THE MIDBEAIN. 121 



In birds and fishes where it is widely spread out on account of the large 

 roof, where also the ventricle comes into relation with not a small part, it is 

 especially favorable for study. One recognizes at once that the central gray, 

 which everywhere incloses the ventricle, covers everywhere the surface of the 

 midbrain-base, which lies next the ventricle. In this gray matter — that is, 

 in the dorsal portion of the midbrain-base — lie several important nuclei. 

 Near either side of the median line may always be seen a number of small 

 masses of cells, which send out ventrally the fibers of the Nervus oculo- 

 motorius (see Fig. 68). Even in the lowest vertebrates these leave the base 

 of the brain, always at the same place, as two not unimportant nerves, which 

 turn toward the eye-cavity. Just posterior to the nucleus of the Oculomo- 

 torius one finds aggregations of cells (Fig. 65) from which the Trochlearis 

 arises. In all animals yet studied the nerve crosses to the other side in the 

 Velum medullMe posticum (Fig. 60). In order to reach this dorsally located 

 decussation, its fibers must, just after their origin, pass somewhat back- 

 ward and then turn dorsally. Thus, the nerve which passes off quite 

 dorsally always appears in the narrow crevice which remains between the 

 midbrain-roof and the cerebellum (see Figs. 56 and 74). 



Lateral from the nuclear origin of this nerve there lies, in the midst of 

 the central gray, a large nucleus: the Nucleus lateralis mesencephali. 'The 

 thin layer of nerve-netted gray matter aroimd the aqueduct in mammals 

 scarcely suggests what important structures are here represented in a process 

 of retrogression. If one dissects off the roof of the teleostean midbrain, 

 one will see, under the same, the protruded part of the cerebellum as a 

 large evagination divided in the median line (see Fig. 86). Laterally from 

 this one finds on either side an elongated, somewhat curved projection, 

 which may not, like the cerebellum, be lifted from the floor of the mid- 

 brain; in fact, it belongs to this. This growth was known to the old anatom- 

 ists, and was designated by them as Torus semicircularis. The Torus arises 

 through the location of the lateral mesencephalic nucleus (especially large 

 in fishes) in the lateral part of the central gray of the midbrain. The same 

 nucleus is also demonstrable in birds, even though it does not reach in them 

 the relative size which it has in fishes. In selaehia its presence is, to me, 

 doubtful; but in reptiles it is evident, and in amphibia it is, through the 

 location at least, to be recognized (Fig. 73). From the nucleus lateralis 

 mesencephali arises a large bundle: the lateral longitudinal bundle. It may 

 be followed through the entire medulla oblongata, and probably passes into 

 the lateral columns of the spinal cord. 



That a part of the longitudinal fiber-system in the midbrain-base arises 

 from the thalamus (Fig. 71) was mentioned above. Three of the bundles 

 which lie here deserve especial mention before we consider the thalamus, 

 because they give to the medullary white matter of the base its characteristic 



