532 THE ORGANS 



of the reticular formation has nearly disappeared. The medial longitudinal 

 fasciculus is much diminished as its ascending iibres terminate in the nucleus of 

 nerve III and many of its descending fibres originate from cells below this level. 



Efferent Suprasegmental Neurones. — The pes pedunculi occupies the same 

 position, and dorsal to it is the diminished substantia nigra. Along the ventro- 

 mesial border of the pes a bundle of fibres can sometimes be distinguished 

 (Fig. 352, Lmp), which at lower levels comes to lie mesial to the medial 

 lemniscus (aberrant peduncular fibres, comp. p. 527). Descending pallial 

 fibres (not distinguishable) also probably form part of the thalamic radiations 

 (pp. 529, 536). 



Fibres of the superior cerebellar peduncle may be seen within and around 

 the nucleus ruber. Some of these terminate in the latter, some pass further 

 forward to end in the thalamus (compare pp. 513, 527). 



The Superior Colliculus is somewhat diminished. The posterior conifnissure 

 passes across in the roof dorsal to the central gray (see preceding section). 



The Thalamus (can hardly be included under the preceding structures). — 

 The corpora geniculata have been mentioned. The pulvinar thalami is a large 

 gray mass dorsal to the medial geniculate body. Fibres passing laterally from it 

 contribute to the retrolenticular portion of the internal capsule (Ctrl). These fibres 

 are a part of the thalamic radiations. 



The nucleus caudatus (a portion of the corpus striatum of the endbrain) is 

 present. 



12. Section through the Interbrain at the Level of the Optic Chiasma. 

 (Figs. 332 and 357.) 



Efferent Peripheral Neurones. — None present. 



Afferent Roots, their Terminal Nuclei, Secondary Tracts, and Tertiary 

 Neurones. — Fibres of the optic nerve are seen decussating and forming the optic 

 chiasma. The further continuation of the optic fibres to their termination is 

 called the optic tract. Both nerve and tract constitute the secondary optic tract, 

 the optic chiasma being analogous to the decussation of the medial lemniscus and 

 of the lateral lemniscus (trapezius). (For further description of optic paths see 



Fig. 358.) 



The medial lemniscus, spino-thalamic, and secondary trigeminal tracts are 



Fig. 357. — Section through the Interbrain at the Level of the Optic Chiasma. (The 

 chorioid plexus of the third ventricle has been removed.) Weigert preparation. (Mar- 

 burg) Ce, capsula externa; Cex, capsula extrema; Chll, chiasma nervorum opticorum 

 (or optic chiasma); Ci, capsula interna; CI, claustrum; Cml, ganghon laterale corp. 

 mammillaris; Cmm, ganglion mediale corp. mammill.; Coa, commissura anterior; Cospm, 

 commissura supramammillaris; Csth, corpus subthalamicum; e, nucleus externus gangl. 

 med. corpor. mammillaris; Fmp, fasciculus mammillaris princeps; Fo, fornix; Fp, fibrse 

 perforantes (pedunculi); j/7/, fasciculus retroflexus (Meynert); Fsp. fasciculus subthal- 

 amico-peduncularis; Fit, fasciculus uncinatus; Ghh, ganglion habenulas; glp, globus palli- 

 dus; H, area tegmenti Forel; ///, pars dorsalis areas tegmenti; ////, pars ventralis areae 

 tegmenti; 7, insula Reillii; /, nucleus internus gangl. medial, corp. mammillaris; Lvil, 

 lamina medullaris lateralis; Narc, nucleus arcuatus thalami; Nc, nucleus caudatus; 

 NL, nucleus I^uysii (nucleus centralis, or median centre, thalami;) A7, nucleus lateralis 

 thalami; Nlv, nucleus lateralis ventralis thalami; Ntg, nucleus ruber tegmenti; Pp, 

 pes pedunculi; P;/,.putamen; SnS, substantia nigra; St, stria cornea; Strz, stratum zonale 

 thalami; Til, tractus opticus; Tbc, tuber cinereum; Tt, taenia thalami; VIII, ven- 

 triculus tertius; Zi, zona incerta. 



