Nos. IAND2.] AX ATOMY OF SCOMBER SCOMBER. 237 



5. Nervus Trochlearis. 



The Nervus Trochlearis {tr) has its usual origin from the 

 superior surface of the brain, there lying between the optic lobe of 

 its side of the head and the overlapping cerebellum. It runs down- 

 ward and forward internal to the hind end of the optic lobe and 

 then forward between the ventral surface of the midbrain and the 

 underlying hypoarium, lying dorsal or slightly lateral to the oculo- 

 motorius. Continuing forward across the tractus opticus it reaches 

 its foramen, at the mesial edge of the alisphenoid, and issuing 

 by it, into the orbit, runs forward dorsal, or dorso-mesial to all 

 the muscles and nerves of the eyeball, lying ventro-mesial to the 

 ramus ophthalmicus superficialis trigemini, as in Amia. It reaches 

 the obliquus superior toward the origin of that muscle, having 

 separated into two principal parts as it approaches it, and then 

 breaks up into several branches which spread upward and down- 

 ward on the ventro-mesial surface of the muscle. One long branch 

 is sent backward and upward along the postero- ventral edge of the 

 muscle, almost to its insertion. 



No indication, whatever, of a communicating branch from this 

 nerve to the ophthalmicus superficialis trigemini could be found; 

 nor could any intracranial branches be found, such as Kingsbury 

 (No. 43, p. 146) describes in Nechirus. 



6. Nervus Ahducens. 



The Nervus Abducens {ah) arises from the floor of the med- 

 ulla as a single strand, lying, at its origin, antero-mesial to the point 

 of origin of the acusticus and postero-mesial to that of the anterior 

 root of the trigemino-facial complex. It runs at first forward and 

 slightly laterally, and then directly forward onto the hind end of 

 the ventral surface of the hypoarium. The nerve here becomes 

 flattened, as if by pressure, and is slightly pressed into the hypo- 

 arium, thus forming a slight impress on the ventral surface of that 

 organ. Before reaching the middle of the hypoarium it turns lat- 

 erally and forward, at a well-marked angle, traverses its foramen 

 in the horizontal wing of the petrosal, enters the eye-muscle canal, 

 and is distributed immediately to the rectus externus, which it 

 innervates. As the nerve approaches 'its foramen Mr. Nomura 



