No. 3.] FISUAL, CEELS’ IN VERTEBRATES: 505 
regards their optical properties, they have axes of maximum 
elasticity at right angles to their lengths. As the red of the 
held color is lowered to a yellow when rod segments lie 
parallel to the a axis of the gypsum plate, it determines the 
long axis of the rod as optically the (c axis) positive direction. 
To obtain an immediate basis of comparison, similar tests 
were made on other organic tissues which seemed, or were 
known, to be of a fibrous structure. Thus naked axis cylin- 
ders from the inner surface of the ox retina gave light reac- 
tions like those of the outer segments. The same was true 
of striped muscle fibers from the crayfish, frog.and ox, con- 
nective-tissue fiber from the ligamentum nuche, vegetable 
fibers, longitudinal and radial fibers in hard wood, and central 
fibers of Spanish moss, etc. The rhabdomes from the com- 
pound eyes of crayfishes showed in general the long axis as 
the negative optical direction, but when it is remembered that 
the fibrous structure of these bodies is at right angles to their 
length instead of being parallel to it, it will be seen that they 
agree with the preceding examples in their optical reactions. 
In the slug Limax maximus the distal portion of the visual 
cell, which corresponds with the outer segment in vertebrates, 
displays somewhat complex reactions. In Figure 46 (Pl. 5) 
I have indicated the character of these reactions as made out 
in a fresh preparation. In a diagrammatic way I have also 
shown .the fibril relations, as figured by Dr. Grant Smith (: 06) 
from fixed preparations. It will be seen from the diagram 
that the character of the reaction depends on the direction 
of the neurofibrils. Where the latter are parallel to the a 
axis of the gypsum plate the field color is lowered to a yellow, 
and where the fibrils are at right angles to this axis the field 
color is raised to a blue. Portions of the body in which the 
fibrils would be seen on end are neutral and so do not affect 
the reaction of fibrils in the plane perpendicular to the direc- 
tion of the transmitted light ray. 
From the above observations on fibrillar bodies, and 
especially on those that are nervous structures in which fibrils 
