438 THE BRAIN OF MAMMALS, BY TH. MEYNERT. 



which one is a portion of the medulla of the tractus, the 

 other of the medulla of the hemispheres. At their adjoining 

 surfaces the medullary laminae connect themselves with the 

 cells of the grey laminse, whilst they, like the enclosed grey 

 substance, are folded in a zigzag manner. In this manner 

 grey and white laminae alternate continuously with each other, 

 as is apparent in every section of the external corpus geni- 

 culatum. The cells of this ganglion are from 30 to 48 JJL in 

 length, and 15 fj. in breadth, and are for the most part 

 coarsely granular and pigmented. The mass of the tractus 

 "belonging to the external corpus geniculatum lying internally 

 to the superficial origin of the optic thalamus, and externally to 

 the deep, is very large (fig. 249, u). 



The connections of the external (and internal) corpus geni- 

 culatum with the occipital (fig. 243) and temporal lobes of the 

 cerebrum have already been given at page 409. 



Situated internally, and in great part also anteriorly, to these 

 external fasciculi of the tractus is the connection of the latter 

 with the oval or fusiform internal corpus geniculatum, by which 

 the fasciculi of the tract traversing it in a meridianal direction, 

 and proceeding from its fusiform cells (the long diameter of 

 which amounts to 25 JJL, and their breadth to 5 p), are conducted 

 in reduced number in the form of an upper and a lower band to 

 the corpora quadrigemina. The superior or indirect connec- 

 tion of the tractus opticus with the nates passes below and 

 somewhat behind the arm of the nates, in a brush-like manner, 

 into the small cells of this body (fig. 249, Gi, W). 



An inferior connection with the corpora quadrigemina is 

 effected by the internal corpus geniculatum in such a manner 

 that fasciculi traceable from the tractus opticus are conducted 

 to the brachia of the testis, and consequently to this ganglion 

 itself. No more striking argument in respect to the signifi- 

 cance of the optic nerve, and in opposition to its being regarded 

 as a peripheric nerve root, can be adduced than this attach- 

 ment of one of its fasciculi to the brachium of the inferior 

 corpus quadrigeminum, which is nothing else than a fasciculus 

 of the corona radiata arising in the cortex of the cerebrum, and 

 terminating in the ganglion of the inferior corpus quadri- 

 geminum (fig. 249, Gi t Bi). 



