58 



chiaria, the middle or the membrana vasculosa Halleri, and the 

 external with its shining pearly lustre passes in front of the iris 

 and produces the gold and silver colours so much admired. The 

 cornea is remarkable for its flatness, the iris for its immobility, and 

 the crystalline lens for its density, magnitude, and sphericity. The 

 aqueous humour is scanty, the ciliary processes are rarely deve- 

 loped, and the yellow spot of Soemmerring is altogether wanting. 

 Between the layers of the choroid, and embracing the optic nerve, a 

 reddish mass of a horse-shoe shape is found in most fishes named 

 choroid gland, which, according to some, secretes the colouring 

 matter ; according to others it is a sort of rete mirabile, and others 

 again look upon it as an enlargement of the optic nerve, whilst the 

 prevailing opinion at present seems to be, that it is a muscle 

 destined to modify in some way the reflection of the rays of light. 

 From the inner layer of the choroid another body, campanula 

 Halleri, passes forwards towards the lens, somewhat analogous to 

 the marsupium of birds. The pupil in fishes is large and round, 

 and the eye is moved by four recti and two obliqui muscles. 



The eyes of the amphibia are very large, and like those of 

 fishes contain a small quantity of aqueous humour, and are flat in 

 front. They enjoy great latitude of motion and are provided with 

 palpebrae and a membrana nictitans; the superior lid is small and 

 scarcely movable; the inferior large and very movable. The 

 opaque integument veils the front of the eye in the proteus ; in 

 most other characters the eyes resemble those of fishes. 



The reptilia present us with characters of eyes well suited to 

 the rare medium they inhabit, such as convexity of cornea from 

 abundance of humours, and a compressed state of the lens. .Like 

 the amphibia, they are furnished with three lids, but unlike them 

 and fish, they are provided with lachrymal apparatus; they ap- 

 proximate the succeeding classes in many respects, as having ciliary 

 processes, a movable iris, generally a vertical pupil, a dark pecten 

 prolonged from the choroid into the vitreous humour, and osseous 

 plates around the anterior margin of the sclerotic. In the chame- 

 leon only a small portion of the eye is to be seen through a narrow 

 vertical slit between the margins of the lids, which conceal a large 

 membrana nictitans. 



We cannot fail to observe how the remarkable peculiarities pre- 

 sented by the organs of vision in birds, coincide with the vigour of 

 their respiratory, circulatory, and locomotive systems. In all the 

 other vertebrate classes we meet with instances, where the eyes, if 

 not absent, are at least rudimentary, but in this class they are 

 remarkable not only for their uniform existence but for their great 

 size and perfect development. From the convexity of the anterior 

 segment of the eyes and their lateral location, birds command an 

 extensive sphere of vision, and in many of the high-flying rapa- 

 cious kind, the orofan is prolonged in front into a tubular form, but 

 in aquatic birds the anterior half is more flattened. The sclerotic 

 is thin, flexible, and elastic posteriorly ; it is divisible into three 



