The _Cerebellum_, or little brain, is situated in the posterior chamber of the skull, beneath the _tentorium_, a tent-like process of the dura mater which separates it from the cerebrum. It is convex, with a transverse diameter of between three and one-half and four inches, and is little more than two inches in thickness. It is divided on its upper and lower surfaces into two lateral hemispheres, by the superior and inferior vermiform processes, and behind by deep notches. The cerebellum is composed of gray and white matter, the former being darker than that of the cerebrum. From the beautiful arrangement of tissue, this organ has been termed the _arbor vitae_. The _peduncles of the cerebellum_, the means by which it communicates with the other portions of the brain, are divided into three pairs, designated as the _superior_, _middle_ and _inferior_. The first pass upward and forward until they are blended with the tubercles of the _corpora quadrigemina_. The second are the _crura cerebelli_, which unite in two large _fasciculi_, or pyramids, and are finally lost in the _pons varolii_. The inferior peduncles are the corpora restiformia, previously described, and consist of both sensory and motor filaments. Some physiologists suppose that the cerebellum is the source of that harmony or associative power which co-ordinates all voluntary movements, and effects that delicate adjustment of cause to effect, displayed in muscular action. This fact may be proved by removing the cerebellum of a bird and observing the results, which are an uncertainty in all its movements, and difficulty in standing, walking, or flying, the bird being unable to direct its course. In the animal kingdom we find an apparent correspondence between the size of the cerebellum and the variety and extent of the movements of the animal. Instances are cited, however, in which no such proportion exists, and so the matter is open to controversy. The general function of the cerebellum, therefore, cannot be explained, but the latest experiments in physiological and anatomical science seem to favor the theory that it is in some way connected with the harmony of the movements. This co-ordination, by which the adjustment of voluntary motion is supposed to be effected, is not in reality a _faculty_ having its seat in the brain substance, but is the harmonious action of many forces through the cerebellum.