XVI 



THE NERVOUS SYSTEM 



29- 



t—Afed. oil 



the wide, more or less triangular fourth ventricle, which com- 

 municates by its tapering posterior end with the central 

 canal of the cord. Its roof is thin and thrown into folds 

 bearing the posterior choroid plexus of blood vessels. At the 

 sides the medulla gives rise to several pairs of cranial 

 nerves. Its lower surface is divided 

 by a median fissure which is con- 

 tinuous with the ventral fissure of 

 the cord. 



The cerebellum consists of a 

 small transverse fold at the ante- 

 rior margin of the fourth ventricle. 

 In most other vertebrates the cere- 

 bellum is an organ of considerable 

 size, but in the frog it is almost 

 rudimentary. 



The optic lobes, which form the 

 dorsal part of the mid-brain, are 

 large rounded bodies lying just 

 in front of the cerebellum. Their Fig. 84. 

 cavities, the optic ventricles, com- 

 municate with each other and also 

 with the channel between the third 

 and fourth ventricles. Below the 

 optic lobes are the crura cerebri, 

 which extend from the medulla to 

 the cerebral hemispheres and form 

 the floor of the mid-brain. 



The thalamencephalon is in the 

 region between the optic lobes 

 behind and the cerebral hemi- 

 spheres in front. Its roof is thin and lined with a vascular 

 membrane, the anterior choroid plexus ; it bears two out- 



\-Sp.cd 



Diagram of a hori- 

 zontal section of a frog's brain. 

 c.c, central canal ; Cer. H, 

 cerebral hemisphere ; Di, dien- 

 cephalon, or thalamencepha- 

 lon ; for. M, foramen of 

 Monro ; /', iter ; Lat.v, lateral 

 ventricle ; Med.obl, medulla 

 oblongata; Nv. i, first, or ol- 

 factory nerve ; Olf. I, olfactory 

 lobe ; Olf.v, olfactory ven- 

 tricle ; Opt.l, optic lobe ; Opt.v, 

 optic ventricle ; Sp. cd, spinal 

 cord ; iA, third ventricle ; lA, 

 fourth ventricle. (After Ecker 

 and Wiedersheim.) 



