MORPHOLOGY OF EYE MUSCLE NERVES 113 



The course of the root fibers dorsally through the brain wall may 

 be due to the course of the fiber tracts through which they run. The 

 position of the nucleus of the nerve relative to the tracts which form 

 the ansulate commissure in typical fishes suggests strongly that the 

 axones from the cells of the trochlearis nucleus may have followed some 

 of these bundles as the path of least resistance. The tracts between 

 the tectum opticum and the base of the oblongata, the tracts be- 

 tween the inferior lobes and the cerebellum, and others, all running 

 more or less dorso-ventrally in the side wall of the brain and decus- 

 sating ventrally at the level of the trochlearis nucleus — ^these bundles, 

 which lie ectal to the nucleus of the trochlearis, may have constituted 

 an effective barrier to the axones of the trochlearis in their attempt 

 to reach the ventro-lateral surface of the brain. The axones may 

 then have turned upward along the ental surface of these bundles 

 until they reached the dorsal surface of the brain. If the fibers were 

 thus directed in their course they would be carried to the mid-dorsal 

 line before gaining an exit from the brain and if they then grew straight 

 on they would pass to the opposite side. 



Filatoff ('07, pp. 366-7) thinks that certain phenomena in the 

 development of the brain may help to explain the peculiar origin 

 of the trochlearis. At the time of the formation of the cephalic 

 flexure the roof of the midbrain, which up to that time formed 

 a part of the thin upper wall, thickened. The production of 

 this thickening may be explained in the following way. By the 

 development of the flexure the cells of the floor of the midbrain 

 become most strongly compressed, since they come to lie directly 

 in the place of flexure, and they seek to elongate themselves in the 

 most direct way where the pressure is a little less strong, namely 

 towards the upper wall. The point at which the trochlearis 

 arises is directly determined by this elongation of the cells. The 

 point of emergence of the trochlearis fibers is shoved from the 

 ventral to the dorsal side. 



Figure 82 of this paper suggests two possible phylogenetic 

 .stages in the development of the trochlear chiasma. An earlier 

 stage is represented on the left of the diagram and a later stage, 

 corresponding essentially to that seen in some elasmobranch 

 embryos, is shown on the right. It is assumed that originally 

 the trochlear, as a somatic motor nerve, was distributed to the 

 myotome of the second somite of its own side after the fashion 

 of typical somatic motor nerves, and that this myotome, when 



JOURNAL OF MORPHOLOGY, VOL. 25, NO. 1 



