Integrative Systems 145 



vertebrate brains (Fig. 135B, C). They are designated by a series of 

 names in which "encephalon" (brain) is combined with appropriate 

 prefixes indicating the relative positions of the several regions. Begin- 

 ning anteriorly, they are the telencephalon (or prosencephalon), 

 diencephalon, mesencephalon, metencephalon, and the most 

 posterior region — transitional between brain and spinal cord — the 

 myelencephalon (myelon, spinal cord). (Confusion sometimes 

 arises from the fact that, in an older terminology, the part behind the 

 mesencephalon was called "epencephalon," and "metencephalon" 

 was applied to the most posterior region.) The lumen of the embryonic 

 neural tube persists in the adult brain, forming a continuous series of 

 cavities known as ventricles (or otherwise designated by"coele" 

 with the prefix appropriate to the region: e.g., mesocoele, metacoele, 

 etc.). 



The gross anatomy of the brain is mainly a matter of formation 

 of local thickenings of the wall of the neural tube, these "lobes" giving 

 the brain its characteristic external configuration. The gross anatomic 

 differences among brains of various vertebrates are due largely to 

 differences in the relative size of the several lobes. The characteristic 

 structural features of the several regions have, to a large extent, 

 received names which were originally used in human anatomy and 

 are not always literally significant as applied to brains other than 

 mammalian. 



The telencephalon is more or less definitely divided into right 

 and left cerebral hemispheres partially separated by an anterodorsal 

 fissure (Fig. 136). The ventral wall of each hemisphere is always much 

 thickened, forming the corpus striatum or basal ganglion (Fig. 138). 

 The dorsal wall, the pallium, varies from a thin and quite non- 

 nervous membrane in many fishes to the most massive part of the 

 brain in man and some other mammals (Fig. 139). Anteroventrally, 

 each hemisphere is continuous with a relatively small olfactory lobe 

 (Figs. 136, 140). The olfactory lobes develop as hollow outgrowths 

 of the telencephalon. Primarily, the telencephalon, especially the 

 corpus striatum, is concerned with the olfactory mechanism but, with 

 increased development of the pallium, the cerebral hemispheres be- 

 come the dominant coordinating mechanism of the brain. 



The diencephalon gives rise to no prominent lobes. Its lateral 

 walls, the thalami (Fig. 138), are thickened by the presence of impor- 

 tant masses of ganglionic material. A large part of its dorsal wall is 

 non-nervous. The anterior region of the roof is a thin membrane which 

 becomes elaborately folded into the adjacent ventricular spaces and 



