SEGMENTS OF TRIGEMINAL NERVE-GROUP 28 1 



The anatomy of the Vth nerve reveals, then, three most striking 

 facts : — 



1 . The motor nucleus of the Vth extends from the very commence- 

 ment of the infra-infundibular region to nearly the commencement 

 of the nucleus of the Vllth ; in other words, the motor nucleus of the 

 Vth extends through the whole prosomatic region, just as it must 

 have done originally if its motor nerves supplied the muscles of 

 the prosomatic appendages. Such an extended range of origin is 

 indicative of the remains of an equally extended series of segmental 

 centres or ganglia. 



2. Of these centres the caudalmost have alone remained lame and 

 vigorous, constituting the nucleus masticator ins, which in the fish is 

 divided into an anterior and posterior group, thus indicating a 

 double rather than a single nucleus ; while the foremost ones have 

 dwindled away until they are represented only by the cells of the 

 descending root, the muscles of these segments being still represented 

 by possibly the tensor veli palati and the other muscles innervated 

 from these cells. 



3. The headmost of these cells takes up actually a position dorso- 

 lateral to the central canal, so that the groups on each side nearly 

 come together in the mid-dorsal line ; a very unique and extra- 

 ordinary position for a motor cell-group, but not improbable when we 

 recall to mind Brauer's assertion as to the shifting of the foremost 

 prosomatic ganglion-cells of the scorpion from the ventral to the 

 dorsal side of the alimentary canal. 



On the sensory side the evidence is also suggestive, the question 

 here being not so much the distribution of the sensory nerves as the 

 number of ganglia belonging to each of the cranial nerves. 



With respect to this question, morphologists have come to the 

 conclusion that there is a marked difference between spinal and 

 cranial nerves, in that whereas the posterior root- ganglia of the 

 spinal nerves arise from the central nervous system itself, i.e. from 

 the neural crest, the ganglia of the cranial nerves arise partly from 

 the neural crest, partly from the proliferation of cells on the surface 

 of the animal ; and because of the situation of these proliferating 

 epidermal patches over the gill-clefts in the case of the vagus and 

 glossopharyngeal nerves, they have been called by Froriep and Beard 

 branchial sense-organs. Beard divides the cranial ganglia into two 

 sets, one connected with the neural ridges, called the neural ganglia, 



