The anterior lobe, of a triangular form, with its apex backwards, is somewhat concave, and rests upon the convex surface of the roof of the orbit, being separated from the middle lobe by the fissure of Sylvius. The middle lobe, which is more prominent, is received into the middle fossa of the base of the skull. The posterior lobe rests upon the tentorium, its extent forwards being limited by the anterior margin of the cerebellum.

  The various objects exposed to view on the under surface of the cerebrum in the middle line are here arranged in the order in which they are met with from before backwards. The Longitudinal Fissure separates the two hemispheres from one another; it divides the two anterior lobes in front; and on raising the cerebellum and pons, it will be seen completely separating the two posterior lobes, the intermediate portion of the fissure being arrested by the great transverse band of white matter, the corpus callosum. Of these two portions of the longitudinal fissure, that which separates the posterior lobes is the longest. In the fissure between the two anterior lobes the anterior cerebral arteries may be seen ascending to the corpus callosum; and at the back part of this portion of the fissure, the anterior curved portion of the corpus callosum descends to the base of the brain.

  The Corpus Callosum terminates at the base of the brain by a concave margin, which is connected with the tuber cinereum through the intervention of a thin laygr of grey substance, the lamina cinerea. This may be exposed by gently raising and drawing back the optic commissure. A broad white band may be observed on each side, passing from the under surface of the corpus callosum in front, backwards and outwards, to the commencement of the fissure of Sylvius; these bands are called the peduncles of the corpus callosum. Laterally, the corpus callosum extends into the anterior lobe.

  The Lamina Cinerea is a thin layer of grey substance, extending backwards from the termination of the corpus callosum above the optic commissure to the tuber ci"ereum; it is continuous on either side with the grey matter of the anterior perforated space, and forms the anterior part of the inferior boundary of the third ventricle.

  The Olfactory Nerve, with its bulb, is seen on either side of the longitudinal fissure, upon the under surface of each anterior lobe.

  The Fissure of Sylvius separates the anterior and middle lobes, and lodges the middle cerebral artery. At its entrance is seen a point of medullary substance, corresponding to a subjacent band of white fibres, connecting the anterior and middle lobes, and called the fasciculus unciformis; on following this fissure outwards, it divides into two branches, which enclose a triangular-shaped prominent cluster of isolated convolutions, the island of Reil. These convolutions, from being covered in by the sides of the fissure, are called the gyri operti.

  The Anterior Perforated Space is situated at the inner side of the fissure of Sylvius. It is of a triangular shape, bounded in front by the convolution of the anterior lobe and roots of the olfactory nerve; behind, by the optic tract; externally, by the middle lobe and commencement of the fissure of Sylvius; internally, it is continuous with the lamina cinerea, and crossed by the peduncle of the corpus callosum. It is of a greyish colour, and corresponds to the under surface of the corpus striatum, a large mass of grey matter, situated in the interior of the brain; it has received its name from being perforated by numerous minute apertures for the transmission of small straight vessels into the substance of the corpus striatum.

  The Optic Commissure is situated in the middle line, immediately behind the lamina cinerea. It is the point of junction between the two optic nerves.

  Immediately behind the diverging optic tracts, and between them and the peduncles of the cerebrum (crura cerebri) is a lozenge-shaped interval, the inter-peduncular space, in which are found the following parts, arranged in the following order from before backwards: the tuber cinereum, infundibulum, pituitary body, corpora albicantia, and the posterior perforated space.

  The Tuber Cinereum is an eminence of grey substance, situated between the optic tracts and the corpora albicantia; it is connected with the surrounding parts of the cerebrum, forms part of the floor of the third ventricle, and is continuous with the grey substance in that cavity. From the middle of its under surface, a conical tubular process of grey matter, about two lines in length, is continued downwards and forwards to be attached to the posterior lobe of the pituitary body; this is the infundibulum. Its canal, funnel-shaped in form, communicates with that of the third ventricle.

  The Pituitary Body is a small reddish-grey vascular mass, weighing from five to ten grains, and of an oval form, situated in the sella Turcica, in connection with which it is retained by the dura mater which forms the inner wall of the cavernous sinus. It is very vascular, and consists of two lobes, separated from one another by a fibrous lamina. Of these, the anterior is the larger, of an oblong form, and somewhat concave behind, where it receives the posterior lobe, which is round. The anterior lobe consists externally of firm yellowish-grey substance, and internally of a soft pulpy substance of a yellowish-white colour. The posterior lobe is darker than the anterior. In the foetus it is larger proportionally than in the adult, and contains a cavity which communicates through the infundibulum with the third ventricle. In the adult it is firmer and more solid, and seldom contains any cavity. Its structure, especially the anterior lobe, is similar to that of the ductless glands.

  The Corpora Albicantia are two small round white masses, each about the size of a pea, placed side by side immediately behind the tuber cinereum. They are formed by the anterior crura of the fornix, hence called the bulbs of the fornix, which, after descending to the base of the brain, are folded upon themselves, before passing upwards to the thalami optici. They are composed externally of white substance, and internally of grey matter; the grey matter of the two being connected by a transverse commissure of the same material. At an early period of foetal life they are blended together into one large mass, but become separated about the seventh month.

  The Posterior Perforated Space (Pons Tarini) corresponds to a whitish-grey substance, placed between the corpora albicantia in front, the pons Varolii behind, and the crura cerebri on either side. It forms the back part of the floor of the third ventricle, and is perforated by numerous small orifices for the passage of blood-vessels, to the thalami optici.

  The Crura Cerebri {Peduncles of the Cerebrum) are two thick cylindrical bundles of white matter, which emerge from the anterior border of the pons, and diverge as they pass forwards and outwards to enter the under part of either hemisphere. Each crus is about three-quarters of an inch in length, and somewhat broader in front than behind. They are marked upon their surface with longitudinal striae, and each is crossed, just before entering the hemisphere, by a flattened white band, the optic tract, which is adherent by its upper border to the peduncle. In its interior is contained a mass of dark grey matter, called locus niger. The third nerves may be seen emerging from the inner side of either crus; and the fourth nerve winding around its outer side from above.

  Each crus consists of a superficial and deep layer of longitudinal white fibres, continued upwards from the pons, separated by a mass of grey matter, the locus niger.

  The Superficial Longitudinal Fibres are continued upwards, from the anterior pyramids to the cerebrum. They consist of coarse fasciculi, which form the free part of the crus, and have received the name of the fasciculated portion of the peduncle, or crust.

  The Deep Layer of Longitudinal Fibres are continued upwards, to the cerebrum, from the lateral and posterior columns of the medulla, and from the olivary fasciculus, these fibres consisting of some derived from the same, and others from the opposite lateral tract of the medulla. More deeply, are a layer of finer fibres, mixed with grey matter, derived from the cerebellum, blended with the former. The cerebral surface of the crus cerebri is formed of these fibres, and is named the tegmentum.

  The Locus Niger is a mass of grey matter, situated between the superficial and deep layer of fibres above described. It is placed nearer the inner than the outer side of this body.

  The posterior lobes of the cerebrum are concealed from view by the upper surface of the cerebellum, and pons Varolii. When these parts are removed, the two hemispheres are seen to be separated by the great longitudinal fissure, this fissure being arrested, in front, by the posterior rounded border of the corpus callosum.