convolutions by irregularly - shaped furrows passing crosswise.
The Peduncles or Crura of the brain are thick bands of fibres which connect cerebrum, cerebellum, and spinal cord. Thus, connected with the cerebellum, there are three pairs of peduncles, two joining it to the spinal cord below, two to the cerebrum above, and two passing crosswise and forming the pons Va. rolii.
The ventricles of the brain are cavities varying in size which exist in different parts. Thus in each half of the cerebrum there is such a cavity, the lateral ventricle. Between the hemispheres of the brain in front is the fifth ventricle, and further back the third ventricle, A considerable space exists between the back of the medulla oblongata and the under surface of the cerebellum which rests upon it. The space is called the fourth ventricle (see Fig. 86), and in its floor are many very important nerve-centres. A small amount of fluid exists in health in these cavities ; but in some dis eases, such as " water in the head ", the quan tity of fluid is enormously increased.
The minute structure of the brain con sists of white matter internally, and gray matter externally. The white matter is more firm and consistent than the gray matter; indeed the latter is so soft as to be readily All these parts of the brain are con nected with each other by numerous nerve-fibres. The fibres from the spinal cord pass upwards in the medulla oblon gata, those from the hinder part going chiefly to the cerebellum, whilst those from the front pass chiefly to the cere brum. In the cerebrum, cerebellum, and ganglia we also find fibres running from one part to another, as from before backwards and across, so as to unite all the parts of the brain and form one complete organ.
A side view of the brain is seen in Fig. 86, which should be studied with the aid of the description.
The cerebellum, or lesser brain, lies in the back part of the skull cavity, and is covered over in man by the hinder lobe of the cerebrum. It is, as it were, slung on the back of the cerebrospinal axis, and consists of two hemispheres with an intermediate portion, or middle lobe, some times called the vermiform process. The whole surface of the cerebellum is divided into washed away from the brain soon after death by a jet of water. The gray matter forms
a layer or web over the surface of the cere brum and cerebellum, and it also exists in masses at various points in the deeper parts, as for example in the corpus striatum, optic thala mus, corpora quadrigemina, pone, medulla, and cerebellum. The white matter constitutes the greater portion of the internal parts of the brain. As brain substance is too soft and too opaque in the natural condition for microscopical examination with high powers, physiologists have devised methods by which portions may be hardened so that thin sections may be cut. These sections are immersed in dilute solutions of such dyes as carmine or logwood, with the effect of staining certain parts of the structure, and thus rendering them visible. When such sections are properly prepared and mounted for microscopical examination it is found that the white matter in the deeper parts of the brain consists of nerve-fibres bound together by fine connective tissue. The gray matter consists of a very fine variety of connective tissue to which the name neuroglia has been given, and in this lie embedded the nerve-cells, already described (p. 128.) The meninges are the membranes which invest and protect the brain and cord. These are : (1) a strong outer fibrous membrane named the dura mater, which closely lines the interior of the skull and forms a loose sheath in the spinal canal ; (2) an inner thin membrane called the pia mater, which is closely adapted to the surface of the brain and spinal cord, and, being crowded with blood-vessels, carries to them their blood-supply; and (3)an in term ediate membran e, the arachnoid, which lies over the pia mater and under the dura mater. The dura mater may be regarded as the protective covering of the brain, and the pia mater as a special membrane on which blood-vessels divide and subdivide, so as to be of very small size before they pene trate the brain ; whilst the arachnoid is really a double membrane, one layer being adapted to the dura mater and the other to the pia mater, the apace between these occasionally containing a small quantity of fluid. These membranes are sometimes the seat of inflam mation.