272 GENERAL ORNITHOLOGY part ii 



parency, which is in turn set between two layers of the hyaloid 

 membrane to be presently noticed. Where these layers of hyaloid 

 separate around the rim of the capsule to form the investment, a 

 small space is left between them ; this circular tube around the lens 

 is the umal of Petit, k, h The lens is stationed in the axis of 

 vision ; some suppose it to be equally stationary in any transverse 

 axis. It is, however, difficult to understand how an object thus 

 suspended in fluctuating humours should be insusceptible of some 

 motion backward or forward as well as of alteration in its degree of 

 convexity ; both of which may be factors in the focusing process. 

 From what has preceded, it is evident that the cavity of the eye is 

 divided into anterior and posterior compartments, or chambers, by 

 the reflection, from the sclerotic wall, of the choroid, hyaloid, and 

 iridian structures, which with the lens form a vertical partition. 

 Each chamber is filled with a fluid of different density and con- 

 sistence. That in the anterior or corneal chamber is thin and 

 watery, and therefore called the aqueous humour; that in the sclerotic 

 cavity is more dense and glassy, and for this reason known as the 

 vitreous humour. There is much less aqueous than vitreous ; but 

 birds have comparatively more of the former than usual, OAving to 

 the relatively greater size and convexity of the cornea. The waters 

 are enclosed in exceedingly delicate membranes ; the vitreous in the 

 hyaloid membrane, e, which, besides lining the posterior chamber and 

 enclosing the lens as already said, sends thin partitions all through 

 the vitreous humour to steady these glassy waters. 



The ojytic nerve, a, of birds is peculiar. In mammals, as a rule, 

 the nerve is a smooth cylinder, proceeding straight to the sclerotic, 

 penetrating the coats of the eyeball directly, near the middle point 

 behind, and then spreading out on the inside of the ball as a large 

 circular concave mirror. This thin, saucer-like expansion of nerve- 

 tissue is the retina. In birds the optic nerve is a fluted column, 

 which approaches the eyeball quite obliquely, strikes it at a point 

 eccentric from the axis of the eye, and does not at once pierce the 

 sclerotic. Tapering to a fine point, and running still obliquely, 

 downward and forward, in a deep groove in the sclerotic that would 

 be a tube were it not split, and through a similar slit in the choroid, 

 a fluting of the nerve rises to attain the cavity of the eye, and the 

 retina spreads out from the sides and end of this fold. But the 

 prime peculiarity of a bird's eye is the " purse " or " comb," mar- 

 supium, pecten, f; a very vascular structure, like the choroid, and 

 likewise painted black ; apparently " erectile," that is, capable of 

 increasing and diminishing in size by influx and efflux of blood. It 

 is attached behind to the nervous structure ; is suspended in the 

 vitreous humour, and runs forward obliquely a part or the whole of 

 the way to the lens, to the envelope of which it may be attached in 



