274 ANIMAL PHYSIOLOGY. [LESSON 85. 



optic ganglia, and these are separated from each other and thrown 

 to either side. 



1231. The cerebellum also is much increased in size, proportion- 

 ally to the medulla oblongata and its ganglia ; there is, however, no 

 appearance of a division into hemispheres. 



1232. The optic ganglia bear a considerable proportion to the size 

 of the cerebrum; they are still hollow, as they are in the embryo 

 condition of man. We shall hereafter see that the brain of the Hu- 

 man embryo bears comparison in many respects to the brain of the 

 bird. 



1233. The great increase of the cerebral hemispheres, arching 

 backwards over the Thalami, and optic ganglia, but destitute of con- 

 volutions, and imperfectly connected by commissures, the large 

 cavity still existing in the optic ganglia, and freely communicating 

 with the third ventricle together with the imperfect evolution of 

 the cerebellum make the correspondence in the two very remarkable. 



1234. In the earlier periods of the Old World, Birds appear to 

 have been connected with the Reptiles, through the flying Lizard, 



FIG 390 or Ptwodactylus, remains of which animal are now only 

 found in a fossilized condition. 



1235. It is very instructive to examine the brain 

 of the chick, after two days of incubation (Fig. 390). 

 We here see that the two halves of the spinal chord (a) 

 are united posteriorly, and form the vesicular enlarge- 

 ment (&), corresponding to the cauda equina and pelvic 

 dilatation of the adult The cervical and dorsal ver- 

 tebrae begin to embrace the anterior (f) portion of the 

 chord, and three vesicular enlargements are seen on the 

 cephalic portion of the nervous axis. The posterior (c) of 

 these enlargements forms the rudimentary lobes of the medulla ob- 

 longata, the middle dilatation (d) constitutes the outline of the optic 

 lobes, the anterior (e) and smaller cephalic enlargement forms the 

 embryo condition of the cerebral hemispheres in the chick, and all 

 these lobes are disposed in a longitudinal direction, as they are found 

 in fishes, and in the embryos of mammalia. 



1236. In the brain of the adult Stork (Fig. 391), the large cere- 

 brum (d, e) is partially divided into lobes ; it covers the optic thala- 

 mi (thalamus, a bed-chamber, bed of the optic nerves), and contains 

 a small ventricle, which extends forwards to the olfactory (nerves of 

 smell) tubercles (/). 



1237. These latter commence from two medullary tracts (h) on 



