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The cerebellum is the largest part of the hindbrain and lies behind the pons and medulla oblongata. It's median portion is separated from the pons and the medulla oblongata by the 4rth ventricle. It lies below the tentorium cerebelli, which separates it from the cerebrum. The cerebellum synchronizes and coordinates the voluntary movements of the body.
Anteriorly it is related to the pons and the medulla oblongata.
Posterio-inferiorly it is related to the squamous part of the occipital bone.
superiorly it is related to the tentorium cerebelli.
the cerebellum weights about 150 grams in an adult man and forms 1/8 part of the cerebrum.
the cerebellum is connected with all parts of the brain stem through 3 pairs of cerebellar peduncles.
with medulla by inferior cerebellar peduncles.
with pons by the middle cerebellar peduncles.
and with the midbrain by the superior cerebellar peduncles.
External features of the cerebellum:-
The cerebellum consists of two cerebellar hemispheres joined by a narrow median strip called the vermis.
Borders: It has two borders.
Anterio-lateral border.
Posterior border which is convex.
Surfaces: It has two surfaces described as follows.
Superior surface: The whole superior surface appears somewhat plane with slight median ridge of the vermis i.e the superior vermis, but generally this surface is directly continous with the cerebellar hemispheres on each side.
Inferior surface: Inferior surface is actually posterio-inferior surface, which is convex from above downwards starting from the posterior border on this surface. The two cerebellar hemispheres are separated from each other by a deep groove called the "vallecula". The inferior surface of vermis projects into the floor of the vallecula. There the vallecula is limited on each side by "sulcus valleculae ".
Notches of the cerebellum:-
The cerebellum has got two notches as explained bellow.
Anterior cerebellar notch is wide but shallow V-shaped that lodges the pons and upper part of the medulla oblongata both of them are separated from the cerebellum by the 4rth ventricle. In the floor of the anterior notch the cerebellar peduncles (superior , middle and inferior cerebellar peduncles) pass into the cerebellum.
Posterior cerebellar notch is narrow but deep interval between the cerebellar hemispheres. This notch contains the "falx cerebelli" of the dura.
Surface features of cerebellum:-
The surface of the cerebellum is every where marked by closely spaced transverse but curved fissures which gives it a laminated appearance. These fissures are comparable to the sulci of the cerebrum. These fissures pass deeply into the cerebellar substance.
Between the two fissures each lamina is named as "folia" (singular is folium). These fissures are variable in depth. Some fissures are deeper than others and divide the cerebellar hemispheres and the vermis into several lobules.
Main fissures of the cerebellum are as follows:
Horizontal fissure: It is the most conspicous fissure of all the fissures of cerebellum. It extends along the lateral and posterior borders of each cerebellar hemispheres. It starts from the middle cerebellar peduncle anteriorly and extends posteriorly to the cerebellar notch. It marks the junction of the superior and inferior surfaces of the cerebellum.
Fissura prima: It lies on superior surface. This is V-shaped with it's apex directed downwards and rather dorsally. It divides the superior vermis into it's anterior 2/3 and posterior !/3. The portion of cerebellum which lies anterior to primary fissure constitutes the anterior lobe and the remainder of cerebellum on both surfaces between flocculo-nodullar and the anterior lobes constitutes the posterior or middle lobe of the cerebellum.
Posterio-lateral fissure: It appears on the inferior surface and separates the flocculo-nodular lobe from the rest of cerebellum.
Divisions of Vermis:
The fissures of the cerebellum continue on both hemispheres of cerebellum also passing over both surfaces (superior and inferior vermis) of vermis and dividing it into different parts.
Divisions on superior surface of Vermis: The superior surface of vermis is divided by short and deep fissures into different divisions in antereo-posterior order:
2-Central lobule.
Relations and divisions of vermis with adjoining cerebellar hemispheres (on superior surface) :
Except lingula each of the division of superior vermis continues laterally with adjoining lobule of the cerebellar hemispheres. The fissura prima cuts the superior vermis between culmen and declive. The lingula and central lobule are separated by postlingual fissure. The central lobule is continous laterally with ala of cerebellar hemispheres on both sides. The ala and central lobule are limited behind by postcentral fissure. Between postcentral fissure and fissura prima lies the culmen of vermis medially and quadrangular lobule of hemisphere laterally. Behind the fissura prima the superior surface of vermis is divided by curved Post-lunate fissure into following portions:
I- Anterior portion: Which consists of the declive of vermis and it's lateral extension of hemisphere the lobulus simplex .
II-Posterior portion: which lies between postlunate fissure and horizontal fissure. Thisportion consists of folium of vermis with adjoining part of cerebellar hemisphere called the superior semilunar lobule.
Divisions on inferior surface of vermis: This includes the inferior surface of vermis and inferior aspect of each cerebellar hemisphere. The inferior surface of vermis bellow the horizontal fissure is divided into four small portions named from behind forewards:
The tuber is continous laterally with inferior semilunar lobule. these parts are bounded behind by horizontal fissure and in front by prepyramidal fissure. the Pyramid is separated from the uvulum by post-pyramidal fissure. The pyramid is continous laterally with bi-ventral lobule on inferior surface of each hemisphere.
Anterior to bi-ventral lobule is a deep fissure called retro-tonsillar fissure. This fissure starts from the sulcus vallecula and curves forewards to anterior part of inferior surface of hemispheres.
Together with the anterior part of sulcus vallecula this retrotonsillar fissure bounds the circumscribed portion of cerebellum called tonsil which converts to uvulum accross the floor of the sulcus vallecula by a strip of cortex called furrowed band.
The nodule is the most anterior part of the inferior surface of the vermis. The nodule is separated behind from the uvulua by the Posterio-lateral sulcus (uvulo-nodular fissure).
On each side the nodule is continous with flocculus of cerebellar hemisphere constituting the flocculo-nodular lobe.
The flocculus is a small partially detached portion of cerebellum. It is somewhat oval in outline with crenated margin. From it's medial end a narrow band of fibers emerges which is called the Peduncle of flocculus.
Phylogenical and Functional subdivisions of cerebellum:-
Archi-cerebellum: It is made up of flocculo-nodular lobe and lingula. This is cheifly vestibular in connections. It is concerned with maintenance of body equillibrium.
Paleo-cerebellum: It constitutes the anterior lobe of cerebellum without lingula but including pyramid and uvula of inferior vermis. The paleocerebellum is concerened with spinal cord and thus is spino-cerebellar in connections. It controls tone of voluntary muscles, posture and crude movements of limbs.
Neo-cerebellum: It constitutes the middle lobe of cerebellum without pyramid and uvula of the inferior vermis. The neocerebellum has extensive connections with cerebral cortex through cortico-ponto-cerebellar fibers. It is primarily concerned with the regulation of the voluntary movements of the body.