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 ANATOMY

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tissues on either side, which ultimately fuse in the medio- brates, enormous in the higher mammals. This is limited dorsal line. With the neurones of this body the cerebellar below and laterally by a rhinal fissure, which marks oft tracts of the cord and several other series of association (4) the rhinencephalon, or smell area, that makes up the rest fibres passing to the mid-brain and thalamus are connected. of this wall. Above, the pahium extends to the margin of The mid-brain, lying in front of the bulb, is the thickened the hemisphere, where it is conterminous with the hippowall of the second cranial vesicle (Fig. 7, sylv.), which in campus. The floor of the hemisphere consists of (5) a man has contracted into a canal, the aqueduct of Sylvius. grey mass, the corpus striatum, in front of which piojects (6) a lobule, the olfactory bulb and pedicle, the latter of Its thickened floor is the pons Varolii, mostly made up of (1) commissural cerebellar fibres crossing from side to side, which often extends backwards, spreading around the traversed longitudinally by (2) bundles of the descending corpus striatum to touch the paraterminal body and fibres of the pyramids passing between the cerebrum and hippocampus on the inner side, and to form a pyriform the spinal cord. These longitudinal fibres pass forwards lobe on the outer side, separated by the rhinal fissure from on each side Trom the hemisphere to the pons, forming a the pallium. All these parts in front of the rhinal fissure large part of the crura cerebri. (3) Between the bundles of are included in the name rhinencephalon. Between the fibres are many nerve-cells, with which some of these fibres two hemispheres, as they grow, commissural neurones arise are directly and synaptically connected. In the ventral floor of the canal in this region are the segmental nuclei of the third, fourth and part of the fifth nerve. The relations of the nerve roots to the cranial vesicles show that the latter are not primitive morphological features, but only secondary mouldings of the brain mass superinduced after the fusion of the original segments of the cephalic region. The quadrigeminal bodies, which in the human brain form the dorsal wall of the second cranial vesicle, are condensations into one irregular mass of several organically distinct senes of neurones and their processes connected directly or indirectly m -with ascending axons from the dorsal roots of the spinal nerves, (2) with the wall of the anterior cianial vesicle, and (3) with the optic nerves. The original cavity of the fore-brain or anterior cranial vesicle becomes in the adult the third ventricle. Its side walls are thickened and form grey masses, the optic thalami. Its roof is membranous and contains no neurones, forming the velum interpositum. Median pouches of this cavity project—(1) upwards and backwards, and (2) downwards and forwards. The former has at its end the pineal gland or epiphysis, the vestige of a median eye, which is functional in Sphenodon and a few other reptiles; the latter which is named the infundibulum, projects towards the sella turcica or fossa above the body of the sphenoid, and there thickens to form the posterior lobe of the hypophysis or pituitary body. This thalamic region is apparently related to the segment of the head which originally rh.f. bore the eyes. The front of the fore-brain is a membranous area, the lamina terminalis. _ The part of the brain which lies above and in front of the thalamic region is the cerebrum (vol. i. p. 872), whose enormous growth in the higher mammals has Cerebrum. cauged it to overlap and alter the relations of Fig. 8.—Scheme of the mammalian cerebral hemisphere. A, Lateral aspect. rh. rhinal fissure ; olf. tr., olfactory tract; olf. bulb, olfactory bulb; tiw. ol., the neighbouring parts of the brain. Its structure presents olfactory tubercle. B, Medial aspect, v.h., vestigia hippocampi hippocampal fissure; f.d., fascia dentata ; /., fimbria ; l.p., pyriform lobe ; par at, many obscure and difficult points, especially m the higher paraterminal body ; to., foramen of Monro ; o.p., olfactory peduncle. mammalia. The cerebrum consists of two hemispheres originally separate from each other, these are at firs and send their axons across in the substance of the lamina hollow outgrowths from the upper, anterior, and lateral terminalis, linking together the hemispheres. These angle of the fore-brain, containing evagmations of the third become grouped in two bundles—one a dorsal commissure, ventricle, which are called the lateral ventricles. These the fornix or psalterium, which joins together the two communicate with the parent ventricle by an aperture, hippocampal areas, and which in higher forms consists of the foramen of Monro, in front of which is the lamina longitudinal as well as transverse fibres. The other is terminalis. The composition of the wall of the cerebra ventral, and its fibres cross the basal region and rhinenhemisphere can only be understood by a careful compara- cephalon, consisting among others of the commissural tive study of its ontology, traced from the lowest verte- fibres of the pallial area. In some animals a third commisbrates to the highest mammals (Fig. 8). In the former sure, the commissura aberrans, crosses the roof of the third this wall of the hemisphere on the medial aspect, m front of ventricle behind the foramen of Monro (not to be conthe lamina terminalis and above it, consists of—- a grey founded with the commissura mollis of an adult human area, the corpus paraterminalis. Above this a faint line o brain). In such animals the roof of the third ventricle demarcation (sulcus limitans) maps out (2) a second area, consists of the velum interpositum, a delicate epithelial the hippocampus, which in some lowly-organized mamma s layer attached on each side to the ventral lip of the forms the entire of the remaining part of the medial wa paraterminal bodies. The lateral margins of this project of the hemisphere. On the outer wall above there is (3) into the foramen of Monro and constitute choroid plexuses. S. I. — 5 i a definite area, the pallium, very small m lower verte-