DISSECTION OF THE EYEBALL. 271 



veins converge in whorls — the vasa vorticosa — to join four or five 

 principal trunks. The ciliary processes have the same structure 

 as the choroid. Each contains a rich plexus of tortuous vessels. 

 The branched cells at the anterior end of each process are without 

 pigment. In the area of the tapetum a thin unpigmented stratum 

 of connective-tissue fibres separates the layer of arteries and veins 

 from the tunica Ruyschiana, which is also free from pigment in 

 this position. In the eyes of albinos the choroid is entirely free 

 from pigment. 



The Ciliary Nerves are efferent branches of the lenticular ganglion. 

 They perforate the sclerotic in company with the ciliary arteries, and 

 run forwards between the sclerotic and cornea. They give branches to 

 the cornea and ciliary muscle, and terminate in the iris. They contain 

 sensory fibres, which are derived from the ophthalmic division of the 

 5th nerve ; motor branches to the ciliary muscle and sphincter muscle 

 of the pupil, which come from the third nerve ; and motor fibres to the 

 dilator muscle of the pupil, which are derived from the sympathetic 

 system. 



Directions. — In the immersed eye from which the cornea and part 

 of the sclerotic have been removed, the portion of choroid exposed 

 is to be torn away with two pairs of forceps from the subjacent 

 retina. The inner surface of the membrane will be seen, through 

 the transparent vitreous humour, in the submerged half of the eye 

 that was frozen. 



The Retina is the most delicate of the coats of the eyeball. It is 

 formed by the radiation of the optic nerve on the inner surface of the 

 choroid, and, like that coat, it is bell-shaped. Its external or choroidal 

 surface is covered by a layer of hexagonal pigment cells, which were at 

 one time referred to the texture of the choroid. Its inner surface is 

 moulded on the vitreous humour. This surface shows a little below 

 and external to the summit of the bell, or of the antero-posterior axis of 

 the eyeball, a disc-like elevation — the papilla optica, which is the point 

 at which the optic nerve begins to expand. From the margin of the 

 papilla fine arterial twigs derived from the arteria centralis retinae radiate 

 outwards in the innermost stratum of the membrane. The nervous 



structures of the retina terminate at a wavy line — the ora serrata 



behind the ciliary processes ; but the retina is continued beneath these 

 processes in the form of an epithelial layer — the pars ciliaris retina;, 

 which forms the edge of the bell. 



In the human eye a yellow spot — the macula lutea — is placed a little 

 external to the papilla optica, and almost exactly in the antero-posterior 

 axis of the eyeball. This is not present in the eye of the horse or in 

 any mammal lower than the quadrumana. 



The perfectly fresh retina is translucent, and of a pale pink colour, 



