No. I.] MORPHOLOGY OF VERTEBRATE HEAD. 97 



sagittal section passes through nearly the entire length of the 

 ophthalmicus superficialis, including the distal distribution of 

 its branches upon the surface of the superior oblique muscle, 

 while the trochlearis, in passing from the brain to the muscle, 

 turns at a right angle where it meets the ophthalmicus super- 

 ficialis. The relation of the nerves to the muscle is shown in 

 Fig. 6, PI. IV. In fact, the trochlearis and the ramus ophthal- 

 micus superficialis trigemini are simply parts of one and the 

 same nerve, from which nerve a branch passes to the surface of 

 the superior oblique muscle. In older embryos the path of the 

 trochlearis becomes more direct, and its connection with the 

 ophthalmicus superficialis then appears as a mere anastomosis 

 between two independent nerves. 



Sec. 7, PI. V, shows the permanent trochlearis when first 

 entirely developed. The drawing is put together from several 

 sections, one of which passes through the fibrous part of the 

 nerve, and one through the distal third of the nerve. The two 

 sections were united, and the median portion of the nerve inter- 

 polated by the aid of the camera. The sections make an angle 

 of 45 with the long axis of the body, which is approximately 

 the plane of the nerve. Sec. 13 shows the structure of the 

 median portion of the trochlearis at about the same stage. The 

 section is drawn especially to show that the body of the nerve 

 is cellular, and that the nuclei belong to the nerve, and not to 

 a mesodermic sheath. 



Returning to the stage when the primary trochlearis and the 

 trigeminus have not yet separated, but are represented in a con- 

 tinuous sheet of cells, and examining the constriction of the 

 brain next anterior to that from which the trochlearis grows, 

 one finds here a cord of nerve cells, which, arising in the mid- 

 dorsal wall of the brain, extend downwards back of the eye, to 

 meet there a line of cells continued forward from the Gasserian 

 ganglion. Dohrn finds in Torpedo a prolongation of the neural 

 crest which is undoubtedly the homologue of this nerve in 

 Acanthias. At the stage described, the neural crest has disap- 

 peared between this nerve and the sheet of cells representing 

 the united trochlear and trigeminal nerves. The nerve passes 

 ventrally into a group of cells that are undoubtedly the rudi- 

 ments of a ganglion. The conection between these cells and 

 the Gasserian ganglion is the path of the future ramus ophthal- 



