IX 



MID- AND INTEE-BEAIN 



489 



including birds, have only corpora bigemina (optic lobes), which 

 probably represent the anterior pair. 



The anterior quadrigeininal bodies are less prominent, but 

 longer and darker, than the posterior (Fig. 229). A small bundle 

 of white fibres, which emerges from the lateral edge of the nucleus 

 and runs towards the corpus geniculatum externum, is a part of the 

 optic nerve. Fibres also spring from the cells of the grey matter 



FIG. 244. Section across mid-brain, through superior corpora quadrigemina. Magnified about 3% 

 diameters. From a photograph. (Schiifer.) Sy, Sylvian aqueduct; c.p., posterior com- 

 missure ; gl.pi., glandula pinealis ; c.q.a., grey matter of one of superior corpora quadrigemina ; 

 c.g.m., corpus genieulatum mesiale ; c.g.L, corpus geniculatum laterale ; tr.opt., optic tract; 

 p.p., pes pedunculi ; p.l.b., posterior longitudinal bundle;/., upper fillet; r.n., red nucleus; 

 n.lll, nucleus of 3rd nerve ; III, issuing fibres of 3rd nerve ; l.p.p., locus perforatus posticus. 



of these eminences, and terminate in the nuclei of the 3rd and 4th 

 pair, where the fibres of the posterior longitudinal bundle also end. 

 Impulses from the optic nerve can thus readily be reflected to the 

 nuclei of the nerves that innervate the muscles of the eye. 



II. The Inter -brain (thalamencephalon) originates, as we 

 have seen, in the 2nd secondary vesicle of the embryonic brain. 

 The optic thalami are the thickening of the walls of this vesicle, 

 the cavity of which shrinks in the adult to the third ventricle. 



