. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 362 CLASSIFICATION OF VERTEBRATES. the aplacental mammals the anterior commissure is especially well developed, and forms the chief connection between the two sides of the brain, while the corpus callosum remains more rudimentary, as in sauropsida. In the placentalia, on the other hand, the corpus callosum or commissure between the two hemispheres becomes the most important connection between the right and left sides, the anterior commissure remaining behind. The longitudinal commissures, the fornix, and the cornua ammonii,
. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 362 CLASSIFICATION OF VERTEBRATES. the aplacental mammals the anterior commissure is especially well developed, and forms the chief connection between the two sides of the brain, while the corpus callosum remains more rudimentary, as in sauropsida. In the placentalia, on the other hand, the corpus callosum or commissure between the two hemispheres becomes the most important connection between the right and left sides, the anterior commissure remaining behind. The longitudinal commissures, the fornix, and the cornua ammonii, are also well developed. The lateral ventricles are large, and in them several sub-regions are distinguished,— anterior and descending, and in the higher mammals posterior cornua. The olfactory lobes are comparatively small, and in the whales are absent. In development a diverticu- lum of the lateral ventricle extends into each olfactory lobe, but except in a few forms, like the horse, this disappears in the adult. The twixt brain and op- tic lobes (corpora bi- or quadrigemini) are poorly developed, and are covered in by the hinder lobe of the cerebrum. The epiphysis is small and lacks any sensory structures. In the cerebellum the vermis is large in the apla- centals, but in the placentalia the lateral lobes of the cere- bellum are in the ascendancy. A corresponding increase from lower to higher is seen in the pons varolii. The spinal cord extends back only to the sacral region, the posterior part of the spinal canal being occupied by a cauda equina formed of the more posterior nerves before their Fig. 349. Brain of rabbit from above, irom Wiedersheim. Bol, olfactorj- bulb; Ftp, pallial fissure ; Gp, pinealis; ////, cere- bellar hemispheres; XH^ medulla; VH, •cerebral hemispheres ; /Fw, Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations
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