THE OCULAR CONTENTS 665 



face of the retina at the porn* opticus (physiologic excavation; optic 

 disk) in the center of the optic pa/ii/ln. Here it divides, its two branches 

 at first pursuing a meridional course het \\een the hyaloid membrane and 

 the retinal surface; soon they pierce the latter to supply the cerebral 

 portion of the retina. No vessels penetrate the neuro-epithelial portion 

 of the retinal layers; these are nourished by the choroid. The vena cen- 

 tralis retinae pursues a course exactly similar to that of the artery. 



THE ORA SERRATA 



At the ora serrata (Fig. 538) the typical layers of the retina, already 

 much thinned, abruptly cease. The first elements to disappear are the 

 rods and cones ; the cones, which become much shorter, practically lacking 

 the outer segment, extend farther toward the ora than the rods. The 

 remaining layers are continued forward only as the double layer of 

 epithelial cells belonging to the pars ciliaris retinas, the inner stratum 

 of which appears to be analogous to and continuous with the cerebral 

 portion of the retina, apparently the sustentacular cells; while the outer, 

 deeply pigmented layer apparently represents the pigmented layer of the 

 retina. For some distance toward the ora serrata, the retina becomes 

 modified by the presence of large vacuoles, probably lymph spaces. 



THE OCULAR CONTENTS 



Within the ocular globe, whose walls are formed by the three coats 

 of the eye, are certain structures which may be collectively considered as 

 its contents. They are : 



1. The aqueous humor. 



2. The crystalline lens. 



3. The vitreous humor. 



4. The hyaloid membrane. 



5. The suspensory ligament. 



THE AQUEOUS HUMOR 



The aqueous humor is a fluid, closely allied to lymph, which occupies 

 the anterior and posterior chambers of the eye. Microscopically it is 

 structureless. Occasional leukocytes, migrants from adjacent lymph 

 channels, may be encountered. 

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