TOO HANDBOOK OF PHYSIOLOGY. 



The Hippocampus major is formed by the folding in of the gray 

 matter from the exterior into the lateral ventricles. The essential points 

 in the structure and arrangement of the various parts of the brain, are 

 diagrammatically shown in the two accompanying figures (Figs. 483, 

 484). 



THE SPECIAL SENSE ORGANS. 



The Eye. Soon after the first three cerebral vesicles have become 

 distinct from each other, the anterior one sends out a lateral vesicle from 

 each side (primary optic vesicle), which grows out towards the free sur- 

 face, its cavity of course communicating with that of the cerebral vesicle 

 through the canal in its pedicle. It is soon met and invaginated by an 

 in-growing process from the epiblast (Fig. 450), very much as the grow- 

 ing tooth is met by the process of epithelium which produces the enamel 

 organ. This process of the epiblast is at first a depression which ulti- 

 mately becomes closed in at the edges so as to produce a hollow ball, 

 which is thus completely severed from the epithelium with which it was 



FIG. 485. Longitudinal section of the primary optic vesicle in the chick magnified (from 

 Remak). A, from an embryo of sixty -five hours; B, a few hours later; C, of the fourth day; 1, 

 the corneous layer or epidermis, presenting in A 3 the open depression for the lens, which is closed 

 in Band C; 2, the lens follicle and lens; 5, the primary optic vesicle, in A and B, the pedicle is shown; 

 in C, the section being to the side of the pedicle, the latter is not shown; 7, the secondary ocular 

 vesicle and vitreo us humor. 



originally continuous. From this hollow ball the crystalline lens is de- 

 veloped. By the in-growth of the lens the anterior wall of the primary 

 optic vesicle is forced back nearly into contact with the posterior, and 

 thus the primary optic vesicle is almost obliterated. The cells in the 

 anterior wall are much longer th'an those of the posterior wall; from the 

 former the retina proper is developed, from the latter the retinal pig- 

 ment. 



The cup-shaped hollow in which the lens is now lodged is termed the 

 secondary optic vesicle: its walls grow up all round, leaving, however, a 

 slit at the lower part. 



Choroidal Fissure. Through this slit (Fig. 487), often termed the 

 choroidal fissure, a process of mesoblast containing numerous blood-ves- 

 sels projects, and occupies the cavity of the secondary optic vesicle be- 

 hind the lens, filling it with vitreous humor and furnishing the lens 

 capsule and the capsulo-pupillary membrane. This process in Mam- 



