VASCULARITY OF THE RETINA. 435 



nus. According to the latter, when the optic nerve has pene- 

 trated through the sclerotic and choroid to the retina, its cylin- 

 ders or nervous tubes spread themselves out on every side, 

 either singly or in bundles ; each cylinder, or collection of 

 tubes bending inwards through the vascular layer, and termi- 

 nating in the form of a papilla on the surface of the vitreous 

 humor. In the eye of the halibut, the fibrous structure of the 

 retina appears very manifest. 



Considering the mode of formation of the retina, as described 

 above, its great sensibility to light, and the known insensi- 

 bility of the optic nerve to that stimulus if the rays-fall on it in 

 any part of its course without first impinging on the retina, this 

 tunic cannot be regarded as a mere expansion of the optic 

 nerve, any more than the nerves themselves can be viewed 

 as a mere expansion of the brain. It may better be viewed 

 as a peculiar structure, expressly provided for the purpose of 

 receiving the impression made by the rays of light ; the nerve 

 being the mere agent by which this impression is transmitted to 

 the brain, where it becomes a sensation. 



Vascularity of the Retina. 



It receives its supply of blood principally from the arteria 

 centralis retina of Zinn, a branch of the ophthalmic which en- 

 ters the optic nerve at a short distance from the ball, and passes 

 through the central foramen in the cribriform plate of Albinus, 

 vide p. 410. Its distribution is principally to the internal vas- 

 cular layer of the retina, and in the healthy state contains only 

 the white portion of the blood. The blood is returned in veins 

 which terminate in a single trunk that passes out through the 

 cribriform plate. 



[Within three or four years a membrane has been discovered 

 by Mr. Jacobs, Demonstrator of Anatomy in Trinity College, 

 Dublin, which is situated between the retina and the tunica 

 choroidea. Its tenuity and delicacy are extreme. It is dia- 

 phanous and is thought by its discoverer to be a serous mem- 

 brane. It extends from the optic nerve as far forwards as the 



