SCHULTZE, ON THE RETINA. 31 
in the relative thickness of the ‘rod-’ and ‘ cone-’ filaments, is not 
apparent in birds or amphibia. How the case may be in those 
reptiles which possess both elements has not yet been ascertained.” 
The author then enters upon the question of the physio- 
logical relations of the “‘rods’” and “ cones ;” and the fol- 
lowing may be taken as a very brief summary of his highly 
interesting observations on this point. 
The organization of the “ yellow spot,” and of the fovea 
centralis, im the human retina, clearly proves that the cones 
alone are not only sufficient for vision, but also that they 
possess certain physiological advantages over the “rods.” 
But it is, at the same time, obvious. that the “ rods” alone 
suffice for the purpose of vision, since the bat and other 
mammals are wholly unprovided with “cones.” But these 
mammals without cones in the retina prefer the dusk or night 
to daylight. The question, consequently, arises, what im- 
pression communicated through the retina in the dusk is 
useless ‘/—by the solution of which we may be guided to some 
conclusion with regard to the peculiar function of the 
““ cones.” 
The visual sense comprises three fundamental impressions, 
which have been termed by Aubert “ Lichtsinn, ‘‘ Farbensinn,” 
and ‘‘ Raumsinn ;” that is to say, “ light-sense,” ‘ colour- 
sense,’ and “ space-sense.” It as at once obvious that the 
light-sense, or the power simply of perceiving luminosity, in- 
cluding [perhaps] quantitative differences in the degree of light, 
is a fundamental requirement in any, even the simplest, visual 
organ. For this purpose, it is clear that a single termination 
of a nerve, or, in other words, in the case of the retina of the 
higher animals, a single rod or cone, would suffice. And it 
may also be admitted that a number of such visual points, 
associated so as to form a single percipient organ, would, in 
addition to the simple perception of light, also give the power 
of estimating space, and consequently of conveying ideas of 
form. These two faculties of the perception of light and of 
space as conveyed by light are inherent in the eyes of all 
vertebrates. The “‘ coneless” retina of the bat, hedgehog, and 
mole, does not, in this respect, differ from the “ rodless”’ retina 
of snakes and lizards, seeing that the ‘cones’ are, at any 
rate, quite as fully percipient of light as the “ rods,” inas- 
much as they equally represent the termination of sentient 
nerves. It may be assumed that the mere sense of luminosity 
is more strongly developed in nocturnal animals, as the bat, 
than it is in the sunshine-loving snake; so that the former 
would find a sufficiency of light when the latter was in dark- 
ness. This would seem to indicate that the ‘“ rods’ were 
