THE BASE OF THE FORE-BRAIN. 83 



The lamina cinerea terminalis (Fig. 27) is most superior 

 of the median structures. It is a thin lamina of ash-colored 

 (cinereum) gray matter closing the end of the neural tube. It 

 extends from the anterior superior surface of the optic chiasma 

 upward and backward to the anterior commissure, just in front 

 of which it becomes continuous with the lamina rostralis of the 

 corpus callosum. Laterally, it is continuous with the cortex 

 of the cerebral hemisphere. Behind it, is the third ventricle; 

 in front of it, a part of the longitudinal fissure of the cerebrum. 



Optic Chiasma. (Chiasma opticum). The optic chiasma is 

 a quadrilateral sheet of nerve fibers whose anterior angles receive 

 the optic nerves and whose posterior angles give off the optic 

 tracts (Fig. 21). With the nerves and tracts attached, it is x- 

 shape. The chiasma is a median structure and is situated beneath 

 the lamina cinerea, in the optic groove of the sphenoid bone. 

 The fibers of the optic nerves and tracts compose it. There are 

 three sets of these fibers, namely, the intercerebral, the direct, 

 and the decussating. A fourth group of fibers, called the inter- 

 retinal and said to be commissural for the retinas, has been hither- 

 to described, but their existence is very doubtful. The inter- 

 cerebral fibers are not found in the optic nerves, but form the 

 inferior commissure (Guddeni) which joins together the medial 

 geniculate bodies (Fig. 43). The direct (or temporal) and decus- 

 sating (or nasal) fibers run through nerve and tract and join the 

 retina with the brain on the same and the opposite side, respectively 

 In most vertebrates below mammals, and in the mouse and guinea 

 pig, it is said that the optic fibers all decussate in the chiasma. 

 Normally in man and the higher mammals, the temporal half 

 of each retina contributes to the chiasma direct fibers and the 

 nasal half crossed fibers (Fig. 67). The optic nerves (nervi 

 optici] extend from the foramen sclerae of each eyeball back to 

 the front of the chiasma, through the optic foramina; they rise 

 in the ganglionar cells of the retinae, which are connected with 

 the rods and cones by the bipolar neurones. The optic tracts 

 (tractus optici) connect the chiasma with the brain. Each tract 

 winds outward and backward around the cerebral peduncle, 

 and divides into a medial and a lateral root (Fig. 43). The roots 



