Both the hemispheres and the median lobe are composed of an assemblage of lamin closely applied to each other. Each lamina consists of a thin layer of white or fibrous matter, between two of grey or vesicular sub stance, which are continuous along the outer margin of the former. Thus the exterior of the cerebellum consists of a stratum of vesicular matter, which forms a cortex to the enclosed white or fibrous substance. The laminal are separated from each other by fissures, and they are covered by pia mater, which adheres closely to them, and penetrates to the floors of the fis sures.
The larnirm are collected into sets on the superior as well as on the inferior surface. Each set forms a lobe. Each lobe is surrounded by a deep fissure, which separates it from the next adjacent lobes.
It is necessary to distinguish the fissures which separate the laminze from those by which the lobes are hounded. The former are very shallow ; the latter are deep, and penetrate quite to the central stein of the hemisphere.
By removing the pia mater carefully from the surface of the hemispheres, and from the deep fissures, the shape and boundaries of the lobes may be clearly demonstrated. Or if a vertical section of a hemisphere be made, the deep fissures may be readily distinguished from the superficial ones which separate the laminw ; and in this way also the lobes may be demon strated.
The floor of each deep fissure is formed by white matter. And as the deep fissures inter vene between the lobes, laminm of the lobes constitute their walls, and the superficial fis sures which separate these laminze open into them.
On the superior surface of the cerebellum two principal lobes rnay be distinguished. These are the square lobe and the posterior superior lobe, according to the nomenclature of Reil, whose descriptions cannot be sur passed in minuteness or accuracy. ( .b'ig. 390, A, P.) The anterior margin of the square lobe over hangs the semilunar fissure ; its posterior mar gin is a little behind the level of the floor of the posterior notch. By careful separation of its lamin or by a vertical section, it may be shewn to consist of eight lobules, each having a stem of fibrous matter derived from the cen tral one of the hemisphere.
The posterior superior lobe (P, fig. 390,) forms the posterior part of the superior surface of the cerebellum ; its posterior margin is that of the hemisphere; the horizontal fissure sepa rates it from the posterior inferior lobe. It is separated from its fellow of the opposite side by the posterior notch.
On the inferior surface of each hemisphere the following lobes are readily distinguishable. (Fig. 391.) We enumerate them, passing from before backwards.
1. The araugdala, so called from its resem blance to an enlarged tonsil. This and its
fellow of the opposite side form the lateral boundaries of the anterior extremity of the valley, and are in great part covered by the medulla oblongata.
2. Behind the amygdala is the biventral lobe, wedge-shaped, narrow towards the valley, wide towards the semilunar fissure. Its laminm are curved with their concavity forwards and inwards, and it is united with its fellow of the opposite side by larninm which cross the val ley forming part of the inferior vermiform process.
3. The slender lobe, which consists of a few laminm curved parallel to the posterior ones of the biventral lobe.
4. The iqferior and posterior lobe, which extends to the posterior edge of the hemi sphere. The inner margin of each of these lobes constitutes the lateral boundaries of the posterior notch.
Such is the constant disposition of the supe rior and inferior surfaces of the cerebellum. A defect of symmetry is sometimes apparent in the inequality of corresponding lobes; hut those above enumerated are always present. So definite an arrangement must obviously have some physiological import. What that may be it is impossible even to conjecture, and we must be, for the present, content with a concise statement of the facts of the anatomy. Some analogy exists between this arrangement and that of the convolutions on the surface of the brain, many of which exhibit a constancy of position and form quite as remarkable.
The median portion of the cerebellum is also composed of laminm, which are continuous with those of the hemispheres, but their arrangement on the superior and inferior surfaces is so diffe rent as to demand a separate description. On the superior surface the latninT are separated from each other by fissures, in the sarne way as those which constitute the hemispheres, and they are collected into sets forming lobes which correspond to and connect those of the lateral hemispheres. These laminw are curved, their anterior margin being very slightly convex (fig. 390). The edges of these famin, as they lie in close apposition, resemble the segments or rings of a worm ; whence the term vermiform has been applied to this as well as the inferior surface of the median lobe. The lamin take for the most part a vertical direction, with the exception of the anterior and posterior ones, which pass gradually to the horizontal, the free rnargins of the former being directed forwards and those of the latter backwards. The pos terior laminm form the floor of the posterior notch : the anterior form, by their adhesion to each other, the layer known by the name of valve of Vieussens, which fills up the interval betvveen the processus cerebelli ad testes.