No. 3.] MUSCLES AND NERVES IN AMIA CALVA. 507 
3. Nervus Opticus. 
The optic nerves (0, Figs. 22-27, Pls. XXIV and XXV) in 
the adult have a well-developed chiasma and a long tractus or 
pedicle, on each side of the head, connecting the chiasma, along 
the side of the brain, with the lower anterior end of the optic 
lobe. 
Whether the chiasma is entirely free from the base of the 
brain or not was not investigated. Each nerve has, as Goro- 
nowitsch states (No. 50, p. 443), the appearance of a much- 
plaited plate, folded upon itself so as to form a cylindrical or 
oval, nearly closed gutter. The open line between the free 
edges of the plate lies along the lateral and posterior edge or 
surface of the nerve, and, proximally, the free edges of the 
plate open and embrace the tractus. The fibres contained in 
the upper of these free edges, in part at least, run upward along 
the anterior edge of the pedicle and do not enter the chiasma 
or cross to the opposite side of the brain. The same is pos- 
sibly true of other fibres of the nerve. The fibres along the 
median and anterior edge of the nerve run directly into the cor- 
responding ones of the nerve of the opposite side. 
The nerves in the adult run outward and forward into the 
extreme hind end of the orbit, with the recti muscles. In 
young specimens they run outward, nearly at right angles to 
the axis of the body, a little in front of the place of origin of 
the superior, inferior, and internal recti, and not far from the 
middle of the orbit, as in adult selachians. They lie imme- 
diately behind the tough, dense pad of tissue already described 
as extending transversely from orbit to orbit. The position of 
this pad, and its appearance in sagittal sections of the young, 
correspond closely to that of the mass of fibres, a.c., described 
and figured by Balfour and Parker, in Lepidosteus (No. 6, p. 
378, and Fig. 45). These fibres in Lepidosteus are considered 
by them to be nervous, and to be probably equivalent in part 
to the anterior commissure of the brain. 
The brain in the young of Amia entirely fills the cranial 
cavity ; in the adult it occupies a portion only of the middle 
and hind part of that cavity, the remaining and larger portion 
