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The Cerebellum

vermis, worm, hemispheres, figs, surface, cerebelli and superior

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THE CEREBELLUM The rhombencephalon is composed of the isthmus, the cere bellum, the pons and the medulla oblongata (Figs. 20, 21, and 33). It is the lozenge-shaped brain. It is evolved from the third primary brain-vesicle; therefore, it is the hind-brain. In con trast with the cerebrum, it is the little-brain.

The narrow connection between the second and third primary brain-vesicles constitutes the isthmus rhombencephali. The isthmus is almost without length; it is little more than the plane of union between the mid-brain and the hind-brain. Through the isthmus pass the various tracts to and from the cerebrum, contained above this level in the base and tegmentum of the peduncle; the superior end of the fourth ventricle is bounded by it and from its dorsal surface the trochlear nerve makes its exit from the brain.

At the fourth week in utero, the third primary brain-vesicle is subdivided into two secondary vesicles, the metencephalon (above) and the myelencephalon (below). The myelencephalon (marrow-brain) is the embryonic medulla oblongata; the meten cephalon develops the pons and cerebellum. The cerebellum is the dorsal and the pons the ventral part of the metencephalon. The preponderant size of the cerebellum is due to the accumula tion of emigrant cells in the dorsal zones and roof-plate of the metencephalon; the cells are derived from the somatic sensory column of the medulla in the region of the rhomboid lip (Her rick). The large cerebellum is characteristic of man. Its weight is 14o grams (5 ounces) slightly more than one-tenth of the whole brain. It is situated in the posterior fossa of the skull, under the tentorium cerebelli and dorsal to the pons and medulla oblongata. Between it and the last two structures is enclosed the fourth ventricle. The cerebellum is distinguished from the cerebrum by its stratification. Its surface is composed of gray substance, the cortex (substantia corticalis); its interior is white and is called the medullary body (corpus medullare, Figs. 104 and 107).

Function.—The cerebellum is first of all a correlation center for the muscle- and static-senses; and, second, a general subcerebral correlation center for all forms of afferent impulses. In response to these impulses it originates impulses which coordinate muscles and maintain equilibrium. Coordination is the one well

established function. The cerebellum also acts as a relay in the indirect afferent and efferent paths. Moreover, physiolo gists claim that it constitutes an augmenter-center, elevating muscle-tone, increasing the power of muscular contraction and accelerating the rate of motor discharges so as to obtain steady, tonic contraction. • Divisions.—The cerebellum is made up of two lateral parts, the hemispheres, and a central part, uniting the hemispheres to gether, called the vermis cerebelli, or worm (Figs. 'or, roe, and io6). In the early embryo the cerebellum is a transverse ridge in the roof of the fourth ventricle, partially divided for a time by a median groove on its ventricular surface; and it remains undifferentiated into medial and lateral parts in many lower animals (Edinger).

The cerebellar hemispheres (hemispheria cerebelli) measure 5 cm. (2 inches) from before backward and about the same in thickness, near the anterior end of the vermis; but they taper rapidly toward the lateral borders (Figs. ror and 102). They present a sharp anterior angle and a rounded lateral angle. The hemispheres are joined together by the worm, or vermis, which forms the central and most elevated part of the cerebellum.

The vermis cerebelli, or worm, is a small elongated lobe, shorter and much thinner than the hemisphere (Figs. ror and ro6). In animals lower than mammals it is not differentiated from the hemispheres and appears to be the only part of the cerebellum present, being very large in birds and swimming reptiles (Edin ger). Its transverse ridges give it a worm-like appearance. It unites the upper half of the medial aspect of the two hemi spheres, their lower halves being separated by an antero-pos terior groove, called the valley or vallecula cerebelli. The upper surface of the vermis is called the superior worm, or vermis superior; and the lower surface, the inferior worm, or vermis inferior. The superior and inferior surfaces are separated from one another at the posterior end of the worm by the great hori zontal sulcus; anteriorly, the medullary body of the cerebellum separates them. At either end of the worm is a notch bounded by the vermis and the hemispheres, the anterior and posterior cerebellar notches.

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