end in the ad- 

 jacent gray matter. 



The gray matter in which the afferent fibres of the general 

 somatic system end is made up of two parts which are, how- 

 ever, continuous with each other. The first of these is the 

 chief sensory nucleus of the trigeminal nerve (nucleus principalis 

 trigemini). It consists of a large mass of gray matter in the 

 region of the entry of the nerve, lying in the lateral part of 

 the medulla in close contact with the mass of sensory fibres 

 (PI. XL), and extending anteriorly a short distance along 

 with the ascending branches of these fibres, so as to receive 

 their terminations. The trigeminal elements are covered 

 superficially by other structures at this level, but a short 

 distance posteriorly they emerge. Continuous with the chief 

 nucleus is a column of gray matter situated immediately 

 medial to the descending root and partly surrounded by it. 

 This is the spinal V nucleus {nucleus spinalis trigemini), 

 which in the rat is as large as the chief nucleus in section, so 

 that no precise line of demarcation can be drawn between 

 them, though it is claimed that they differ in function (Pis. 



^Meyersohn finds that, if cross sections at the level of the pyramidal 

 decussation be magnified so as to make their diameters equal, the size 

 of the spinal V root in the rat has the ratio to that in man of 1.21: 0.25. 



