Anatomy of the Brain. 567 



the spinal cord from its lower extremity to its upper end, where it merges 

 into the medulla oblongata. It also continues through this latter sec- 

 tion till it opens into the Fourth Ventricle. The hind end of this ven- 

 tricle is a sharp point, made so by the divergence from each other of the 

 posterior pyramids. From its shape it is called the calamus scriptorius 

 ( or writing pen ). Across the floor of this space are transverse striae, 

 or lines, which are called the barbs of the calamus scriptorius. The 

 calamus scriptorius is covered over by a medullary membrane of trian- 

 gular shape, called the valve of Tarin ( Valvula Tarini). It is fixed by 



FIG. 27Q.-'Cast Showing Form and rel- 

 ative position of Ventricles of the Brain. 

 ( Top view.) ( From Gray's Anatomy, by 

 permission.) 



1, 2. Lateral Ventricles. 



3. Third Ventricle. 



4.- Fourth Ventricle. 



etc. Anterior cornu of lateral ventricles. 



joe. Posterior 



ic. Inferior or descending cornu of lat- 

 eral ventricles. 



fm. Foramen of Munro. 



aq. Aqueduct of Svlvius. 



c, . Canal of the Spinal Cord, or sixth 

 ventricle. 



its lateral edges to the restiforme 

 bodies, and by its third side, or 

 base, to the posterior vermiform 

 process of the cerebellum. The 

 rest of the roof of the fourth ven- 

 tricle is formed by the cerebellum, 

 as the greater part of the cavity is 

 filled by its middle lobe, or vermi- 

 form process. The lateral walls 

 are formed by the restiform bodies, 

 and the floor by the fibres of the 

 posterior pyramids, which here be- 

 gin to form the dorsal part of the 

 crura cerebri. The fourth ventri- 

 cle is bounded at its front end by the valve of vieussens, beneath which 

 is the entrance into the next ventricle, the aqueduct of sylvius. This 

 channel passes underneath the corpora quadrigemina and above the crura 

 cerebri, and opens at its front end into the Third Ventricle. The third 

 ventricle has for its rear terminus the posterior commissure of the optic 

 thalamus, beneath which is the entrance into the aqueduct of sylvius. 

 Its roof is the middle or gray commissure, and its floor the crura cerebri. 

 Its sides are the inner surfaces of the optic thalami. Near its front 

 end, which is wider than the rear, is a depression in the floor and an 

 opening leading downward into the interior of the tuber cmereum, and 

 thence to the infundibulum. In front of this, and at the extreme end, 

 there is a thin lamina of gray matter, called the lamina cinerea. The 



FIG. 276. 



