The roof is arched; it is formed by the valve of Vieussens and the under surface of the cerebellum, which presents in this situation four small eminences or lobules, two occupying the median line, the nodulus and uvula, the remaining two, the amygdala, being placed on either side of the uvula.

  The anterior boundary, Floor, is formed by the posterior surface of the medulla oblongata and pons. In the median line is seen the posterior median fissure; it becomes gradually obliterated above, and terminates below in the point of the calamus scriptorius, formed by the convergence of the posterior pyramids. At this point is the orifice of a short canal terminating in a cul-de-sac, the remains of the canal which extends in foetal life through the centre of the cord. On each side of the median fissure are two slightly convex longitudinal eminences, the fasciculi teretes; they extend the entire length of the floor, being indistinct below and of a greyish colour, but well marked and whitish above. Each eminence consists of fibres derived from the lateral tract and restiform body, which ascend to the cerebrum. Opposite the crus cerebelli, on the outer side of the fasciculi teretes, is a small eminence of dark grey substance, which presents a blueish tint through the thin stratum covering it; this is called the locus caeruleus; and a thin streak of the same colour continued up from this on either side of the fasciculi teretes, as fat-as the top of the ventricle, is called the taenia riolacca. The lower part of the floor of the ventricle is crossed by several white transverse lines, lineae trausversae; they emerge from the posterior median fissure; some enter the crus cerebelli, others enter the roots of origin of the auditory nerve, whilst some pass upwards and outwards on the floor of the ventricle.

  The Lining Membrane of the fourth ventricle is continuous with that of the third, through the aqueduct of Sylvius, and its cavity communicates below with the sub-arachnoid space of the brain and cord through un aperture in the layer of pia mater extending between the cerebellum and medulla oblongata. Laterally, this membrane is reflected outwards a short distance between the cerebellum and medulla.

  The Choroid Plexuses of the fourth ventricle are two in number; they are delicate vascular fringes, which project into the ventricle on each side, passing from the point of the inferior vermiform process to the outer margin of the resti-form bodies.

  The Grey Matter in the floor of the ventricle consists of a tolerably thick stratum, continuous below with the grey commissure of the cord, and extending up as high as the aqueduct of Sylvius, besides some special deposits connected with the roots of origin of certain nerves. In the upper half of the ventricle is a projection situated over the nucleus, from which the sixth and facial nerves take a common origin. In the lower half are three eminences on each side for the roots of origin of the eighth and ninth nerves.

  Structure. If a vertical section is made through either hemisphere of the cerebellum, midway between its centre and the superior vermiform process; it will be found to consist of a central stem of white matter, which contains in its interior a dentate body. From the surface of each hemisphere, a series of plates of medullary matter are detached, which, covered with grey matter, form the laminae; and from its anterior part arise three large processes or peduncles, superior, middle, and inferior, by which it is connected with the rest of the encephalon.

  The Lamina are about ten or twelve in number, including those on both surfaces of the organ, those in front being detached at a right angle, and those behind at an acute angle; as each lamina proceeds outwards, other secondary laminae are detached from it, and from these tertiary laminae. The arrangement thus described gives to the cut surface of the organ a foliated appearance, to which the name arbor vita has been given. Each lamina consists of white matter, covered externally by a layer of grey substance.

  The white matter of each lamina is derived partly from the central stem; in addition to which white fibres pass from one lamina to another.

  The grey matter resembles somewhat the cortical substance of the convolutions, consisting of two layers, the external one, soft and of a greyish colour, the internal one, firmer and of a rust colour.

  The Corpus Dentatum, or Ganglion of the Cerebellum, is situated a little to the inner side of the centre of the stem of white matter. It consists of an open bag or capsule of grey matter, the section of which presents a grey dentated out line, being open at its anterior part. It is surrounded by white fibres; white fibres are also contained in its interior, which issue from it to join the superior peduncles.

  The Peduncles of the cerebellum, superior, middle, and inferior, serve to connect it with the rest of the encephalon.

  The Superior Peduncles (Processus e Cerebello ad Testes) connect the cerebellum with the cerebrum; they pass forwards and upwards to the testes, beneath which they ascend to the crura cerebri and optic thalami, forming part of the diverging cerebral fibres: each peduncle forms part of the lateral boundary of the fourth ventricle, and is connected with its fellow of the opposite side by the valve of Vieussens. Behind, it is continuous with the folia of the inferior vermiform process, and with the white fibres in the interior of the corpus dentatum. Beneath the corpora quadrigemina, the innermost fibres of each peduncle decussate with each other, so that some fibres from the right half of the cerebellum are continued to the left half of the cerebrum.

  The Inferior Peduncles (Processus ad Medullam), connect the cerebellum with the medulla oblongata. They pass downwards, to the back part of the medulla, and form part of the restiform bodies. Above, the fibres of each process are connected chiefly with the laminas, on the upper surface of the cerebellum; and below, they are connected with all three tracts of the half of the medulla, and, through these, with the corresponding half of the cord, excepting the posterior median columns.

  The Middle Peduncles (Processus ad Pontem), the largest of the three, connect together the two hemispheres of the cerebellum, forming their great transverse commissure. They consist of a mass of curved fibres, which arise in the lateral parts of the cerebellum, and pass across to the same points on the opposite side. They form the transverse fibres of the pons Varolii.