Comparative Anatomy of the Brain. 587 



the roof of the ventricle, above the hippocampus, and converges to- 

 ward its fellow from the other hemisphere. Their junction constitutes 

 a union, or commissure, between the two hemispheres. Fibres are con- 

 tinued forward, radiating from this upon the inner or mesial surface of 

 the forepart of the hemispheres, while others bend downward, forming 

 the anterior pillars of the fornix. This common meeting ground con- 

 stitutes the hippocampal commissure, and it forms the beginning of the 

 great commissure, the corpus callosum. The two hippocampi are rela- 

 tively very large in the echidna and the marsupials, and the hippocampal 

 commissure connecting these becomes, therefore, a correspondingly im- 

 portant tie between the two hemispheres. This tie is, however, con- 

 fined to the lower part of the cerebrum, the part above the ventricles 

 not being directly joined together until the corpus callosum is formed, 

 in the higher species, the marsupial being in this respect in the same 

 condition as the monotremes and birds. The anterior commissure is 

 large in marsupials and monotremes. 



( The anterior commissure which appears to connect the two corpora 

 striata, in reality does not. It is composed of two parts ; the front 

 part forms a connection between the olfactory tracts. This division of 

 the commissure is especially large and well marked in animals with 

 large olfactories, but is small in man and monkey. The posterior divi- 

 sion of the commissure passes outwards, downwards and backwards, 

 under the lenticular portion of the corpus striatum, and goes on to the 

 hippocampal lobule and the nucleus amygdalae, where its fibres spread 

 out and terminate. It therefore connects the two opposite hippocampal 

 lobules, &c.) (Ferrier.) 



In the next advance of brain development, the corpus callosum is 

 present. There is, however, but little advance in other respects, for 

 the cerebral hemispheres show no other very marked superiority over 

 those below. They are relatively larger, however, and grow over the 

 optic lobes, and also partially conceal the olfactory lobes. The hemi- 

 spheres are smooth, except a few of the highest in the sub-class. Owen 

 names this sub-class the LissencepJiala. The orders included in this 

 sub-class are the Bruta, or Edentata, the Cheiroptera, Insectivora, and 

 Rodentia. These animals still possess some of the characteristics of 

 birds and reptiles. For example, the three-toed Sloth has cloaca, con- 

 voluted trachea, supernumerary cervical vertebrae, and floating ribs. x 

 The Ant-eaters have long, slender, beak-like, toothless jaws, and a giz- 

 zard. The Pangolins (Manis), of Hindostan and Africa, ( Edentula of 

 the order Bruta) have both gizzard and gastric glands, like those of the 

 birds, and have imbricated (or guttered) scales, and they are also tooth- 

 less. The Armadillos have plates on their back, like the scaly lizzards. 

 ^Almost all mammals have seven cervical vertebrae. 



