MIDBRAIN 215 



formation is far advanced in the frog, for Aronson and Noble ('45) 

 report that the warning croak uttered during mating is abohshed by 

 extensive injury of the inferior colHcuH, though it is not lost after 

 complete ablation of the hemispheres, diencephalon, superior col- 

 liculus, cerebellum, and anterior parts of the tegmentum. 



In Anura the optic tectum is greatly enlarged so as to cover the 

 dorsal convexity of the midbrain. The nucleus posterior is also 

 enlarged under the influence of the well-developed lateral lemniscus, 

 and it is folded into the aqueduct as the so-called "corpus posticum" 

 or "torus semicircularis." In some reptiles and in all mammals the 

 definitive inferior colliculus reappears on the dorsal surface. 



INTERMEDIATE ZONE 



The distinctive characteristics of the intermediate zone of Amblys- 

 toma are more clearly shown in the midbrain than elsewhere. This 

 band of subtectal tissue is well differentiated from the overlying 

 tectum and the underlying peduncle and isthmic tegmentum and is 

 given a distinctive name. 



TEGMENTUM DORSALE 



The dorsal tegmentum is separated from the isthmus by the sulcus 

 isthmi (s.is.) and from the peduncle by the limiting sulcus (s.) of the 

 nucleus of the tuberculum posterius (fig. 2). On the external and 

 ventricular surfaces there is no visible boundary between this zone 

 and the overlying tectum, but the internal structure is sharply con- 

 trasted. The gray of the tectum is irregularly laminated, with 

 plaques of cells separated by thin sheets of neuropil; that of the sub- 

 tectal area is more homogeneous (figs. 34, 36, 93, 94). The texture 

 of the white substance is even more distinctive. 



The neurons are rather small and are imbedded in dense neuropil, 

 which is continuous with that of adjoining areas. Typical illustra- 

 tions are shown in figures 24, 48, and 49 (see also '42, figs. 29, 30, 44, 

 45, 48). Their dendrites ramify among dorsal and ventral tegmental 

 fascicles and into the tectum, peduncle, and isthmic tegmentum. 

 The axons arise from the dendritic tree, and most of them are short, 

 arborizing locally and in adjoining areas. Many divide with ascend- 

 ing and descending branches. Axons of the larger elements (fig. 49) 

 descend to the isthmic tegmentum in the dorsal tegmental fascicles 

 (fig. 21, tr.tegds.), these fibers being the largest component of fascicles 

 of group (7). All these neurons seem to be concerned with local cor- 

 relation. 



