Turning now to the evidence as to the number of ventral segments, _i.e._ the motor and sensory supply to the prosomatic appendages afforded by the trigeminal nerve, we must, I think, come to the same conclusion as Dohrn, viz. that if there were originally seven dorsal or somatic segments in this region represented by: 1, Anterior cavity, muscle lost; 2, 3, 4, 5, muscles of the premandibular cavity, _sup. rectus_, _inf. rectus_, _int. rectus_, _inf. oblique_, supplied by IIIrd nerve; 6, 7, muscles of the mandibular cavity, _sup. oblique_, supplied by IVth nerve and muscle lost, there must have been also seven corresponding ventral or splanchnic segments supplied by the trigeminal. At present the evidence for such segments is nothing like so strong as for the corresponding somatic ones; there are, however, certain suggestive {280}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 considered at present to be motor, and is believed to supply, as Kölliker suggested, among other muscles, the _m. tensor tympani_ and the _m. tensor veli palati_. It is thus described by Obersteiner--