No.3: MUSCLES AND NERVES IN AMIA CALVA. 523 
contain any fibres that are homologous with those that form 
the r. ophthalmicus superficialis trigemini of Amia, teleosts, 
and selachians, for in all these latter fishes the trochlearis lies 
under the nerve. It is more probably the homologue of the r. 
ophthalmicus profundus of other fishes, that nerve, when found, 
lying always under the trochlearis, as will be shown later. 
The abducens in Petromyzon arises (Ahlborn) close in front 
of the ventral, motor root of the trigeminus, not ventrally near 
the median line as in other fishes, runs upward inside the cranial 
wall, and issues (Firbringer) dorsal to — but, as shown in his 
Fig. 19, behind — the trigeminus. It must, therefore, acquire 
this position by crossing first the ventral and then the lateral 
or posterior surface of the trigeminus while the two nerves are 
still inside the cranium. That it runs upward internal to, that 
is anterior to, the trigeminus, and then crosses the upper sur- 
face of that nerve from before backward, seems wholly improb- 
able. The descriptions, however, are not concise. It lies below 
the trochlearis, sends a short branch to the rectus externus, and 
then continues downward and outward behind that muscle to 
the rectus inferior,in which it ends. The rectus superior is 
innervated by a posterior branch of the oculomotorius, sent out- 
ward, according to Fiirbringer, below the ramus ophthalmicus, 
a wholly unaccountable position if the ramus ophthalmicus be 
the ophthalmicus profundus of other fishes, as its relation to the 
nervus trochlearis seems to indicate. The rest of the oculomo- 
torius, the inferior division of the nerve, runs forward under 
the opticus and there separates into several branches, all of 
which enter the obliquus inferior, that muscle being called by 
Fiirbringer the obliquus anterior. The branches all enter the 
muscle on its outer surface, according to Fiirbringer’s text, but 
on its upper or inner surface, as his Fig. 19, giving the arrange- 
ment in Petromyzon marinus, plainly shows. Of the several 
branches that enter the obliquus, two pass through it to supply 
the rectus internus, which lies immediately below the obliquus, 
and arises not far from it near the front end of the orbit. 
In Dipnoi the muscles of the eye are small and the obliqui 
are said to be wanting in certain species. In Protopterus 
annectens, where all six muscles are found (No. 88, p. 281), 
