The encephalon, or brain, is the portion of the cerebro-spinal axis which is lodged within the cranial cavity and constitutes (by weight) about 98 per cent, of the whole. It consists of the cerebrum, the cerebellum, the pons Varolii, and the medulla oblongata. The medulla is continuous with the spinal cord at the decus- sation of the pyramids. The encephalon, taken as a whole, is ellipsoidal or ovoidal in form, presenting above a tolerably even convexity formed by the cerebral hemi- spheres, but below a more irregular surface corresponding to the fossee in the base of the skull. The cerebrum comprises the cerebral hemispheres containing the lateral ventricles (prosencephalon), the optic thalami with the third ventricle between them (thalamencephalon), and the mesencephalon. It occupies the upper compartment of the cranial cavity, resting on a floor formed by the anterior and middle cranial fossae and the tentorium cerebelli. The cerebellum occupies the posterior cranial fossa, and is placed above and behind the fourth ventricle. It is connected by three pairs of peduncles or crura to the cerebrum, pons, and the medulla respectively. The fourth ventricle is bounded below and in front by the pons and medulla.
In every part of the encephalon two distinct kinds of nervous substance are to be met with, termed grey and white matter. The grey matter is chiefly disposed upon the siu"face, as in the convolutions of the cerebrum and cerebeUum ; but it is also met with in detached or partially detached masses or nuclei, such as the corpora striata and optic thalami in the cerebrum, the nuclei of the medulla and pons, and the corpus dentatum of the cerebeUum. It is composed of groups of gpmglion cells, which possess the power of originating nervous impulses ; or of receiving impulses produced by the action of external stimuli on the end- organs of nerves ; or of modifying and redirecting such impulses. The white matter occiu-s in the greatest quantity in the central parts of the cerebral hemispheres. It is composed of meduUated nerve-fibres, which conduct the impulses to and from the ganglion cells in the grey matter. Both grey and white matter are supported by a delicate interstitial tissue termed neuroglia.
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