THE MYOLOGY OF POLYODON 109 
which arises from the cranial wall behind the foramen of the 
optic nerve and in front of the ganglion of the trigeminus. There 
is no canal for this part of the muscles to pass through. As 
they diverge from one another, the external and superior rectus 
are the more dorsal and at the same time the external and infe- 
rior are the more lateral. The superior rectus is inserted immedi- 
ately under and in part posterior to the insertion of the superior 
oblique. The inferior rectus, which is usually the largest of them 
all, is inserted ventrally at a point somewhat posterior to the 
insertion of the inferior oblique. The external rectus has its 
insertion near the posterior corner of the eye about midway 
between the insertions of the two foregoing. The internal rectus 
is more central than any of the others. It is also the smallest of 
the eye muscles. It passes forward ventrally to the optic nerve 
and is inserted slightly anterior to its point of emergence from 
the eye. 
Innervation. The muscles are innervated by the usual nerves. 
The trochlear nerve emerges from the cranium through a small 
foramen above and somewhat anterior to that of the optic nerve. | 
It runs forward some distance closely appressed between the 
cranial wall and the protractor hyomandibularis muscle. Ante- 
riorly it comes out across the ventral face of the muscle and 
crosses into the orbit to supply the superior oblique. The oculo- 
motor nerve also passes through a foramen of its own in leaving 
the cranium. Medial to the trigeminus it comes into intimate 
relation with the abducens. An anastomosis between the two 
may take place but it could not be demonstrated. As it enters 
the orbit a dorsal branch is supplied to the superior rectus while 
the main portion of the nerve continues outward along the antero- 
mesial margin of the inferior rectus. It supplies numerous twigs 
to this muscle and gives rise to a rather complicated plexus in 
the floor of the orbit from which branches rise to supply the 
internal rectus and inferior oblique. The abducens nerve leaves 
the cranium beneath the posterior part of the trigeminal ganglion. 
It runs forward median to the ganglion and nerve and comes 
into relation with the oculomotor as described above. It is 
distributed to the external rectus. 
