96 PLATT. [Vol. V. 



that the nerve, after losing its primary attachment to the brain, 

 should grow back through so great a distance to the point 

 whence it originally arose, unless its continuity had been but 

 seemingly interrupted. In the second place, it seems improba- 

 ble that the nerve should result by the fusion of two outgrowths, 

 one from the ganglion, and one from the brain. 



In regard to the first of these objections, it will be noticed 

 that the position of the primary trochlearis is not the same as 

 that of the permanent nerve (compare sees. 9 and 7). The 

 former lies close against the ectoderm, the latter deep in the 

 mesoderm. Further, at the time when the primary trochlearis 

 atrophies, it is a compact cord, measuring three or four cells in 

 diameter, and when it breaks into fragments, as mentioned 

 above, they retain the width of the primary nerve (sec. 8). In 

 order that the final nerve should be the same as the original 

 trochlearis, it would therefore be necessary to bridge two, or 

 sometimes three, wide breaks in a solid cord, the cells of which 

 still retain their primary rounded contour, and are not prolonged 

 into fibres. Evidently such connection, if it existed, could not 

 be overlooked. 



In regard to the second objection, the cells of the permanent 

 trochlearis cannot arise from the brain, because no cells are to 

 be found in the root of attachment, as would certainly be the 

 case if cells migrated from the brain to the distal portion of the 

 nerve. The fibrous root of the nerve cannot be composed of 

 prolongations from the distal cells, because the fibrous or prox- 

 imal part of the permanent trochlearis arises before the distal 

 and cellular part ; further, the fibrous root is thickest as it 

 comes from the brain, becoming gradually attenuated as it pro- 

 ceeds into the mesoderm. 



Therefore the permanent trochlearis is a new nerve, arising 

 from two sources.. 



Before the permanent trochlearis is formed, branches of the 

 ramus ophthalmicus superficialis trigemini (all that is left of 

 the original trochlearis) are quite widely distributed in the 

 region surrounding the anterior limit of the superior oblique 

 eye-muscle. When the permanent trochlearis is established, its 

 relation to the superior oblique muscle, and to the ramus oph- 

 thalmicus superficialis, is such, that the ophthalmicus, rather 

 than the trochlearis, appears to supply the muscle. A single 



