28 SCHULTZE, ON THE RETINA. 
It would appear that in the sheep and other mammals the 
“rods” are not developed until the differentiation of the 
other parts of the retina has advanced some way, nor before 
the end of embryonal dife; but in some instances, as in the 
rabbit and cat, this is seen in a far more striking manner. 
Neither of these animals at birth present any trace of “ rods” 
and ‘cones.’ The blindness, therefore, of the new-born 
rabbit and kitten does not depend solely upon the closure of 
the lids, but is also associated with an undeveloped state of 
the retina itself. The “rods” and “ cones” do not appear 
to be fully developed before the thirteenth day, when they 
are in the same condition as in the calf or lamb at birth. 
The development of the “rods” and “cones” in man 
appears to follow the same course as in the ruminants above 
named. 
The fourth section relates to the differences between the 
“rods”? and “ cones,” with respect more especially to their 
functions. And in it is given a recapitulation of the principal 
anatomical facts upon which the physiological conclusions 
or suppositions are based, in the following words : 
“ With the enlargement of our knowledge of the structure and dis- 
position of the two elements composing the percipient layer of the 
retina—the ‘rods’ and ‘ cones’—arises the question whether we are - 
thus in a condition to attempt the problem of the hitherto unknown 
physiological distinction between them. We hear that, at any rate, 
the direction in which the solution of this question is to be sought 
may now be indicated with some degree of certainty, and I will en- 
deavour briefly to state my views, as follows: 
«The anatomical facts upon which we have to rely, shortly recapi- 
tulated, are these: 
“1, The difference in size andform. This is manifested more parti- 
cularly in the so-termed inner segment, which in the ‘ rods’ is always 
sharply defined from the outer segment, but which may also be dis- 
tinguished as a separate element also in the ‘cones.’ The inner 
segments in both the ‘rods’ and ‘ cones,’ in the perfectly fresh con- 
dition, consist of an apparently almost structureless substance, but 
which very rapidly, after death and in all preservative media, coagu- 
lates into a more or less distinctly granular matter. This substance, 
to judge from micro-chemical reactions, most nearly resembles 
albuminous matter, as, for instance, the protoplasm of young cells. 
An essential distinction between the substance of the inner segment 
of the ‘rods’ and of the ‘cones’ is manifest in the circumstance that 
solutions of osmic acid of a certain strength produce in that of the 
cones a very distinct parallel striation, which, under similar conditions, 
I am unable to perceive in the inner segments of the ‘rods.’ No 
universal distinction exists in-the absolute diameter of the inner 
segments, as, for instance, in the human retina; for although the 
cones throughout by far the greater part of the retina are fully twice 
as thick as the rods, their inner segments in the fovea centralis are quite 
as slender as those of the ‘rods.’ The outer segments or shafts consist 
of a more highly refracting substance, which after death coagulates in 
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