Nos. IAND2.] ANATOMY OF SCOMBER SCOMBER. 235 



4. Nervus Ocniouiotorius. 



The Nervus Oculomotorius (ocm) arises as a single strand, 

 near the median line, from what is undoubtedly the floor of the 

 mid-brain ; the surface from which it arises seeming, however, in 

 gross dissections, to be the extreme anterior end of the medulla 

 rather than a part of the brain anterior to the medulla. The ven- 

 tral surface of the medulla seems here to be continued forward 

 beyond the lateral surface of the structure, and to form a some- 

 what conical surface, with a rounded anterior end which reaches 

 almost to the middle transverse line of the optic lobes. This part 

 of the brain is covered ventrally by the hypoaria, the nervi oculo- 

 motorii thus lying, at their origin, immediately internal to, that 

 is, dorsal to those structures. From here each nerve runs at first 

 forward and laterally and then forward, and issues from between 

 the hypoarium of its side and the floor of the mid-brain at the 

 antero-lateral corner of the hypoarium, between it and the hind 

 edge of the tractus opticus. Continuing forward it issues from 

 the cranial cavity by the olfactorius foramen, between the adjoin- 

 ing edges of the basisphenoid, alisphenoid and petrosal. Immedi- 

 ately after issuing from its foramen it separates into its inferior 

 and superior divisions. 



The superior division of the nervus (Fig. 66) is much the smaller 

 of the two. It separates at once into two principal parts, one of 

 which goes to the dorso-mesial, and the other to the ventro-lateral 

 surface of the rectus superior muscle, the nerve innervating that 

 muscle alone. It lies, at its origin, mesial to, or ventro-mesial to, 

 the truncus ciliaris profundi. 



The inferior division of the nervus is large, and separates 

 at once into two portions, both of which turn mesially, down- 

 ward and forward across the lateral surface of the rectus superior, 

 between that muscle and the rectus externus. The most dorsal 

 portion of the nerve soon separates into two parts, one of which 

 enters and supplies the rectus inferior, the other running dorsal 

 to that muscle and entering and supplying the rectus internus. 

 Both parts break up into several branches as they approach their 

 respective muscles, and both have a long, terminal branch. The 

 terminal branch of the nerve that supplies the rectus inferior runs 



