The cerebellum (fig. 324, a) is at once recognizable from its position and singleness. In the Perch its form is that of a blunted cone, with the summit directed slightly backward; but the shape and relative dimensions of this part of the brain are extremely variable. It consists, in fishes, only of the central portion (processus vermiformis), so that there are neither lateral lobes nor pons Varolii: its surface is composed of cineritious substance, and in its centre is a ramified medullary axis containing a ventricle.
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