THE ORGANS OF SENSE. 119 



pigmented layer being a continuation forwards of the pigment layer of 

 the retina, and the superficial layer consisting of columnar cells. 



THE CONTENTS OF THE EYEBALL. 



The cavity of the eyeball is divided into two unequal portions by the 

 crystalline lens and its suspensory ligament. The larger, posterior 

 space is occupied by the vitreous humor (body) ; the smaller, anterior 

 space by the aqueous humor. 



The crystalline lens is a biconvex, transparent structure, and lies 

 immediately behind the pupil. It is composed of concentrically 

 arranged fibrous laminae, made up of prismatic fibres with serrated 

 edges. The central portion of the lens is firmer and denser than the 

 peripheral portion, wjiich is more jelly-like. The posterior surface has 

 a higher degree of convexity than the anterior surface, and rests in the 

 hyaloid fossa of the vitreous body. The lens is enclosed in a structure- 

 less capsule, which, towards the equator, is continuous on the anterior 

 surface with the suspensory ligament of the lens. The latter is 

 prolonged backwards as the zonula ciliaris, to be attached to the ciliary 

 processes. 



The vitreous humor is transparent and resembles a thin jelly, contain- 

 ing a few scattered delicate fibres. It is enclosed in a delicate capsule, 

 the hyaloid membrane, which in the neighbourhood of the ciliary 

 processes is thickened to form the zonula ciliaris. The latter splits in 

 front into two layers, the anterior being the suspensory ligament of the 

 lens, and the posterior lining the concavity in the vitreous body, the 

 hyaloid fossa, in which the lens rests. A canal runs through the 

 vitreous humor from the optic disc to the posterior pole of the lens, and 

 is lined by a continuation of the hyaloid membrane. This canal 

 contains the hyaloid artery in the embryo. 



The space in front of the lens is divided into an anterior and a 

 posterior chamber by the iris, and contains the aqueous humor, which 

 is a watery fluid, alkaline in reaction, and containing salts, chiefly 

 sodium chloride, with a trace of protein. 



THE NUTRITION OF THE EYEBALL. 



The vascular tunic of the eyeball receives the long and short 

 posterior ciliary arteries and the anterior ciliary arteries. The outer 

 layers of the retina receive nourishment by means of lymph derived 

 from the blood-supply to the choroid. The inner layers of the retina 

 have a direct blood-supply through the distribution of the central artery 

 of the retina, which enters the eyeball with the optic nerve. The 



