124 ANATOMY OF THE CENTRAL NERVOUS SYSTEM. 



which pass from the thalamus to the cerebellum; furthermore, fibers from 

 the Decussatio transversa (Fig. 72), which cross posterior to the chiasma 

 and turn backward to the midbrain-wall on either side until they reach 

 the most posterior region of the midbrain-roof, probably ending in the 

 ganglia of the posterior quadrigeminal bodies or in the Ganglion isthmi. 



Having presented the gradual development of the mesencephalon, let 

 us, without a more detailed account, turn to a summary of the most im- 

 portant features. 



Summary. — From the roof of the mesencephalon arises the optic tract 

 from cells which, through their dendritic processes {Endstdtten), are in com- 

 munication with the great roof commissure and with the bundles to the 

 sensory terminal nuclei in the oblongata and the spinal cord. From the 

 gray matter ventral to the aqueduct arise fibers of the Nervus oculo-motorius 

 and of the Fasciculus longitudinalis. 



The base is occupied mostly with longitudinal bundles, tracts, and 

 fasciculi to the spinal cord and to the cerebellum; and the whole is em- 

 braced ventrally by the decussation of the deep medullary stratum and 

 laterally by the imcrossed fibers of the same, — the fillet. 



The structure of the midbrain is, as far as linown, in all animals the 

 same, except that those portions of the fibers which pass downward from 

 the roof of the organ, — i.e., the Traetus opticus and the fillet, — are much 

 more highly developed in fishes and birds than in mammals. In the latter, 

 therefore, there has taken place a relative retrogressive development. 



In other places a relative increase of the volume of the midbrain has 

 taken place; not, however, conditioned upon a change in the structure of the 

 midbrain itself, — that remains the same, — but upon the tracts which pass 

 through the organ. In mammals large tracts arise destined for the pons or 

 spinal cord. These tracts find no room in the structure described above 

 as typical for lower vertebrates. They pass quite ventral to that, giving rise 

 to a new external ventral layer: the cerebral peduncles. Lying dorsal to this 

 is the tegmental system, common to all vertebrates; the cerebral peduncles 

 represent a novum adclitum which appears only late in the animal kingdom. 



So the midbrain offers again a good example of the fact that in the 

 animal series (Taxonomic Series) no one segment of the brain undergoes 

 a step-by-step progressive development which is even approximately parallel 

 to the rank of the animal as determined by its general structure. There is 

 a particular organ gradually developed, which may be highly developed in 

 organisms of median rank, while in organisms of higher rank it may be 

 weakly developed, as wo have seen to be the case in the midbrain. The 

 case may be further complicated by the association, here and there, of new 

 tracts arising in brain-segments which, in particular classes, are especially 

 developed. 



