528 THE ORGANS 



Internal arcuate fibres from the gray matter of the superior colliculus pass 

 through the reticular formation, and form an oblique decussation. This 

 decussation is the dorsal, or fountain-like decussation of Meynert. The fibres 

 originate from cells in the superior colliculus (tectum opticum), and after 

 decussation form the descending colliciilo-btdbar and spinal tract (tecto-spinal 

 or predorsal tract) (see also below). 



The Anterior Corpus Quadrigeminum or Superior Colliculus. — In this four 

 principal layers may be distinguished besides the usual covering of neuroglia 

 cells and fibres: (i) An outer white laj'cr, stratum zonale. This consists of fine 

 nerve fibres coming from the superior brachium, possibly fibres from the optic 

 tract and cerebral cortex. Among them are small nerve cells, mostly horizontal 

 and with tangential or centrally directed axones. (2) A gray layer, the stratum 

 cinereum. This consists of radially arranged nerve cells with their larger den- 

 drites proceeding outward, and their axones inward. The largest cells lie deep- 

 est. In this layer the optic fibers principally terminate. (3) The stratum 

 opticum consists principally of optic fibres which send their terminals mostly 

 into the preceding layer, but also into the deeper layers. It also contains cells 

 whose axones pass into the next layer. (4) Deep gray-white layer, or stratum 

 lemnisci, because it is stated to contain fibres from the medial lemniscus which 

 terminate in the superior colliculus (denied by some). This layer contains large 

 and medium stellate cells whose axones, together with axones from cells in 

 the more superficial layers, either pass across to the opposite colliculus or 

 sweep ventrally around the central gray, decussate in the raphe and proceed 

 caudally as the colHculo-bulbar and spinal tract. The above relations have 

 been principally ascertained by the Golgi method. The superior colliculus 

 also possibly receives fibres from the lateral lemniscus and spino-thalamic tract. 

 It also receives fibres from the occipital and temporal cortex cerebri (pallio- 

 collicular fibres). 



Belonging to the midbrain is the posterior commissure (not in the section) the 

 fibres of which cross in the roof just anterior to the superior colliculus. Its 

 fibres originate from coUicular cells (in turn receiving optic fibres), decussate and 

 terminate in the nucleus of the medial longitudinal fasciculus and other nuclei 

 in the reticular formation. 



The colliculus thus consists essentially of (a) afferent fibres from the retina 

 (optic tract), the pallium and possibly other parts of the nervous system, and/&) 

 efferent neurones to other parts of the brain and cord brought into various 

 relations with each other in the colliculus either directly or by (c) the association 

 cells of the colliculus, the axones of which do not leave the latter. 



Forebrain or Prosencephalon 



Interbrain (diencephalon or thalamencephalon) 



! 



In the interbrain or diencephalon, three parts may be distin- 

 guished ; the thalamus, epithalanius, and hypothalamus. The epithala- 

 mus consists principally of the pineal body, the habenulas, and striae 

 thalami. The hypothalamus consists mainly of the structures in 



