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ANATOMY
These are, therefore, not parts of the wall of the hemisphere but of the fore-brain, and behind them is the aberrant commissure, which separates the choroid plexus from the membranous roof behind it (joaraphysis), and lies at the caudal end of the paraterminal body. Behind the paraphysis is the commissure of the habenula in front of the pineal eye. The predominant characteristic of the brains of man and of the higher mammals is the enormous overgrowth of Character ^ie pallium, which consists of those neurones istics of that govern the lower mechanisms and whose human functions are to initiate actions. Coincidentally brain. -with this growth, remarkable changes take place in the other areas of the cerebral surface. The hippocampus becomes indented by the development of a longitudinal sulcus on its median aspect, and is differentiated in structure from the surrounding parts by the formation within it of a compact layer of pyramidal cells. The bottom of this sulcus projects into the lateral ventricle, and there makes a swelling to which the name hippocampus is often limited, while the part containing the column of cells at the margin of the hippocampal area which still remains on the median surface of the hemisphere close to the paraterminal body is called fascia dentata. These pyramidal cells are probably receptive cells connected with the olfactory nerve fibrils. The growth of the pallium towards the paraterminal body pushes the hippocampus backwards and causes its anterior end to become rudimentary, a mere pre-commissural vestige of the part which in the reptile and amphibian made up almost the whole mesial and part of the lateral wall of the hemisphere. With this enormous growth of the governing mechanism of the pallium there is correlated a growing necessity for bilateral co - operation, and therefore an increased growth of commissural neurones. But the space through which the original pallial commissural axons (in the ventral commissure) pass is limited by the growth of the ganglionic masses of the corpus striatum and their approximation to the thalamus (wall of the fore-brain). These new axons must therefore seek a new and more direct path, and they find this along the track occupied by the fibres of the now much reduced dorsal commissure. This newly - developing pallial commissure or corpus callosum, which is peculiar to the eutherian brain, is quite distinct from the hippocampal dorsal commissure, whose place it to some extent takes, and which it exceeds in size in proportion to the predominance of the pallium over the hippocampus. A portion of the paraterminal body is intercepted between the corpus callosum and the fornix, becoming the septum lucidum (vol. i. p. 876), which is thus not pallial but paraterminal in its nature. A fissure which runs from the margin of the olfactory bulb to the genu of the corpus callosum is the real boundary of the pallium, and the band which borders this furrow above, the gyrus sub-callosus, is also a surviving part of the paraterminal body. The ventral commissure, which continues to carry the uniting axons of the pyriform tubercle and olfactory bulb, is separated in the human brain by the anterior commissure. From these the pallial fibres which accompanied them in the marsupials as the most dorsal band of the ventral commissure have in the eutheria followed in their increase a course of least resistance and occupied the space vacated by the recession of the hippocampi. This change is foreshadowed in the brains of diprotodont marsupials by the separation of this band of the anterior commissure from the rest and its passing to the pallium through the internal capsule, as if to relieve the tension of the area of the anterior commissure. The convolutioning of the surface of the adult cerebral hemisphere has been described at length in vol. i. p. 873. These convolutions
are folds produced by the enormous growth of the grey surface of the pallium, and first appear about the fourth month of foetal life. Arnold and others have described a series of transitory fissures which they supposed to arise and to become obliterated before the growth of the permanent set, but these have been shown to be the result of accident and reagents by Hochstetter and Elliot Smith. (For further particulars concerning the cerebrum see Cunningham, Trans. Royal Irish Acad. 1892, and the numerous papers in the Journal of Anatomy by Elliot Smith, 1898-1901. For the visceral nerves and their courses see Harman’s papers, Journal of Anat. 1898, 1899, 1900. For views as to the general constitution of a nerve see Gaskell, Jo urn. of Physiol. 1886. For the distribution of sensory nerves see Head, Brain, 1893-94. The anatomy of the sense organs has been sufficiently treated in Ency. Brit. vol. i. pp. 884-899. An excellent account of recent work on the subject is given by Schafer in Quain’s Anat. iii. 1894.) IV. Organs of Assimilation.—From appearances presented in the early stages of the vertebrate embryo, it is probable that there were two preceding forms of alimentary canal which were functional at different periods in the archaic ancestry of the group before the evolution of the existing or hypoblastic digestive organs. Of the earlier a vestige remains in the central canal of the spinal cord; of the second the notochord is possibly a trace. When the hypoblastic digestive canal becomes closed in, it has at first no communication with the outer surface, being closed both at the head and tail ends; but within the first few weeks of existence two superficial pits form, one on the ventral side of the fore-brain region and one on the ventral side of the tail. These, too, by the rupture of the membranes which separate them from the gut become respectively the mouth and the cloacal opening. The former of these pits is called the stomodaum, and gives rise to the cheeks and lips, the alveolar arches, and the nasal passages. In it the palate arises as a horizontal partition, which grows from each side across its cavity, dividing the nasal from the oral cavity. Sometimes this partition fails to unite with its fellow in the middle line, giving rise to the malformation called cleft palate. The bottom part of the pouch of the stomodseum projects through the tissue which is forming the base of the skull, and fuses with the hypophysis to form the pituitary body. The pharynx (see vol. vii. p. 222) is the part of the alimentary canal which lies in front of the continuous coelomic cavity, having on each side of it the Mouth remains of the primitive upper aortic arches, which in man have become the carotid arteries. It was in early embryonic life the region of visceral clefts like those which persist in fishes, but these disappear in man, except the first, which remains as the outer ear passage and the Eustachian tube. The teeth, described in vol. vii. p. 232, are papillae of the same order as those of the skin, only differentiated because they are more discrete, larger, and arise on the dermis covering the jaw arches. Investigators are not agreed as to the nature of heterodontism— some (Osborn and Cope) teaching that the multituberculate molar has originated from the development of secondary tubercles on an originally simple protocone. Others, like Rose, believe the compound teeth to arise from the fusion of several papillae. The balance of evidence is in favour of the latter view. Some mammals, such as the porpoise and other toothed whales, have but one dentition of homodont teeth; these are all animals with long narrow jaws and simple dental papillae. In others, which are heterodont, all the first set persist together with several premolars of a secondary set. In others the milk set of teeth are replaced; but the permanent molars, which are the latest part of the primary dentition, persist and are not replaced, as in man. Man is descended from mammals which had three incisors on each side of each jaw, and three premolars. In all the primates, however, the lateral incisor has aborted, leaving only two; and likewise the hindmost of the premolars has generally
