260 THE RETINA, BY MAX SCHULTZE. 



magnitudes that are not beyond the limits perceptible and measurable 

 by the microscope, the statements above made will scarcely appear 

 to be too venturesome. Comparative anatomical investigations will 

 obviously prove of the very greatest value in this research. Whatever 

 modifications the eyes of animals may present in regard to their 

 structure and development, we may presume there is just such an 

 agreement between the structure of the terminal organs of the nerves 

 and the acessory apparatus, adapting them for the conversion of 

 luminous waves into nerve force, as that which we see in the auditory 

 organ, where delicate hairs auditory hairs project into a fluid. We 

 shall here therefore endeavour to give a short resume of the state of 

 our information respecting the terminal apparatus of the optic nerves 

 in animals, with a reference at the same time to the physiological 

 value of the differences observed. 



All Vertebrata that are able to see, with the single exception perhaps 

 of the Amphioxus, the eyes of which present a much lower grade of 

 development, possess a retina with a layer of rods and cones similar 

 to that of Man. Now, although as a general rule the cones are recog- 

 nizable by their ventricose internal segment and conical outer segment, 

 and are mingled with rods, as in the retina of Man, there being no 

 difficulty in distinguishing between the two either in him or in the 

 Monkeys, Pig, Ruminants, and most Osseous Fishes ; still cases occur 

 in which the rods and cones resemble each other much more closely, as 

 in the Guinea-pig and Rabbit, where the inner segments of the cones 

 are scarcely thicker or in any other way unlike the rods, so that they 

 can only be distinguished by the characters of their outer segments. 

 Transitional forms are moreover seen in the Tritons* and in a less de- 

 gree in the Frog, in both of which the outer segments of the rods are 

 conical. In Birds the cones are very slender and rodlike. The ex- 

 ternal segment is much elongated, and is not always very distinctly 

 conical. Though it would appear from this that the distinction 

 between the rods and cones is not always sharply marked in the 

 animal kingdom, there are nevertheless always other characteristics 

 by which the two structures may be distinguished from one another. 

 Amongst these special characteristics are the highly refractile spheroids 

 of fatty substance found in the Birds, which for the most part 

 have a red or yellow colour, and are present in all the cones, whilst 

 they are absent in the rods. They are situated in the internal 

 segment, at the point of its junction with the outer segment, and 

 are so large, and so completely fill each cone at this point, that it 



* Max Schultze, Archiv fur Mikroskop. Anatomie, Band iii., p. 237. 



