280 THE ORIGIN OF VERTEBRATES 



facts which point distinctly in this direction in connection with both 

 the motor and sensory parts of the trigeminal. The origin of the 

 trigeminal motor fibres in the central nervous system is most striking. 

 We may take it for granted that a nucleus of cells giving origin to 

 one or more segmental motor nerves will possess a greater or less 

 longitudinal extension in the central nervous system, according to 

 the number of fused separate segmental centres it represents. Thus 

 a nucleus such as that of the IVth nerve or of the facial is small 

 and compact in comparison to the extensive conjoint nucleus of 

 the vagus and cranial accessory. 



Upon examination of the motor nucleus of the trigeminal, we 

 find a compact or well-defined nucleus, the nucl. masticator ius, the 

 nerves of which supply the masseter, temporal, and other muscles, 

 so that the anatomical evidence at first sight appears to bear out 

 van Wijhe's conclusion that the motor trigeminal supplies at most 

 two segments. Further examination, however, shows that this is not 

 all, for the extraordinary so-called descending root of the Vth must 

 be taken into consideration in any question of the origin of the 

 motor elements, just as the equally striking ascending root enters 

 into the consideration of the meaning of the sensory elements of 

 the Vth. 



It is not necessary here to discuss the controversy as to whether 

 this descending root is motor or sensory. It is universally con- 

 sidered at present to be motor, and is believed to supply, as 

 Kolliker suggested, among other muscles, the m. tensor tympani and 

 the m. tensor veli palati. It is thus described by Obersteiner — 



" From the region of the mid-brain the motor root receives an 

 important addition of thick fibres, which form the cerebral or 

 descending root. The large, round vesicular cells from which the 

 fibres of the descending root arise form no single compact group, but 

 are partly single, partly arranged like little bunches of grapes, as far 

 as the region of the anterior corpora quadrigernina. The further we 

 go brainwards, the smaller is the number of fibres. In the region 

 of the anterior corpora quadrigernina, the few cells of origin are 

 found more and more median ; so that the uppermost trigeminal 

 fibres descend in curves almost from the mid-line, as is shown by the 

 exceptional occurrence of one or more of the characteristic cells above 

 the aqueduct. At the height of the posterior commissure one finds 

 the last of these trigeminal cells." 



