BRAIN. 



145 



fibre tracts soon to be mentioned, some with nerves arising from this 



region. 



Most noticeable of these ganglia are the olivary bodies (oliva) near the roots of 

 the hypoglossal or first spinal nerves; the nuclei of the cuneate and slender funiculi 

 connected with the posterior columns of the cord; the eminentia medialis in the 

 floor of the fourth ventricle, connected with the anterior and lateral columns; and 

 the tuber acusticum, an enlargement connected with the eighth nerve; its anterior 

 end in the ichthyopsida is specialized as the lobe of the lateral line system. 



The cerebellum is developed from the dorsal zones and the roof 

 plate, the latter invaded by nerve cells from the sides. In front it dips 

 deeply into the fourth ventricle, its anterior portion being vertical and 

 together with part of the roof of the isthmus, forming the valve of 



cb 



FIG. 150. Diagrammatic longitudinal section of brain, ac, anterior commissure in 

 lamina terminals; aq, aqueduct; c, cerebrum; cb, cerebellum; cp, chorioid plexus; cs, corpus 

 striatum; cv, ( cerebellal> ventricle; h, hypophysis; he, habenular commissure; ip, inferior 

 chorioid plexus; i, mesencephalon ; ml, myelencephalon; p, pinealis; pa, paraphysis; pc, 

 -posterior commissure; pe, parietal eye; v, valve of Vieussens; i't, velum transversum with 

 aberrant commissure. 



Vieussens (velum medullare anterius,fig. 150). In the ichthyopsida 

 and low r er reptiles there is no special differentiation of parts in the cere- 

 bellum, but in the higher reptiles and in the birds a central portion, the 

 vermis, and a pair of lateral lobes, the flocculi (fig. 161) occur. In the 

 mammals the cerebellum is still farther enlarged, chiefly by the develop- 

 ment of large cerebellar hemispheres between vermis and flocculi, the 

 latter being forced by them to the lower side of the cerebellum. In the 

 walls of each hemisphere, besides others, there is a large nerve centre, 

 the nucleus dentatus, connected with the posterior peduncle of the 

 cerebellum to be mentioned shortly, and with the fibres which go 

 farther forward in the brain. 



The mesencephalon is relatively largest in the lower vertebrates, 

 less conspicuous and tending to be covered by cerebrum and cerebellum 

 in the higher groups. On its dorsal surface are the two optic lobes 

 (transversely divided in the mammals) each connected with an optic 



IO 



