66 DR KNOX on the Comparative Anatomy of the Eye. 



membrane, which may be considered as an expanded transparent 

 marsupium *. Viewing the anatomy of these parts in the com- 

 parative range of animals, we observe, that, in the cat's eye, the 

 retina passes onwards to its destination unaided by any very dis- 

 tinct membrane ; the vitreous humour is eminently transparent, 

 and the circular point by which the optic nerve penetrates the 

 sclerotic is translucent. If the choroid transmit any membrane 

 across the termination of the optic nerve, it must be excessively 

 thin, and, to common glasses, altogether inappreciable. But, al- 

 ready in the horse, the passage of the choroid over the optic 

 nerve, and the escape of the medullary substance, to form or be 

 connected with the retina by innumerable small foramina, is vi- 

 sible to the naked eye. In the eye of some fishes, as the cod, 

 the principle is the same, though somewhat varied ; the retina 

 is divided into two portions, betwixt which a prolongation of the 

 choroid passes, to form a distinct, though partly colourless 

 marsupium. Lastly, In birds, we find a true coloured marsu- 

 pium f. 



I have found it more difficult to decide on the functions of 

 the marsupium than on its true anatomy ; but if we cannot shew 

 what it is, we can at least demonstrate what it is not. As it is 

 in no way muscular, it cannot serve to alter the relation of the 

 internal parts of the eye to each other, but it may assist in re- 

 taining the lens in its situation, and give support to the numer- 

 ous vessels proceeding to it. It has no nerves. 



* May not this membrane be the same with the dark coloured tunic, formed be- 

 tween the retina and vitreous humour, in the larger varieties of the cephdlopodous 

 mollusca ? 



f- It has appeared to me, that, in many animals, and even in some of the rumi- 

 nantia, the central artery of the retina does not send any branch through the centre 

 of the vitreous humour. 



