CHAMBER OF AQUEOUS HUMOR. 663 



The posterior ciliary veins (fig. 231) are commonly four in number, 

 and the brandies entering these trunks form arches (h) (vasa vorticosa) 

 on the surface of the choroid coat: they perforate the sclerotic layer at 

 separate points, midway between the cornea and the optic nerve, and end 

 in the ophthalmic vein. 



Anterior ciliary veins begin in a plexus within the ciliary muscle, and 

 accompany the arteries through the sclerotic to end in the ophthalmic: 

 they communicate with the venous space of the sinus circularis iridis (p. 

 656). 



The ciliary nerves (fig. 231, /") are derived from the lenticular ganglion, 

 and the nasal nerve (p. 55). Entering the back of the eyeball with the 

 arteries, they are continued with the vessels between the sclerotic and 

 choroid, nearly as far as the ciliary muscle: at this spot the nerves send 

 offsets to the cornea, and piercing the fibres of the ciliary muscle, enter 

 the iris, but their manner of ending is unknown. Offsets from the nerves 

 supply the ciliary muscle and the choroid. 



CHAMBER OF THE AQUEOUS HUMOR (fig. 227). The space between 

 the cornea in front and the lens behind, in which the iris is suspended, 

 contains a clear fluid named the aqueous humor. In the foetus before the 

 seventh month this interval is separated into two by the iris and the pu- 

 pillary membrane, but in 'the adult it is only partly divided, for the two 

 communicate through the pupil. The boundaries of the two chambers 

 may be seen in the eye on which a section has been made. 



The anterior chamber is the larger part of the space; it is limited in 

 front by the cornea, and behind by the iris. 



The posterior chamber (i) is a narrow interval behind the iris at the 

 circumference, which is bounded in front by the iris; behind by the lens 

 capsule, and by a piece of the membrane (suspensory ligament of the lens) 

 on the front of the vitreous humor : and at the circumference by the 

 ciliary processes. 



The aqueous humor is quite transparent, and consists nearly of pure 

 water. A small quantity of chloride of sodium, with some extractive 

 matter, is in solution in it. 



THE RETINA (fig. 227, e). This layer (tunica nervea) is in part con- 

 tinous with the optic nerve, and is the most delicate of all the coats of the 

 eyeball. On it the image of objects is formed in the bottom of the eye. 



Dissection. The retina can be satisfactorily examined only on an eye 

 which is obtained within forty-eight hours after death. To bring it into 

 view on the eyeball in which the choroid coat was dissected, the choroidal 

 covering must be torn away carefully with two pair of forceps, whilst the 

 eye is immersed in water or spirit. 



The retina is the most internal of the three concentric strata in the 

 globe of the eye, and is situate between the choroid coat and the trans- 

 parent mass (vitreous) in the interior. It is moulded upon, and supported 

 by the vitreous body; and its form is that of a segment of a sphere, with 

 a large aperture in front. Beginning behind at the optic nerve (fig. 227, 

 c) the thin layer extends forwards to the ciliary processes (their outer ex- 

 tremities), where it ends in a wavy border the ora serrata. 



Where the retina ceases in front, a thin gray layer (ciliary part of the 

 retina) composed of elongated nucleated cells, which are not nerve ele- 

 ments, is continued on as far as the tips of the ciliary processes (fig. 235, 

 j) on which it ends. 



The retina is of a pinkish gray color, and is semitransparent when 



