394 ANATOMY OF THE CBNTEAL NEEVOUS SYSTEM. 



arise, not in the motor nucleus, but high up in the quadrigeminal region, 

 where laterally from the aqusedtictus Sylyii (in Fig. 199 above to the left) 

 scattered ganglion-cells give rise to the radix mesencephalica nervi trigemini. 

 These cells continue downward always in the lateral wall of the canal, and 

 one can detect a more considerable collection of them through the thin 

 ventricular ependyma, forming a dark bunch on either side at the beginning 

 of the fourth ventricle. It is there called the locus ccsruleus. 



The nucleus of the motor nerves of mastication is made up much the same as 

 the other motor centers in the spinal cord. Lately, however, S. Eam6n y Cajal has 

 discovered in it a peculiar arrangement. The root from the midbrain, coming from 

 the large, swelled cells around the aqueduct, gives oS to the large motor main 

 nucleus a great number of extremely fine collaterals. Each cell there is surrounded 

 by a very thick net of such fibrils. The discoverer of this remarkable arrangement 

 is of the opinion that it is possible by these collaterals for a relatively weak impulse, 

 originating in the higher nucleus, to be transformed in the main, or lower, nucleus 

 to a powerful stimulus. The large motor cells might be, as it were, laden, and at 

 times simultaneously discharged, bringing about the strength and the co-ordination of 

 the act of mastication. 



The main portion of the nervus trigeminus, the sensory, arises, without 

 doubt, from the cells of the Gasserian ganglion, whose peripherally directed 

 branch forms the nerve, just as in the spinal ganglion-cells, while a thick 

 ''root" running centrally enters the pons. It penetrates this and disappears, 

 to a small extent, in the nucleus there situated (sensory trigeminal nucleus). 



As the fibers enter the nucleus the majority of them split into a 

 ■delicate ascending branch and a descending one. The former terminates 

 soon in the part of the nucleus above where the fiber enters, forming 

 a delicate arborization; the latter terminates more gradually, giving off 

 large numbers of collaterals, and the nucleus in which this occurs, the caudal 

 continuation of the nucleus just mentioned, is unusually long and extends, 

 as the nucleus terminalis nervi trigemini, as far downward as the cervical 

 cord. Throughout its extent it is accompanied by the mass of trigeminal 

 fibers, naturally growing continually thinner. Their cross-section is seen 

 in all the series from the cervical cord up, as a slender, crescentic bimdle. 

 This tractus bulbo-spinalis nervi trigemini is in close proximity to the long, 

 vitreous column of the nucleus, as far as the posterior horn of the cervical 

 cord, and there finally vanishes. 



In Fig. 251 is shown a schematic representation of the tracts of the 

 separate trigeminal roots. Many of the details included are after prepara- 

 tions by Eamon y Cajal. 



From the long bulbar nucleus of the sensory portion of the trigeminus 

 arises the secondary trigeminal tract. Several years ago, based on investiga- 

 tions in comparative anatomy, it was demonstrated that there is a decus- 



