THE SOMATIC MOTOR DIVISION. I95 



which probably arises from the trigeminus motor nucleus. The 

 source of the muscle — whether from dorsal or lateral mesoderm — ■ 

 is uncertain. The nerve innervating the obhquus superior muscle 

 is the IV or trochlearis, which is the ventral nerve of somite 2. 

 This also is comparable with a ventral spinal nerve except that 

 its root starting from a ventral motor nucleus runs upward in 

 the brain wall, decussates with its fellow^ in the roof of the brain 

 and emerges from the dorsal or lateral surface between the cere- 

 bellum and tectum mesencephali. No satisfactory explanation 

 has yet been found for the curious course of this nerve. The 



Fig. 105. — A transverse section through the nucleus of origin of the III nerve in 

 a cyclostome, Lampetra. it. Ill, nucleus of III nerve; b.M., bundle of Meynert 

 (tractus habenulo-peduncularis) ; d.b.M., decussation of the same; n.b.M., end- 

 nucleus of the same; g.M., one of the giant cells of Mauthner. 



nerve which innervates the four muscles derived from the first 

 somite is the III or oculomotorius, which arises from a ventral 

 motor nucleus in the base of the mesencephalon. It is a noticeable 

 peculiarity in the origin of this nerve that a large part of its fibers 

 arise from the nucleus of one side and cross to enter the root of 

 the opposite side (Fig. 105). The same arrangement is found in 

 the roots of other ventral nerves but to a much less degree. 



All the somatic motor nerves arise from a portion of the gray 

 matter which lies latero-ventral to the central canal or ventricle. 

 In the brain region it is usually marked by a pair of special grooves 

 or furrows in the floor of the ventricle, one at either side of the 



