NERVOUS CENTRES OF MAMMALIA SPINAL CORD. 495 



look at the brain from above downwards : as we see in the Encephalon 

 of the Sheep (Plate. II., Figs. 14, 15, d). The Cerebral hemispheres 

 increase, not only in size ; but also in complexity of structure, both 

 external and internal. Their exterior, instead of remaining smooth, is 

 marked by convolutions ; which serve to extend very greatly the amount 

 of surface over which blood-vessels can pass into the gray substance. 

 Their internal structure becomes more complex, in-the same proportion 

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

 all these conditions present themselves in a far higher degree than in 

 any other animal. The number of commissural bands, connecting the 

 two hemispheres with each other transversely, and uniting their anterior 

 arid posterior portions, is very greatly increased ; and in fact, a large 

 proportion of their mass is composed, in Man and the higher Mammalia, 

 of fibres of this character. In proportion to the increase of the Cere- 

 bral hemispheres, there is a relative diminution in the size of the ganglia 

 of special sense ; but their dimensions, as compared with the entire bulk 

 of the animal, are by no means reduced, but are even increased. The 

 Olfactive ganglia (Fig. 14, a) are always readily discoverable ; being 

 separated from the remainder of the encephalic masses -by a peduncle 

 on each side. The Optic ganglia (Fig. 15, c), on the other hand, are 

 so completely covered-in by the Hemispheres, that it is only when the 

 latter are turned aside that we can discern them. They differ in exter- 

 nal aspect from the optic ganglia of Birds and the lower Vertebrata ; 

 being divided by a transverse furrow into anterior and posterior emi- 

 nences, whence they are known as the Corpora Quadrigemina. The 

 Auditory ganglia are lodged in the substance of the Medulla Oblongata, 

 forming the "gray nuclei" of the strands termed the "posterior pyra- 

 mids ;" and similar nuclei in the "restiform bodies" are the ganglionic 

 centres of the Glosso-pharyngeal nerves, and perhaps minister to the 

 sense of Taste. Besides these, however, are the two large bodies termed 

 the Corpora Striata and Thalami Optici, which have been commonly 

 considered as appendages of the Cerebrum, but which must undoubtedly 

 be regarded as independent of it, and as themselves constituting gan- 

 glionic centres, whose development bears no constant proportion to that 

 of the Cerebrum. From the peculiar relation presently to be described 

 ( 901), which these bodies bear on the one hand to the Spinal Cord, 

 and on the other to the rest of the Encephalon, there seems strong 

 reason to believe that they together constitute the ganglionic centre of 

 the sense of Touch, and of the motions which are automatically prompted 

 by it. The Cerebellum is chiefly remarkable for the development of its 

 lateral parts or hemispheres, and for the intricate arrangement of the 

 gray and white matter in them (Fig. 15, d) ; the central portion, some- 

 times called the vermiform process, is relatively less developed than in 

 the lower Vertebrata, where it forms the entire organ. The Spinal 

 Cord is much reduced in size, when compared with other parts of the 

 nervous centres ; the motions of the animals of this class being more 

 dependent upon their will or guided by their sensations ; and the simply 

 reflex actions bearing a much smaller proportion to the rest. The 

 development of ganglionic enlargements, in accordance with the presence 

 or absence of high locomotive powers in the extremities, follows the 

 same rule as in the preceding classes. 



