The hindbrain or rhombencephalon consists of the medulla (myelencephalon), pons (metencephalon), and the cerebellum as its largest structure. The cerebellum consists of two hemispheres joined medially by a relatively narrow vermis, sits in the posterior cranial fossa of the skull beneath the tentorium cerebelli, and is separated from the medulla and pons by the fourth ventricle. The cerebellar cortex has many curved transverse fissures in the form of narrow infoldings called folia. Structurally, the cerebellum is covered by a cortex of gray matter with a medulla of white matter, which holds four intrinsic pairs of nuclei (see below). Observation of the superior surface shows two deep transverse fissures, the primary and the posterior superior fissures. Viewed from the ventral surface, the cerebellum is divided approximately into superior and inferior halves by the horizontal fissure. Three pairs of cerebellar peduncles connect the cerebellum to the three lower brain segments. The inferior, middle, and superior cerebellar peduncles connect it to the medulla, pons, and midbrain, respectively.
The superior vermis lies between the hemispheres as a longitudinal ridge; it is more clearly differentiated visually from the hemispheres on the ventral surface, where it is divided by fissures into the nodule, uvula, and pyramid. A stalk extends from the nodule on each side to the flocculus, which forms the flocculonodular lobe. The tonsil is a lobule that lies over the inferior vermis. The inferior medullary velum is exposed if the tonsil is removed.
From an embryological and functional viewpoint, the cerebellum can be divided into three main parts. (i) The archicerebellum, or flocculonodular node, is made of the pairs of flocculi and their peduncular connections. The flocculonodular node is the most ancient part of the cerebellum, present in fish as well as humans, and is connected with the vestibular nuclei and system. It is connected particularly with the dentate nucleus, one of the intrinsic medullary cerebellar nuclei. (ii) The paleocerebellum, or anterior lobe of the cerebellum, lies dorsal to the primary fissure. The lobe also includes the pyramid and uvula of the inferior vermis. The anterior lobe receives inputs via the spinocerebellar tract, originating in stretch receptors, and is the lobe most involved in the control of involuntary muscle tone. This lobe is connected principally to the globose and emboliform nuclei, which project to the red nucleus, and thence to the central tegmental, rubroreticular, rubsospinal, and rubrobulbar efferent pathways. The paleocerebellum evolved in terrestrial vertebrates, which need to use limbs to support the body against the pull of gravity; therefore its connections are mainly spinal, and its functions are concerned with such stereotyped movements such as posture, locomotion, and muscle tone. (iii) The neocerebellum, which, as its name implies, is the phylogenetically newest part of the cerebellum, communicates with the thalamus and motor cortex. This lobe is made up of virtually all the posterior lobe, except for the pyramid and uvula of the vermis. The neocerebellum modulates non-stereotyped, learned behavior such as the learning of manual skills.