| From Cat. R.C.S. England. Fig. 18.—Section of Brain of Turtle (Chelone). |
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).[1] 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 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.
| 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).
| From Cat. R.C.S. England. Fig. 20.—Mesial and Lateral Views of the Brain of Ornithorynchus. |
| From Cat. R.C.S. England. Fig. 2l.—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.)
| From Cat. R.C.S. England. Fig. 22.—Dorsal and Lateral Views of the Brain of a Ratel (Mellivora indica). |
Embryology.