THE INTEEBRAIN- OH THALAMENCBPHALON. 



139 



We know at present that parts of the optic nerve arise from the large 

 ganglion-cells of the retina (S. E. y Cajal, Monakow), and we know that an- 

 other part passes from the roof of the midbrain itself to be distributed in 

 the retina. The retina itself may be looked upon as a system of neurons 

 arranged in strata one above the other. A part of these neurons stand in 

 direct relation to the Opticus, as just stated; while a part are connected 

 with other cells, of the second, third, and fourth orders, propagating the 

 stimulus received by the peripheral neurons of the first order. In the midst 



Doc. poab-chiasmatica 

 Dec. transversa 

 Basal opticus root 



Tr. strio-tbalam. 



Dec. supra-infundibularis 



Ggl. ectomamillare 



N. ocnlo-motorius 

 Tr. quinto-thalamicus 



Fasc. longit. post. 



N. trigeminus 



Pig. 87. — ^Horizontal section through the hypothalamus, chiasma, and 

 medulla of a lizard. 



lie many cells the distribution of whose processes make it evident that they 

 connect one part of the retina to another.^ 



From the eyeball the optic nerve passes into the cranial cavity. In 

 fishes whose eyes are relatively much larger than man's, also in birds, the 

 nerves in question are enormous, and in brain-sections of these animals 

 dominate the field. At the base of the skull they cross in the chiasma, 

 which lies, as above stated, just anterior to the hypothalamus. This decus- 

 sation is, in most animals, complete, but in some birds and probably in the 



'For a more exact description see S. E. y Cajal's monograph on "The Retina." 

 There is a German translation by Dr. Greef. 



