614 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
their outer portions. The opposite ends pass rather abruptly into the 
very slender rod fibers. Each fiber somewhere in its course expands 
to inclose the nucleus, and finally terminates in a pyriform enlarge- 
ment. The nucleus in preserved specimens may have its chromatin 
arranged in a few broad transverse bands. 
Every cone cell consists of a cone, a cone fiber, and a nucleus. The 
cones like the rods are divisible into outer and inner segments. The 
outer segment is usually shorter than that of the rod (12 ») and 
tapers somewhat to its rounded extremity. It never contains visual 
purple, but otherwise, as for example in breaking into transverse 
disks, it resembles the outer segment of the rod. The inner cone seg- 
ment bulges like the body of a flask. It is divided into an outer, 
longitudinally fibrillar edpsoid portion, and an inner contractile 
myoid portion. The noncontractile ellipsoid is said to become 
strongly eosinophilic 
in the dark. Because 
of the myoid  sub- 
stance the cones, un- 
like the rods, may 
alter their length. 
The contractility is 
said to be less in man 
than in the pig, and 
less in the latter than 
in some amphibia and 
fishes where the myoid 
segment is reported to 
Fic. 1.—A, diagram of human rod cells and cone cells from shorten from 50 p to 
the equatorial part of the retina. B, cone cells from the 5 [le The nuclei are 
fovea, drawn on the same scale. ‘ 
found in a mass of 
protoplasm near the base of the cone; beyond the nucleus the pro- 
toplasm forms a cone fiber which is thicker than that of a rod and 
which ends in a branched and expanded base. 
The stimuli received by the outer segments of the rods and cones 
are transmitted through their fibers to the nerve cells of the retina, 
and thence to the brain. <A single retinal nerve cell receives the 
stimuli from several rods and cones. 
Since rods and cones are believed to have different relations to 
the perception of color, their distribution in man and other animals 
should be significant. In the peripheral portion of the human retina 
rods are in excess, so that in sections three or four rods appear be- 
tween every two cones. Near the depression, or fovea, where vision is 
most acute, rods and cones are equally abundant, and in the fovea 
itself only cones are found. These cones, however, are strikingly 
