162 



FUNCTIONS OF THE NERVOUS SYSTEM. 



ganglia; and it is sometimes marked with transverse lines, which indicate 

 the intermixture of gray and white matter in its substance ; there is as yet, 

 however, no appearance of a division into hemispheres. On drawing apart 

 the hemispheres of the Cerebrum, the Corpora Striata, Optic Thalami, and 

 Tubercula duadrigemina or Optic Ganglia, are seen beneath them; the 

 size of the last still bears a considerable proportion to that of the whole 

 Encephalon. The Optic Ganglia are still hollow, as they are in the embryo 

 condition of Man. Indeed the Brain of the Human foetus about the twelfth 

 week will bear comparison, in many respects, with that of the Bird. The 

 Cerebral hemispheres, much increased in size, and arching back over the 



Fig. 31. 



e..- 



Brain of human embryo at twelfth week; A, seen from behind; B, side view; c, sectional view a cor- 

 pora quadrigemina; b b, hemispheres ; rf, cerebellum; e, medulla oblongata ;/, optic thalamus; g, floor of 

 third ventricle ; i, olfactory nerve. (After Tiedemann.) 



Thalami and Optic ganglia, but destitute of convolutions and imperfectly con- 

 nected by commissures, the large cavity still existing in the Optic ganglia, 

 and freely communicating with the third ventricle, and the imperfect evolu- 

 tion of the Cerebellum, make the correspondence in the general condition of 

 the two very considerable. 



218. The Brain of the lowest Mammalia presents but a slight advance upon 

 that of Birds, in regard both to the relative proportions of its parts, and to their 

 degree of development. Thus, in the Marsupialia, the Cerebral hemispheres 

 exhibit no convolutions ; and the great transverse commissure, the Corpus 



Fig. 32. 



Agr 



Upper and under surface of Brain of Rabbit; A, B, , as before ; o I, olfactive lobes; o Coptic nerve- 

 m o motor ocuh ; c m corpora mamillaria ; c c, crus cerebri ; ,, pons varolii ; p a, patheticus ; t r i, trifa' 

 cial ; a b, abducens ; fa c, facial ; a u, auditory ; a g, vagus ; ,, spinal accessory ; h y p, hypoglossal. 



