688 ORGANS OF SENSE. 



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-celh 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-corpuscles 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 Ileinrich 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 connected 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 areola3 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 retinue 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 mem- 

 brane 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 ter- 

 minate 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 circumference, 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 capillaries 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 resem- 

 bles more a staining of the constituent parts. 



AQUEOUS HUMOR. 



The Aqueous Humor completely fills the anterior and posterior chambers of 

 the eyeball. It is small in quantit}' (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 i><>s/,'ri<ir clntmher, smaller than the anterior, is bounded in front by the 

 iris; behind, by the capsule of the lens and its suspensory ligament, and the 

 ciliary processes. 



In the adult, these two chambers communicate through the pupil; but in the 



