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. masticatorius, 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 palatt. 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 quadrigemina. The further we 
go brainwards, the smaller is the number of fibres. In the region 
of the anterior corpora quadrigemina, 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.” 
