810 HANDBOOK OF PHYSIOLOGY. 



The next to appear is the parieto-occipital or perpendicular fissure; 

 these two great fissures, unlike the rest of the sulci, are formed by a curv- 

 ing round of the whole cerebral mass. 



In the sixth month the fissure of Rolando appears: from this time 

 till the end of foetal life the brain grows rapidly in size, and the convo- 

 lutions appear in quick succession; first the great primary ones are 

 sketched out, then the secondary, and lastly the tertiary ones in the 

 sides of the fissures. The commissures of the brain (anterior, middle, 

 and posterior), and the corpus callosum, are developed by the growth of 

 fibres across the middle line. 



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. 502, 503). 



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 toward 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 



Fig. 504. 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; c, 

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

 closed in B and C ; I, the lens follicle and lens ; pr, 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 ; v, 

 the secondary ocular vesicle and vitreous humor. 



ingrowing process from the epiblast (fig. 504), 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 

 originally continuous. From this hollow ball the crystalline lens is 

 developed. The way in which this occurs has been indicated in a pre- 

 vious chapter under the head of structure of the lens. By the ingrowth 

 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 vesi- 



