The cerebellum, or hinder brain, consists of a body, and of three pairs of crura or peduncles, by which it is connected with the rest of the cerebro-spinal axis. The cerebellum is covered with a gray cortical substance, rather darker than that of the cerebrum. Its greatest diameter is transverse, and extends to about three and a half or four inches; its width from before backwards is about two or two and a half inches; and its greatest depth is about two inches, but it is much thinner round its outer border. It consists of two lateral hemispheres joined by a median portion called the vermiform process, and other structures therewith connected, etc. Minute structure: The cortical gray substance is composed of an external clear gray layer, an inner grayish-red "granule" layer, and between the two a single layer of large cells with long processes, termed the corpuscles of Porkinge (after the man who first described them). Outside all is the layer of fibers and vessels of the pia mater. The external layer consists of a delicate matrix, probably of the nature of connective tissue, consisting of cells and fibers, etc. The cerebellum is probably concerned in the coördination of movements. Its functions seem especially connected with afferent impulses proceeding from the semicircular coats. The spinal cord is a cylindriform column of nerve substance connected above with the brain, through the medium of the medulla oblongata, terminating below, about the lower border of the first lumbar vertebra, in a slender filament of gray or vesicular substance, the filum terminale, which lies in the midst of knots of many nerves forming the codæ equina. Through the center of the cord, running in a longitudinal direction, is a minute canal, which is continuous through the whole length of the cord, and opens above into the space at the back of the medulla oblongata and pons varolii, called the fourth ventricle; the aqueduct of silvius connects it with the third ventricle, lateral and fifth ventricles, near the base of the brain. The cerebro-spinal fluid circulates in the interior of these ventricles and spinal cord. What precise mechanical function it subserves is only surmised, not known. The cerebro-spinal axis is protected by three membranes, named also meninges. They are: 1. An external fibrous membrane, named dura mater, which closely lines the interior of the skull, and forms a loose sheath in the spinal canal; 2. An internal areolo-vascular tunic, the pia mater, which accurately covers the brain and spinal cord; and, 3. An intermediate membrane, the arachnoid, which lies over the pia mater, the two being in some places in close connection, and in others separated by a considerable space.