492 Miscellaneous, 



MISCELLANEOUS. 



Observations on the Structure of the Retina in certain Animals. 

 By H. MuLLER. 



I HAVE shown in my work upon the Retina, that this part furnishes 

 microscopic characters which may be employed in the systematic 

 distribution of vertebrated animals, to such an extent, that it is often 

 possible to determine the class, the order, and even the genus of an 

 animal from a small fragment of its retina. 



In general the more marked the systematic characters are in the 

 different divisions of a class of Vertebrata, the more do we observe 

 variations in the microscopic characters of the retina. The retina of 

 the Sturgeon presents one of the most remarkable examples of this. 

 In a recent examination I found that the layer of batons in this fish 

 is constituted in accordance with a type foreign to the other Fishes, a 

 type which occurs moreover in the class of Birds. There are two 

 elements, the cones and the batons. The latter are truncated ex- 

 ternally, whilst the internal part passes into a conical point. The 

 fatty drops, which have been mentioned by other observers, do not 

 belong to the batons but to the cones, which I had formerly suspected, 

 and as may be seen in my work above referred to. The cones are com- 

 posed of an internal thicker, and an external thinner part, as in Birds. 

 At the extremity of the former part is the fatty drop, which, except 

 in its less brilliant colour, exactly resembles those which are found in 

 the cones of Birds. We do not at present know any other Fish, of 

 which the retina exhibits this arrangement of cones and batons, 

 exactly similar to that of Birds. But on the one hand it is very re- 

 markable that this type of the retina, proper to Birds, also occurs in 

 certain Reptiles, namely the Tortoises, which, themselves, in this 

 respect differ widely from the other sections of the Reptiles. On the 

 other hand, I may remark, that, amongst Fishes, it is exactly in the 

 orders which also possess the most peculiar characters, that we find the 

 most distinct variations in the elements of the retina. In the Sturgeons 

 the layer of cones and batons is constituted in accordance with the 

 type of Birds ; in the Cyclostoma, as appears from my previous re- 

 searches, there are only simple cones, without batons ; in the Pla- 

 giostoma, on the contrary, I have only found batons and no cones. 

 In the class of Reptiles, we also find very important differences between 

 the Batrachia, the Sauria, and the Tortoises, whilst in the Birds and 

 Mammalia there is a greater uniformity in the general type of the 

 elements referred to, and only slighter modifications. 



Another remarkable point is the presence of nervous fibres with 

 double outlines in the retina in certain animals. It is well known 

 that in the eye of the Rabbit there is a beautiful white radiation 

 especially on the two sides of the entrance of the optic nerve, and 

 many observers have remarked, that fibres are sometimes found else- 

 where which contain a kind of medulla. But, besides the Rabbits, 

 there are many animals in which the optic fibres present a medulla 

 with dark outlines, in a very marked degree. 



I have found that in the Sturgeon, the optic fibres which extend 



