ENCEPHALON OF MAMMALIA. 



285 



with the Spinal Cord ; so that in Man the Tubercula Quadrigemina are abso- 

 lutely smaller than they are in many animals of far inferior size. The inter- 



Fiz. 149. 



feSB** 



' F**\/YIf 



J^CL 



ff Mf ^ 



VU, 



Upper and under surface of Brain of Rabbit. A, B, r, as before ; ol, olfactive lobes ; op, optic nerve ; 

 mo, motor oculi ; cm, corpora mamillaria; cc, crus cerebri ; pv, pons varolii ; pa, patheticus ; tri, trifa- 

 cial ; ab, abducens; fac, facial ; aw, auditory ; rag-, vagus ; s, spinal accessory ; hyp, hypoglossal. 



nal structure of the hemispheres becomes more complex, in the same propor- 

 tion as their size and the depth of the convolutions increase; and in Man all 

 these conditions present themselves in a far higher degree, than in any other 

 animal. In fact it is only among the Ruminantia, Pachydermata, Carnivora, 

 and Quadrumana, that regular convolutions can be said to exist. The cor- 

 respondence between the bulbous expansion of the Olfactive Nerves in Mam- 

 malia, and the Olfactive lobes of the lower Vertebrata, is made evident by the 

 presence, in both instances, of a cavity which communicates with the lateral 

 ventricle on each side ; it is in Man only that this cavity is wanting. The 

 external form of the Corpora Quadrigemina of Mammalia, differs from that of 

 the Optic ganglia of Birds, owing to the division of the former into anterior 

 and posterior eminences, (the nates and testes;) and there is also an internal 

 difference, occasioned by the contraction of the cavity or ventricle, which now 

 only remains as the Aqueduct of Sylvius. The Cerebellum is chiefly re- 

 markable for the development of its lateral parts or hemispheres ; the central 

 portion, sometimes called the vermiform process, is relatively less developed 

 than in the lower Vertebrata, in which it forms the whole of the organ. 



4. General Functions of the Spinal Cord. Reflex Action. 



363. The functions of the Nervous System in Vertebrated Animals are so 

 complex in their nature, and our means of analyzing them are so imperfect, 

 that the inquiry is confessedly one of the greatest difficulty, and needs all the 

 light which can be thrown upon it from any source. The great accession to 

 our knowledge of them, which has been made within the last few years, 

 chiefly by the labours of Sir C. Bell, and Dr. M. Hall, has so far changed 

 the aspect of this department of Physiological Science, as to render it neces- 

 sary for those who had previously studied it, to begin de novo. This is espe- 

 cially the case in regard to the actions dependent on the Spinal Cord ; which 

 it seems desirable to consider in the first instance, in order that it may be 



