724 



THE N^R VOL'S SYSTEM 



Dissection.— If the student has fullowed out the dissection previously reconiuiended, the 

 brain will now he rediieed to the medulla, the pons, the mesencephalon, and small portions of the 

 liemispheres (chietiy thalamenceplialon). He should now tear oft" the transverse fibres of the 

 pons on one side of the mid<lleline, so as to bring into view the flattened bundles which Ibrm the 

 continuation of the pyramidal body through the jions. By tearing these pyramidal fibres in a 

 sagittal direction, the dissector will be able to follow them through the crusta into the inner cap- 

 sule. La.stly the dissector should make a series of transverse sections through the remaining por- 

 tion of brain-substance. An inspection of figs. 423, 424, 429, 430, and 431 will materially aid 

 him in comprehending the somewhat faint indications which he will observe in his sections. 



The following are the principal jjoints which can be made out by the unaided eye in an 

 ordinary brain, hardened in spirit, and treated iu the above-mentioned manner. (In a brain 

 hardened in Miiller's fluid nnuiy additional details can be made out.) In the uitiiermost sections 

 the tegmental or red nuck'us is readily recognised by its rounded outline ; it lies below the optic 

 thalamus, and extends backwards for a considerable distance (fig. 423). If the section pass through 

 the external geniculate body, curved bands of white matter intersecting the grey matter can be 

 seen. Sections through the upper part of the mesencephalon show the red nucleus in the teg- 

 mentum, the substantia nigra, the crusta, and the aqueduct of Sylvius. Lower down, the decus- 

 sation of the superior peduncles of the cerebellum forms a conspicuous white centre in the teg- 

 mentum ; the grey matter surrounding the aqueduct is distinct in colour from the rest of the 

 section, and on the ventral side of this grey matter the posterior longitudinal bundle can be faintly 

 made out. Sections through the inferior corpora quadrigemina show the posterior longitudinal 



Ficf. 430. — Transverse Section of the Pons near the Centre of the Foirtu 



Ventricle. (Schwalbe. ) 



SEVESTH NERVE 



NUCLEI'S OF SIXTH 



SEVENTH 

 NEB VE 



KA I'HE 



NUCLEUS OF EIGHTH 



FORMA TIO 

 RETICULARIS 



NUCLEUS OF 

 SEVENTH 



ASCENDING 



ROOT OF 



FIFTH 

 AlI'irORY 



NERVE 

 SUPERIOR 



OLIVE 



SIXTH NERVE 



CONTINUA TION OF 

 PYRAMIDS 



bundle more distinctly ; the superior peduncles of the cerebellum lie on the ventral side of the 

 bundle, touching one another at the raphe, but commencing to rise up around the sides of the 

 aqueduct ; ventral t(j these the tract of the fillet may be seen as a white band ; and ventrally to 

 the fillet the longitudinal fibres of the pons are well seen intersecting the transverse fibres. 

 Sections through the valve of Vieussens (fig. 429) show the su)>erior peduncles of the cerebellum 

 in the form of distinct white crescents placed laterally to the floor of the fourth ventricle. The 

 posterior longitudinal bundle is now very distinct. It appears pear-shaped in cross-section with 

 the narrow end jiointing (jutwards and abutting against the substantia ferruginea ; the latter stands 

 out as a conspicuous dark spot. The tract of the fillet is easily recognised as a white flattened 

 oval bundle. The area majiped out between the posterior longitudinal bundle, the sufjcrior 

 ])eduncles of the cerebellum, and the tract of the fillet, is occupied by the formatio reticularis. 

 In sections distal to this, the i>ost.erior longitudinal bundle and the tract of the fillet rajndly lose 

 their distinctness, and in the medulla little can be nnide out as a rule excei)t the olivary nucleus. 

 The latter appears as a thin wavy grey line occui)ying the interior of the olivary body. 



Internal structure of the medulla. — Transverse sections of the iiiedulla show 

 tliat it is composed of syminctrical halves se])arated by a central ra])he. Tlie raplie 

 is entirely formed by decussating fibres. Eacli lateral half is composed of both 

 grey and \yhite matter. The white matter will be dealt with in common with the 

 tracts of the spinal cord and with the cranial nerves. 



The arrangement of the grey matter in the medulla is, for the most part, 



