No. 3.] MUSCLES AND NERVES IN AMIA CALVA. 5i7 
larvae from 12 mm. in length upward the muscles have, as 
already described, a similar origin, arising immediately below, 
or in the younger larvae immediately posterior to, the fenestra. 
The rectus externus (ve) arises, as already stated, near the 
hind end of the eye-muscle canal. The three other recti 
muscles (7s, 77¢, and vzf) arise from the lateral face of the 
basisphenoid, and hence at the external opening of, or partly 
within, the orbital opening of the eye-muscle canal. In larvae 
the internus has the most anterior origin, the inferior next, and 
the superior the most posterior. The rectus inferior is said 
by Sagemehl (No. 106, p. 87) to arise in the Characinidae in 
the anterior portion of the eye-muscle canal. 
The obliquus superior is innervated by the nervus troch- 
learis (¢7), which pierces the lining membrane of the upper, 
lateral chamber of the eye-muscle canal, and enters that 
chamber in front of and slightly above the main trigeminal 
root and the root of the ophthalmicus profundus (Fig. 26, 
Pl. XXV). It lies, while in the chamber, internal to the 
ophthalmic branches of the facialis and trigeminus, and leaves it 
through the upper end of the large orbital opening of the canal. 
It then runs forward along the inner wall of the orbit, dorsal 
to all the recti muscles, and separating into two nearly equal 
parts, each of which again separates into two parts, it enters 
the obliquus superior on its upper surface, near the middle of 
its length. In its course through the orbit it lies below and 
internal to the ophthalmic branches of the facialis and tri- 
geminus, above and internal to the profundus ganglion, and 
above the oculomotorius. No branches of it except those 
entering the muscle were found, nor were there any commu- 
nicating or connecting branches to or from any of the other 
nerves or ganglia. The same is true of the nerve in the 
smallest larvae examined ; the nerve, however, often passes in 
sections so close to the portio ophthalmici profundi that it is 
impossible to say that there is there no interchange of fibres. 
No ganglion or ganglion cells were found at any point in the 
nerve, either in the adult or in larvae. 
The rectus externus is innervated by the nervus abducens 
(a6), which runs forward and then forward and outward in the 
