KETINAL ELEMENTS. 427 



on which they end by slightly-expanded extremities. These radial 

 fibres support the whole structure of the retina, passing between the 

 nervous elements of the inner layer, and also between the capillary 

 network. They may merely be modifications of connective tissue ; but 

 others of the nuclear fibres are distinctly connected with the rods and 

 cones on the one hand, and with the proper nervous elements of the 

 deepest layer on the other, and are themselves probably true nervous 

 structures. The internal, or nervous, layer consists partly of the ex- 

 panded fibres of the optic nerve, which pierce the baoillary and granu- 

 lar layers at the optic colliculus, and then spread out, to form the 

 retinal network, in which the nerve-fibres, losing their double outline, 

 and retaining only their central or axial fibres, are arranged in fine 

 meshes among the radiating fibres of Miiller. The nerve-fibres are 

 here very fine, measuring only from g^oo^ 1 to 6oJt)(i tn f an ^ ncn ^ n 

 diameter. Between this network and the granular layer, is found a 

 stratum of large gray ganglionic vesicles or nerve cells, with ramified 

 offsets, similar to those found in the gray substance of the brain ; 

 hence this layer is sometimes named the vesicular layer. The offsets 

 or processes of these cells, are said to be connected, on the one hand, 

 with the nuclear fibres proceeding to the rods and cones, and on the 

 other, with the axial fibres of the retinal expansion of the optic nerve ; 

 whilst some of the fine nerve-fibres are also said to be traceable directly 

 into certain of the nuclear fibres, and, through them, to be connected 

 with the rods and cones. The capillary vessels chiefly lie in the neigh- 

 borhood of the layer of ganglionic vesicles. 



From the intimate connection of all these complex elements of the 

 retina, most physiologists are of opinion, that whilst some of the ra- 

 diating fibres are possibly only supporting structures, others, as well 

 as the rods and cones, are either actual nervous elements, or impor- 

 tant appendages of the extremities of the nerves. 



At the fovea centralis of the yellow spot, only certain of the retinal 

 elements are present, viz., the cones of the columnar layer, which are 

 here smaller and set closely together, a stratum of gray ganglionic 

 nerve-cells, and the membrana limitans. At the elevated margin of 

 the yellow spot, the other ordinary retinal structures, which are ab- 

 sent in the fovea centralis, begin to appear. At the optic colliculus, 

 the only elements present are the nerve-fibres, radiating from that 

 point. 



The transparent humors of the eyeball are, the vitreous humor, the 

 crystalline lens, and the aqueous humor. 



The vitreous humor, Fig. 83, v, so named from its glass-like trans- 

 parency (vitrum, glass), occupies about the posterior f ths of the entire 

 globe, and measures from before backwards about half an inch. It is 

 a colorless, transparent, jelly-like mass,, inclosed in a clear membrane, 

 called the hyaloid membrane, processes of which also traverse it. The 

 vitreous humor consists of a specially modified connective tissue, 

 called jelly like or mucous, arranged in segments, like an orange. It 

 is composed almost entirely of water, in which are some salts, arid a 

 little animal matter. Behind and around, the vitreous humor is con- 



