THE BRAIX AXD ITS MEMBRANES. 743 



quadrigemina the fibres of each peduncle decussate with each other, so that fibres 

 from one half of the cerebrum are continued in the other half of the cerebellum. 

 The fibres thus pass upward into the optic thalamus, surrounding, as they go, 

 the red /tu<-J>-t(s. from and to the cells of which they receive and give fibres. 

 (4) Certain fibres from the olivary nucleus of the medulla: above, these are trace- 

 able into the internal capsule of the hemisphere. 



The red nucleus (Fig. 451), or nucleus of the tegmentum, is a cylindrical mass 

 of gray matter on each side of the middle line. On cross-section it is seen as a 

 reddish circle in about the middle of the tegmentum ventral to the aqueduct of 

 Sylvius. 



The following structures are all grouped on the dorsal surface of the mid-brain. 

 They belong to the tegmentum, since they are dorsal to the substantia nigra. 

 This dorsal surface is limited by the sulcus lateralis on each side. 



The corpora or tubercula quadrigemina are four rounded eminences placed in 

 pairs, two in front and two behind, and separated from one another by a median 

 longitudinal and a transverse groove. 



They are situated immediately behind the third ventricle and posterior com- 

 missure, and beneath the posterior border of the corpus callosum. Below, they 

 rest upon a layer of white matter, the quadrigeminal lamina, immediately beneath 

 which, in the median line, is the aqueduct of Sylvius. The anterior or upper 

 pair are the larger, oblong from betore backward, and of a gray color. The pos- 

 terior or loicer pair are hemispherical in f >rm and lighter in color than the pre- 

 ceding. From the outer side of each of these eminences a prominent white 

 band, termed braeki/um, is continued. Those from the anterior pair (brachia 

 iinti-riora) are long and slender, and each passes at first obliquely outward, and 

 then curves backward, downward, and forward around the posterior extremity 

 of the optic thalamus, which overhangs it, and then between the inner and 

 outer geniculate bodies into the optic tract. Those from the posterior pair 

 (In-ti'-lda posterior d) are comparatively short and broad, and each passes to an 

 oval prominence, the internal geniculate body, beneath which it apparently dis- 

 appears. Both pairs of these bodies are composed of white matter externally 

 and gray matter within. In fishes, reptiles, and birds these bodies are only two 

 in number, and are called the optic lobes, from their connection with the optic 

 nerves ; but in mammalia they are four in number, as in man. In the human 

 fcetus they are developed at a very early period, and form a large proportion 

 of the cerebral mass. 



These bodies are apparently connected with the cerebellum by means of the 

 superior peduncles of the cerebellum, which are continued onward to the optic 

 thalami through the tegmentum, as already mentioned. 



Arching over the upper ends of these peduncles is a flattish triangular-shaped 

 band of white fibres, the lemniscus or lower fillet, which issues from beneath the 

 transverse fibres of the pons to pass obliquely round the upper end of the superior 

 peduncle of the cerebellum and become lost in the inferior quadrigeminal body. 



The corpora geniculata are two small, flattened, oblong masses, placed on the 

 under and back part of each optic thalamus, and named, from their positions, 

 corpus geniculatum externum and internum. The two bodies are separated from 

 one another by the brachium anterius from the anterior corpus quadrigeminum. 



Structure of the Corpora Quadrigemina and Geniculata. The peripheral 

 ///"// matter of the corpora quadrigemina differs somewhat in the posterior and 

 anterior bodies. The posterior are composed almost entirely of gray matter, 

 covered over by a thin stratum of white matter, and separated from the central 

 .'//"// matter of the aqueduct by tracts of transverse white fibres derived from, and 

 forming part of, the lower fillet. The anterior are covered superficially by a thin 

 stratum of white matter : beneath this is a layer of gray matter, termed the 

 xtrntuni 'i/'creutn, and consisting, as well as the gray matter of the posterior 

 l<>bes, of small multipolar cells imbedded in a fine network of nerves. Beneath 

 this, again, is a characteristic mass of gray matter, termed the stratum opticum. 



