The cerebellum occupies a posterior position with respect to the cerebral hemispheres. Externally, it can be divided into a vermis and two cerebellar hemispheres, or into lobes (rostral, caudal and flocculonodular) which can be further divided into lobules.
Functionally and philogenetoically, the cerebellum can be divided into a spinocerebellum (paleocerebellum), cerebrocerebellum Ineocerebellum) and vestibulocerebellum (archicerebellum). The spinocerebellum corresponds to the lingula, central, culmen, pyramis and uvula of the vermis, and to the quadrangular lobule, paramedian lobule, dorsal and ventral paraflocculus of the cerebellar hemispheres. It helps to maintain muscular tone and posture. A lesion in the spinocerebellum causes increased tone in the antigravity or extensor muscles resulting in opisthotonus and extensor rigidity in the limbs without altering the state of consciousness. This situation is known as decerebellate rigidity. The cerebrocerebellum corresponds with the declive, folium and tuber of the vermis, and to the simplex lobule and ansiform lobules. It participates in the planning and initiation of movements involving multiple joints. Lesions located in the cerebrocerebellum causes intention tremor (exaggerated movement of flexion and returned to ground with excessive extension). Anatomically, the cerebrocerebellum it is connected with associative and sensorimotor cerebral cortical regions. The vestibulocerebellum corresponds to the nodulus of the vermis and the flocculus of the cerebellar hemisphere (together they form flocculonodular lobe). It regulates eye movement and balance.
The gray matter forms an external layer, the cerebellar cortex with a lot of folds, and the cerebellar nuclei. These are located within the white matter. They are: the fastigial nucleus, the interpositus nucleus and the dentate nucleus (or lateral nucleus of the cerebellum). The fastigial nucleus controls posture by acting on the antigravity muscles. The interpositus nucleus control the amplitude and frequency of the movements. The dentate nucleus controls the initiation of movement.
The white matter is composed of fibers that look like a tree, and for this reason it is known as arbor vitae.
The cerebellum receives information from the locomotor apparatus and maintains connections with the pyramidal and extrapyramidal systems. It has an ipsilateral relationship with the muscles and a contralateral one with the pyramidal and extrapyramidal centers. In the case of the vestibular nuclei (balance) the connections are mostly ipsilateral. The principal function of the cerebellum is to maintain muscular tone and posture. It is also in charge of assigning automatic control of coordination or synergism between different muscular groups (agonists and antagonists).
The cerebellar peduncles connect the cerebellum with the brain stem. On each side the rostral peduncle (or brachium conjuntivum) connect it with the mesencephalon, the middle peduncle (or brachium pontis) connect it with the pons, and the caudal peduncle (or corpus restiforme and corpus juxtarestiforme) connect it with the medulla oblongata. The rostral and caudal peduncles are formed by afferent and efferent fibers. The middle peduncle is only composed of afferent fibers.
The rostral cerebellar peduncle is formed mainly by efferences to the red nucleus and thalamus. The afferences of the rostral peduncle come from the spinal cord. The information crosses inside the cerebellum.
The middle cerebellar peduncle is formed just by from contralateral afferences from the pontine nuclei. These receive the inputs from the corticopontine tract.
The caudal cerebellar peduncle receives afferences from the olivary nucleus (contralateral), the vestibular nuclei and vestibular receptors of the inner ear (ipsilateral), and from the spinal cord (ipsilateral and contralateral). The efferences (ipsilateral and contralateral) are to most nuclei of the brain stem.
 From the Latin, cerebellum, “small brain”.
 From the Latin, vermis, “worm”.
 From the Greek opisthen, “behind, in the back”; tonos, “tension” a form of spasm that consists of the hyperextension of the body in a form in which the head is flexed backwards.
 From the Latin fastigium, “ceiling, top, the highest".
 From the Latin interpositio, “interposition”.
 From the Latin dentatus, “similar to a tooth”.
 From the Latin “tree of life”.
 See Chapter 6.
 From the Latin ipse, “same”; latus, “side”.
 See Chapter 9
 From the Greek synergia, “cooperation”.
 From the Latin pedunculus, “small foot”.
 Lateral portion for afferent fibers.
 Medial portion for the vestibulocerebelar and cerebelovestibular fibers.
 From the Latin afferens, -ntis, ad, “toward” + ferre, “to take”.
 From the Latin efferens, -ntis, ex, "out" + ferre, "to take".