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 Sylvius, and from the nervous tissue in their walls the optic nerves derive their fibres. From the front of the prosencephalon or anterior vesicle the olfactory nerves come off, and at the base of each of these are two hollow swellings; the larger and more anterior is the olfactory bulb, the smaller and more posterior the cerebral hemisphere. Both these swellings must be regarded as lateral outgrowths from the blind front end of the original single vesicle of the brain as seen in Amphioxus, and from the anterior subdivision or prosencephalon in the lamprey. The anterior vesicle, however, is now again subdivided, and that part from which the cerebral hemispheres bud out, and the hemispheres themselves, is called the telencephalon, while the posterior part of the original prosencephalon is known as the thalamencephalon, or more rarely the diencephalon. On the dorsal surface of the thalamencephalon are two nervous masses called the ganglia habenulae; the right is much larger than the left, and from it a stalk runs forward and upward to end in the vestigial pineal body (or epiphysis), which contains rudiments of a pigmented retina and of a lens, and which is usually regarded as the remains of one of a pair of median eyes, though it has been suggested that it may be an organ for the appreciation of temperature. From the small left ganglion habenulae a still more rudimentary pineal stalk projects, and there are signs of a third outgrowth (paraphysis) in front of these. On the floor of the thalamencephalon the blind pouch-like infundibulum is in contact with the pituitary body, an outgrowth from the combined pituitary and olfactory pouch, which in the adult opens on to the top of the head just in front of the pineal area. The anterior closed end of the nerve-tube, in front of the foramina of Munro or openings from which the hemispheres have grown out, is known as the lamina terminalis, and in this is seen a little white commissure, connecting the hemispheres of opposite sides and belonging entirely to the telencephalon, known as the anterior commissure. The roof of the telencephalon is mainly epithelial, and contains no traces of cortical structure. In the posterior part of the roof of the thalamencephalon is the small posterior commissure (Ahlborn, Zeits. wiss. Zool. Bd. xxxix., 1883, p. 191). In the Elasmobranch Fish, such as the sharks and rays, the cerebellum (Cer. fig. 17) is very large and contains the layers found in all the higher vertebrates. In the mesencephalon fibres corresponding with those of the fillet of higher vertebrates can be seen, and there is a nucleus in the hinder part of the corpora bigemina foreshadowing the separation into corpora quadrigemina. There is only one pineal stalk in the roof of the thalamencephalon, and the ganglia habenulae—very constant structures in the vertebrate brain—are not so marked as in Petromyzon, but are, as usual, connected with the olfactory parts of the cerebrum, with the surface of the optic lobes (tectum opticum), and with the corpus interpedunculare (Meynert’s bundle). They are united across the middle line by a small superior or habenular commissure. In the floor of the thalamencephalon are two masses of ganglionic tissue, the optic thalami. The infundibulum dilates into two rounded bodies, the lobi inferiores, while the pituitary body or hypophysis cerebri has two lateral diverticula known as sacci vasculosi. Ganglia geniculata are found for the first time in connexion with the optic tracts in the lower part of the thalamus. The olfactory lobes (fig. 17, Olf. Bulb) are very large and often separated by long stalks from the cerebral hemispheres, which are comparatively much larger than those of the Cyclostomata; their roof or pallium is nervous, but devoid of cortical structure, while in the floor in some species large anterior basal ganglia or corpora striata are found (Miklucho-Maclay, Beiträge z. vergl. Neurol., 1870; Edinger, Arch. mikr. Anat. Bd. lviii., 1901, p. 661, “Cerebellum”). The Teleostean Fish are chiefly remarkable for the great development of the optic lobes and suppression of the olfactory apparatus. The pallium is non-nervous, and the optic tracts merely cross one another instead of forming a commissure. A process of the cerebellum called valvula cerebelli projects into the cavity of each optic lobe (Rabl. Ruckhard, Arch. Anat. u. Phys., 1898, p. 345 [Pallium]; Haller, Morph. Jahrb. Bd. xxvi., 1898, p. 632 [Histology and Bibliography]). The brain of the Dipnoi, or mud fish, shows no very important developments, except that the anterior pineal organ or paraphysis is large (Saunders, Ann. and Mag. Nat. Hist. ser. 6, vol. iii., 1889, p. 157; Burkhardt, Centralnervensystem v. Protopterus, Berlin, 1892).

In the Amphibia the brain is of a low type, the most marked advances on that of the fish being that the anterior commissure is divided into a dorsal and ventral part, of which the ventral is the true anterior commissure of higher vertebrates, while the dorsal is a hippocampal commissure and coincides in its appearance with the presence of a small mass of cells in the outer layer of the median wall of the pallium, which is probably the first indication of a hippocampal cortex or cortex of any kind (Osborn, Journ. Morph. vol. ii., 1889, p. 51).

In the Reptilia the medulla has a marked flexure with a ventral convexity, and an undoubted cerebral cortex for the first time makes its appearance. The mesial wall of the cerebral hemisphere is divided into a large dorsal hippocampal area (fig. 18, Hip.) and a smaller ventral olfactory tubercle. Between these two a narrow area of ganglionic matter runs forward from the side of the lamina terminalis and is known as the paraterminal or precommissural area (Elliot Smith, Journ. Anat. and Phys. vol. xxxii. p. 411). To the upper lateral part of the hemisphere Elliot Smith has given the name of neopallium, while the lower lateral part, imperfectly separated from it, is called the pyriform lobe. In the Lacertilia the pineal eye, if it be an eye, is better developed than in any existing vertebrate, though even in them there is no evidence of its being used for sight. Behind the so-called pineal eye and its stalk is the epiphysis or pineal body, and sometimes there is a dorsal sac between them (see fig. 18). The middle or soft commissure appears in certain reptiles (Crocodilia and Chelonia), as does also the corpus mammillare (Edinger, Senckenberg, Naturf. Gesell. Bd. xix., 1896, and Bd. xxii., 1899; Haller, Morph. Jahrb. Bd. xxviii., 1900, p. 252). Among the birds there is great unity of type, the cerebellum is large and, by its forward projection, presses the optic lobes down toward the ventro-lateral part of the brain. The cerebral hemispheres are also large, owing chiefly to the great size of the corpora striata, which already show a differentiation into caudate nucleus, putamen and globus pallidus. The pallium is reptilian in character, though its cortical area is more extensive. The geniculate bodies are very large (Bumm, Zeits. wiss. Zool. Bd. xxxviii., 1883, p. 430; Brandis, Arch. mikr. Anat. Bd. xli., 1893, p. 623, and xliii., 1894, p. 96, and xliv., 1895, p. 534; Boyce and Warrington, Phil. Trans. vol. cxci., 1899, p. 293).

Among the Mammalia the Monotremata have a cerebellum which shows, in addition to the central lobe of the lower vertebrates, a flocculus on each side, and the two halves of the cerebellum are united by a ventral commissure, the pons varolii. The pallium is reptilian in its arrangement, but that part of it which Elliot Smith has named the neopallium is very large, both in the Ornithorynchus and Echidna, a fact very difficult to account for. In the latter animal the cortical area is so extensive as to be thrown into many and deep sulci, and yet the Echidna is one of the lowliest of mammals in other respects. A well-marked rhinal fissure separates the pyriform lobe from the neopallium, while, on the mesial surface, the hippocampal fissure separates the neopallium from the hippocampal area. Just below the hippocampal fissure a specially coloured tract indicates