the first appearance of the fascia dentata (see fig. 20). The anterior commissure is divided, as in reptiles, into dorsal and ventral parts, of which the latter is the larger (fig. 20, Comm. V. and D.), while just behind the dorsal part is the first appearance of the fimbria or fornix. In addition to the two fissures already named, there is, in the Echidna, one which in position and mode of formation corresponds with the Sylvian fissure of higher mammals. Elliot Smith, however, wisely refuses to homologize it absolutely with that fissure, and proposes the name of pseudosylvian for it. The pineal body is rudimentary, and the optic lobes are now, and throughout the Mammalia, subdivided into four corpora quadrigemina.
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| From Cat. R.C.S. England. |
| Fig. 19.—Ventral and Dorsal Views of the Brain of Ornithorynchus. |
Among the Marsupialia the Tasmanian devil (Sarcophilus) gives a very good idea of a generalized mammalian brain, and shows a large development of the parts concerned in the sense of smell. The most important advance on the monotreme brain is that the calcarine fissure has now appeared on the posterior part of the mesial surface and causes a bulging into the ventricle, called the calcar avis or hippocampus minor, just as the hippocampal fissure causes the hippocampus major (Gervais, Nuov. Arch. Mus. tom. v., 1869; Ziehen, Jenaische Denkschr. Bd. vi., 1897).
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| From Cat. R.C.S. England. |
| Fig. 20.—Mesial and Lateral Views of the Brain of Ornithorynchus. |
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| From Cat. R.C.S. England. |
| Fig. 21.—Mesial and Lateral Views of the Brain of the Tasmanian Devil (Sarcophilus). |
In the Eutheria or mammals above the marsupials, the cerebellum gradually becomes more complex, owing to the appearance of lateral lobes between the flocculus and the vermis, as well as the paraflocculus on the outer side of the flocculus. The corpus callosum now first appears as a bridge between the neopallia, and its development leads to the stretching of the hippocampal formation, so that in the higher mammals the hippocampus is only found in the lower and back part of the ventricle, while the rudiments of the dorsal part remain as the striae longitudinals on the corpus callosum. The dorsal part of the original anterior commissure becomes the fornix, and the paraterminal area is modified to form the septum lucidum. The first appearance of the fissure of Rolando is probably in some of the Carnivora, in which, as the sulcus crucialis, it forms the posterior boundary of the “ursine lozenge” described by Mivart (Journ. Linn. Soc. vol. xix., 1886) (see fig. 22, Sulc. Cru.). In the higher apes or Anthropoidea the human fissures and sulci are largely recognizable, so that a gibbon's brain, apart from all question of comparative anatomy, forms a useful means of demonstrating to a junior class the main gyri and sulci of Man in a simple and diagrammatic way. The main points of difference, apart from greater simplicity, are that the central lobe or island of Reil is exposed on the surface of the brain, as it is in the human foetus, and that the anterior part of the occipital lobe has a well-marked vertical sulcus, called the simian sulcus or Affenspalte; this often has a semilunar shape with its convexity forward, and is then called the sulcus lunatus. It is usually concealed in European brains by the overgrowth of the surrounding gyri, but it occasionally remains, though less frequently than in the brains of Egyptian fellaheen. Its relation to the white stria of Gennari is especially interesting, and is recorded by Elliot Smith in the Anatomischer Anzeiger, Bd. xxiv., 1904, p. 436. The rhinal fissure, which is so characteristic a feature of the lower mammals, almost disappears in Man, and is only represented by the incisura temporalis (see fig. 11, i.t). The hippocampal fissure persists with little modification all through the mammalian class. The calcarine fissure remains with many modifications from the marsupials to man, and in view of the famous controversy of 1864, in which Owen, Huxley and the then bishop of Oxford took part, it is interesting to note that its hippocampus minor can now be clearly demonstrated, even in the Marsupialia. Another very ancient and stable sulcus is the orbital, which is a simple antero-posterior line until Man is reached (see fig. 23, Sulc. Orb.). The great point of importance, however, in the evolution of the mammalian brain is the gradual suppression of the olfactory region, and the development of the neopallium, a development which takes a sudden stride between the Anthropoid apes and Man. (For further particulars of this and other points in the comparative anatomy of the brain, see Catalogue of the Physiological Series of the Museum of the Royal College of Surgeons of England, vol. ii. 2nd ed., by R. H. Burne and G. Elliot Smith, London, 1902.)
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| From Cat. R.C.S. England. |
| Fig. 22.—Dorsal and Lateral Views of the Brain of a Ratel (Mellivora indica). |
Embryology.
The brain, like the rest of the nervous system, is developed from the ectoderm or outer layer of the embryo by the formation of a groove in the mid-dorsal line. The lips of this medullary groove unite to form a canal beginning at the place where the neck of the embryo is to be. The part of the neural canal in front of the earliest union forms the brain and very early becomes constricted into three vesicles, to which the names of prosencephalon, mesencephalon and rhombencephalon are now usually given. The simple tubular brain we have seen as a permanent arrangement in Amphioxus, but the stage of the three vesicles is a transitory one, and is not found in the adult of any existing animal. From the sides of the prosencephalon, the optic vesicles grow out before the neural tube is completely closed, and eventually form the optic nerves and retinae, while, soon after this, the cerebral hemispheres bulge from the antero-dorsal part of the first primary vesicle, their points of evagination being the foramina of Munro. From the ventral parts of these cerebral hemispheres the olfactory lobes are
