Now turn forward the cerebellum; pull out the vascular _choroid plexuses_ of the pia, which fill the fourth ventricle; and bring the upper surface of the _medulla oblongata_ into view. The _fourth ventricle_ is a triangular depression terminating in a posterior point called the _calamus scriptorius_. (Here a very fine probe may pass into the central canal of the spinal cord.) The lateral boundary of the ventricle on either side is formed by the _restiform body_ or _column_, which runs into the cerebellum, forming its _inferior_ or _posterior peduncle_ on that side. Including the calamus scriptorius by their divergence, the posterior columns of the spinal cord continue into the medulla as the _fasciculi graciles_. These are at first separated from the broad restiform bodies by a slight groove. But this disappears anteriorly, and the 'slender' and 'ropelike' strands soon become outwardly indistinguishable. Turn next to the ventral surface of the medulla, and note the _anterior pyramids_, two roundish cords, one on either side of the slight _median groove_. The pyramids are crossed and closed over anteriorly by the _pons Varolii_, a broad transverse band which surrounds them like a collar, and runs up into the cerebellum on either side, forming its _middle peduncles_. The pons has a slight median depression and its posterior edge is formed by the _trapezium_ on either side. The trapezium consists of fibres which, instead of surrounding the pyramid, seem to start from alongside of it. It is not visible in man. The _olivary bodies_ are small eminences on the medulla lying just laterally of the pyramids and below the trapezium. [Illustration: FIG. 35.--Fourth ventricle, etc. (Henle). _III_, third ventricle; _IV_, fourth ventricle; _P_, anterior, middle, and posterior peduncles of cerebellum cut through; _Cr_, restiform body; _Fg_, funiculus gracilis; _Cq_, corpora quadrigemina. ] Now cut through the peduncles of the cerebellum, close to their entrance into that organ. They give one surface of section on each side, though they receive contributions from three directions. The posterior and middle portions we have seen: the _anterior peduncles_ pass forward to the _corpora quadrigemina_. The thin white layer of nerve-tissue between them and continuous with them is called the _valve of Vieussens_. It covers part of the canal from the fourth ventricle to the third. The cerebellum being removed, examine it, and cut sections to show the peculiar distribution of white and gray matter, forming an appearance called the _arbor vitæ_ in the books.