226 NUNNELEY, ON THE RETINA. 
regarded it as a protecting membrane, imterposed between 
the nervous retina and the choroid. This notion became 
commonly adopted, and was much extended by other anato- 
mists, some of whom, particularly the late Mr. Dalrymple, 
argued very strenuously for its being a true serous membrane, 
constituting a shut sac like the pleura or arachnoid, a view 
he supported by arguments, drawn from analogy and patho- 
logical conditions, which he thought satisfactory. Of course, 
now that its structure is so far known, this idea must be 
altogether dismissed. It is most easily detached in trans- 
lucent shreds from the other elements of the retina as soon 
as decomposition begins, or on immersion in water. When 
this occurs it is evident that the structure has undergone a 
change ; the rods are then altered. (Plate X, fig. 3.) 
2. The cones or bulbs constitute the second layer of the 
retina; however, as these bodies when found are always placed 
amongst the rods towards their inner end, they are hardly to 
be considered as forming a distinct layer, though for con- 
venience of description it is necessary to speak of them 
separately. Regarding them there is even more difficulty 
and uncertainty than with the rods. As to the latter no one 
can doubt their existence in the whole of the vertebrate 
division, whatever differences of opinion may be entertained 
as to their form and connexion, but the very existence of the 
bulbs, in by far the greater number of animals, is open to 
considerable doubt; and of their form, number, and con- 
nexion, to much more. Hannover, who has given the most 
elaborate description and figures of them, from their form, 
denominates them cones jumeaux, coni gemini—twin cones, 
and says they consist in the fish, where they are most 
developed, of double cylindrical bodies, two or three times as 
large as the rods, placed side by side: that each of these 
cylinders is divided into two equal parts; an internal one, 
smooth and round, as though enclosed in a delicate capsule, 
separated from the external half by two transverse lines ; 
and an external moiety, composed of a mass of minute 
granules, and terminating outwardly in two conical points. 
That after a time, or on the addition of a liquid, the inner 
cylindrical portion becomes larger and fusiform, bilobate like 
a coffee-berry and granular, while the conical points fall off, 
curve themselves into hooks, and often altogether disappear. 
That the cones jumeauex, like the rods, are planted perpen- 
dicularly to the other elements of the retina and choroid, 
that each cone jumeau is completely surrounded by a regular 
number of the rods, and that each of the two conical points 
is, like the filaments of the rods, received into a membranous 
