THE RABBIT. 377 



cavity, bounded laterally by the dorsal pyramids of the 

 medulla, and roofed in by the cerebellum and the pia 

 mater. Its floor is marked by a median groove, ending 

 posteriorly in a pointed depression, the calamus scrip- 

 torius, which leads into the central canal of the spinal 

 cord. 



523. The valve of Vieussens (v.vn), a thin transverse 

 band, forming a sort of ledge over the anterior end of the 

 fourth ventricle, and connected by its anterior edge with 

 the testes ; the fourth pair of nerves spring from its upper 

 surface : beneath it is a small aperture leading into the 

 aqueduct of Sylvius ( 526). 



524. The anterior (a.pn\ middle, and posterior 

 (p-pn) peduncles of the cerebellum : the anterior connect 

 it with the testes, and so bound the valve of Vieussens 

 laterally ; the middle are continuous on each side with the 

 pons Varolii ; and the posterior connect it to the dorsal 

 side of the medulla oblongata, and are the anterior ends of 

 the restiform bodies. 



525. The arbor vitae, or tree-like appearance produced 

 in a section of the cerebellum by the intermixture of its grey 

 and white matter. 



LXIII. Pass a guarded bristle from the fourth ventricle 

 through the aperture beneath the valve of Vieussens 

 ( 5 2 3)5 an d cut down upon it by removing the 

 optic lobes, so as to expose : 



526. The aqueduct of Sylvius (Fig. 72), or iter a 

 tertio ad quartum ventriculum, a narrow passage by 

 which the third and fourth ventricles are put in communica- 

 tion with one another. It is bounded above by the optic 

 lobes, below by the crura cerebrL 



527. Note at the same time the absence of optic ven- 

 tricles or cavities in the optic lobes. 



