772 



DISSECTION OF THE BRAIN. 



inner or 

 deep part. 



Optic tract 

 arises from 

 anterior 

 quadrige- 

 niinal and 

 external 

 geniculate 

 bodies, and 

 thalarmis. 



decussate in the lamina quadrigemina, above the aqueduct of Sylvius, 

 with those of the opposite side. The inner fibres of the fillet main- 

 tain their deep position, and are continued upwards with the 

 tegmentum to the cerebral hemisphere. 



The ORIGIN OF THE OPTIC TRACT can now be seen (fig. 281). At 

 the outer side of the cms cerebri the optic tract forms a bend (genu), 

 and then divides into two parts. The inner and smaller of these 

 springs from beneath the internal geniculate body ; while the outer is 

 continued into the external geniculate body and the optic thalamus, 

 receiving also the brachium of the anterior quadrigeminal body. 



The proper visual fibres pass to the grey matter (1) of the pulvinar 

 of the optic thalamus (2) of the external geniculate body, and (3) 

 of the anterior corpus quadrigeminum. The fibres passing to the 



up. Irac/i. 



nj". lij-ach . 



tania. semic. 



ext.ycn.b. 



opk.nirve 



FIG. 281. ORIGIN OP THE OPTIC TRACT. THE MESENCEPHALON is DIVIDED 



CLOSE ABOVE THE PONS. 



internal geniculate body, the most posterior fibres of the tract (see 

 p. 728) are, apparently, not associated with vision. 



Posterior The POSTERIOR COMMISSURE (fig. 277, e) is the thin foremost part 



commissure. O f ^ ] am j na quadrigemina, which is folded back so as to present a 

 rounded margin in front towards the third ventricle, above the open- 

 ing of the aqueduct of Sylvius (fig. 274, p. 757). On each side it 

 joins the optic thalamus, and to its upper part the stalk of the pineal 

 body is attached. It consists mainly of decussating fibres con- 

 tinuous with those of the fillet ; but some are said to be comniis- 

 sural, uniting the tegmenta of the two sides. 



The PINEAL BODY (conarium ; fig. 274) is ovoidal in shape, 

 like the cone of a pine, and about a quarter of an inch in length. 

 It lies with its base turned forwards in the groove between the 

 anterior quadrigeminal bodies. It is surrounded by pia mater; 

 and its base is attached by a hollow white stalk, below to the 

 posterior commissure, and above to the optic thalamus on each 



Pineal 

 gland ; 



