436 THE BRAIN OF MAMMALS, BY TH. MEYNERT. 



geniculata, namely, that with the tractus opticus. This relation 

 of the corpora quadrigemina is given here in anticipation of a 

 further description of them, because, for one thing, the consi- 

 deration of the optic thalamus, as well as generally of those 

 ganglia of the cerebrum which terminate in the region of the 

 cms cerebri, will thus be completely finished, whilst the corpora 

 quadrigemina constitute a boundary region belonging to the 

 pons, which will be considered in our next section. 



In the planes of origin of, or rather of convergence of fibres 

 from the tractus opticus, the posterior extremities of the optic 

 thalami, the pulvinaria, by their divergence, retreat from the 

 median portion of the territory of the crus cerebri, allowing 

 the corpora quadrigemina to press in between them. In place 

 of lying upon the latter, the pulvinar lies upon the two corpora 

 geniculata (fig. 249, Th, Gi, Ge), and the stratum zonale sends 

 its fascicuh' towards the tractus opticus, by means of which it 

 partly arises from the retina, and partly from the medulla of 

 the temporal lobe, in an arch parallel to the tractus, and 

 destined for quite identical regions of the optic thalamus. 

 There are thus two connections of the tract with the optic 

 thalamus : (1) a superficial one, that has just been mentioned ; 

 and (2) a deep-seated one, already described on p. 431. 



The external corpus geniculatum .presents in transverse sec- 

 tion a plump heart-shaped, and consequently bilobed form, 

 which is not quite distinctly enough expressed in fig. 249, Ge. 

 J. Wagner must have taken one of these lobes for a special 

 structure when he enigmatically speaks of a well-defined optic 

 nucleus in the optic thalamus, which he, in his drawing, 

 confuses with the lamination of alternate grey and white 

 substance peculiar to the external corpus geniculatum. The 

 grey matter of the external corpus geniculatum is not a 

 rounded mass, like that of the ganglia, but a lamina, which 

 however, as if for the purpose of its being included in a 

 complete medullary capsule, is folded in a zigzag manner, 

 as is shown by successful longitudinal sections. If we recall 

 the laminated optic nerve of fishes, which is likewise folded 

 upon itself in its sheath, we may explain the structure of the 

 external corpus geniculatum morphologically as follows : A 

 lamina of grey matter lies between two medullary laminae, of 



