C22 ORGANS OF THE SENSES. 



course towards the anterior margin of the retina the "bundles interlace, forming a 

 delicate net, with flattened elongated meshes. The nerve fibres which form this 

 layer differ from the fibres of the optic nerve in this respect ; they lose their dark 

 outline, and their tendency to become varicose, and consist only of the central 

 part or axis of the nerve tubes. The mode of termination of the nerve fibres is 

 unknown. According to some observers, they terminate in loops ; according to 

 others, in free extremities. Recent observers have stated, that some of the nerve 

 fibres are continuous with the caudate prolongations of the nerve cells external to 

 the fibrous layer. The nerve cells are placed on both sides of the fibrous layer, 

 but chiefly upon its inner surface, and imbedded within the meshes formed by the 

 interlacing nerve fibres ; they are round or pear-shaped transparent cells, nucleated, 

 with granular contents, furnished with caudate prolongations, some of which join 

 the fibres of the optic nerve, whilst others are directed externally towards the 

 granular layer. It is probable that these cells are identical with the ganglion cor- 

 puscles of vesicular nervous substance. 



An extremely thin and delicate structureless membrane lines the inner surface 

 of the retina, and separates it from the vitreous body ; it is called the membrana, 

 limitans. 



The radiating fibres of the retina, described by Heinrich Miiller, consist of 

 extremely fine fibrillated threads, which are connected externally with each of the 

 rods of the columnar layer, of which they appear to be direct continuations ; and, 

 passing through the entire substance of the retina, are united to the outer surface 

 of the membrana limitans. In their course through the retina, they become con- 

 nected with the nuclear particles of the granular layer, and give off branching 

 processes opposite its innermost lamina ; as they approach the fibrous expansion 

 of the optic nerve, they are collected into bundles, which pass through the areolaa 

 between its fibres, and are finally attached to the inner surface of the membrana 

 limitans, where each fibre terminates in a triangular enlargement. 



The arteria centralis retinas and its accompanying vein pierce the optic nerve, 

 and enter the globe of the eye through the porus opticus. It immediately divides 

 into four or five branches, which at first run between the hyaloid membrane and 

 the nervous layer ; but they soon enter the latter membrane, and form a close 

 capillary network in its substance. At the ora serrata they terminate in a single 

 vessel, which bounds the terminal margin of the retina. 



The structure of the retina at the yellow spot presents some modifications. 

 Jacob's membrane is thinner, and of its constituents only the cones are present ; 

 but they are small, and more closely aggregated than in any other part. The 

 granular layer is absent over the fovea centralis. Of the two elements of the 

 nervous layer, the nerve fibres extend only to the circumference of the spot ; but 

 the nerve cells cover its entire surface. The radiating fibres are found at the cir- 

 cumference, and here only extend to the inner strata of the granular layer. Of 

 the capillary vessels, the larger branches pass round the spot, but the smaller capil- 

 laries meander through it. The color of the spot appears to imbue all the layers 

 except Jacob's membrane ; it is of a rich yellow, deepest towards the centre, and 

 does not appear to consist of pigment cells, but resembles more a straining of the 

 constituent parts. 



HUMORS OF THE EYE. 

 AQUEOUS HUMOR. 



The Aqueous Humor completely fills the anterior and posterior chambers of 

 the eyeball. It is small in quantity, scarcely exceeding, according to Petit, four or 

 five grains in weight, has an alkaline reaction, in composition is little more than 

 water, less than one-fiftieth of its weight being solid matter, chiefly chloride of 

 sodium. 



The anterior chamber is the space bounded in front by the cornea ; behind, by 

 the front of the iris and ciliary ligament. 



The posterior chamber, smaller than the anterior, is bounded in front by the 



