154 THE NERVOUS SYSTEM. 



The pituitary body (Hypophysis cerebri] is a flattened sac, placed 

 behind the tuber cinereum and continuous with it by means of the 

 infuudibulum. 



ft. Internal structure. The aqueduct of Sylvius, after commu- 

 nicating with the ventricles of the optic lobes, again contracts 

 (Fig. 105), but still remains somewhat larger than before. In the 

 thalamencephalon the Sylvian aqueduct opens into the third 

 ventricle, which gradually assumes the form of a vertical slit with 

 the walls bulging slightly outwards in their upper parts. The 

 thin roof of this ventricle, where complete, contains a band of 

 transverse fibres. The floor is depressed both before and behind 

 the part immediately above the optic chiasma, the posterior de- 

 pression lying above the tuber cinereum, which here descends 

 towards the infundibulum : a transverse section through this por- 

 tion of the third ventricle has the form of a square standing on 

 one angle, the superior angle being produced into the vertical 

 slit of the general cavity. The anterior depression is formed 

 by the general cavity being prolonged downwards and forwards to 

 the lamina termiualis in the form of a narrow and shallow slit. 



The white and grey matter of the thalamencephalon are only so 

 far distinct in that the portion immediately surrounding the cavity 

 is darker than the rest of the section. In the immediate neighbour- 

 hood of the cavity are many small cells and nuclei, which become 

 scarcer further from the ventricle ; they are arranged in rows, 

 separated by a fibrillated matrix. On either side is a distinct bundle 

 of longitudinal fibres, the ' round bundle ' of Koppen, which come 

 from the optic lobes but appear to arise from either the pars com- 

 missuralis or the medulla oblongata, and to receive additional fibres 

 from the optic thalami ; they pass forwards to the base of the 

 cerebral hemispheres (Stieda). A second set of longitudinal fibres 

 arises in the substance of the tuber cinereum and passes forwards 

 to the hemispheres (strand of the Tuber -cinereum and Thalamus, 

 Koppen) ; this band, together with the ' round bundle/ form a cms 

 cerebri (Koppen). 



The commissural fibres of the thalamencephalon are : (i) a com- 

 missura transversa Halleri in the posterior portion of the organ ; 

 (2) an optic commissure, consisting of fibres arising from the thala- 

 mencephalon (thalamencephalic root) and crossing the median line 

 to join the optic tract of the opposite side ; (3) a probable com- 

 missure between the optic nerves just in front of the chiasma 

 ; the existence of the latter is not yet proved beyond 



