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In the hippocampus impressions of smell are brought into relation with those of other senses (probably taste); and from the hippocampal cells fibres are emitted to form a system known as the fornix, which establishes connexions with the hippocampus of the other hemisphere and with the hypothalamus, thalamus, and more distant parts of the brain.
The rudiment of the hippocampal formation that develops on the medial surface begins in front, alongside the place where the stalk of the olfactory peduncle (which becomes the trigonum olfactorium) is inserted; it passes upwards to the superior end of the lamina terminalis, from the rest of which it is separated by a triangular mass of gray matter called the corpus paraterminale (Fig. 555); and then it proceeds backwards, fringing the fissura chorioidea in the whole of its extent, ending below in the temporal region alongside the posterior part of the area piriformis. The anterior part of this great hippocampal fringe of the pallium does not attain its full development in the human brain and remains as a more or less vestigial aborted
FIG. 554.-MEDIAL ASPECT OF THE RIGHT CEREBRAL HEMISPHERE, WITH THE OLFACTORY PARTS COLOURED.
structure; but the posterior part undergoes a peculiar transformation. The tertiary olfactory neurones, coming mainly from the posterior part of the area piriformis, enter the margin of the hippocampal formation, and the small cells which receive these incoming fibres multiply rapidly during the third month, and arrange themselves in a densely packed row of granules, which represent the distinctive feature of the fascia dentata (Fig. 556). At first this cell-column is continuous at its peripheral margin with a much more loosely packed column of larger and less numerous cells, which represent the hippocampus; and these in turn give place to the more diffusely arranged and laminated cells of the typical cortex cerebri, which we call the neopallium. As development proceeds both the dentate and hippocampal columns of cells rapidly increase in length, and both appear to push their way towards the ventricle (Fig. 556, B) into the substance of the wall, which becomes correspondingly thickened. The ventricular swelling thus formed is the hippocampus; and it is important to recognise that this swelling is not produced by any invagination of the surface, such as is usually described under the name of the
fissura hippocampi. There is no fissura hippocampi in the human brain. What is usually described under this name is an artificial cleft made by pushing the handle of a scalpel into the hippocampal formation at the edge of the exposed part of the fascia dentata (Fig. 556, B and C, at x) and separating the morphological surface of the hippocampus from that of the buried part of the fascia dentata. Cleavage readily occurs along this line because there are numerous nerve-fibres, hippocampal and dentate respectively, upon each side of it.
As development proceeds a break occurs in the cell-column at the junction of its hippocampal and dentate parts, and the two columns (Fig. 556, C) become partially interlocked.
The axons of the hippocampal cells collect upon its ventricular surface to form the alveus, the fibres of which converge towards
the margin of the fascia Hippocampal commissure.
dentata, where they bend into the longitudinal direction (ie. parallel to the edge of the pallium and the lamina chorioidea) to form a prominent white marginal fringe, the fimbria.
The fibres of the fimbria pass upwards and forwards (Fig. 555), and ultimately reach the upper end of the lamina terminalis, which provides a bridge to conduct a certain number of them across the median plane into the fornix or
The broken red lines indicate the paths taken by callosal fibres in the neopallium to reach the upper end of the lamina terminalis.
FIG. 556.-DIAGRAMS REPRESENTING THREE STAGES IN THE DEVELOPMENT OF THE HIPPOCAMPAL FORMATION.
fimbria of the other hemisphere, so as to link together in functional association the two hippocampi. These crossing fibres are known as the commissura hippocampi.
Most of the fibres that go up in the fimbria from the hippocampus do not pass into the hippocampal commissure, but bend downwards in the anterior lip of the foramen interventriculare to enter the thalamic region. They are collected into a vertical rounded column, which is called the columna fornicis ; when it reaches the hypothalamus it bends backward to end in the corpus mamillare.
The olfactory bulb and tract, the area piriformis, tuberculum olfactorium, corpus paraterminale, and the formatio hippocampalis together form a part of the hemisphere, which is concerned mainly with the function of smell. Hence they may be grouped together as the rhinencephalon; but this term has been used in so many different ways that it is of doubtful utility.
In the lowest vertebrates the whole hemisphere is practically rhinencephalon. Nevertheless, fibres coming from other parts of the nervous system and conveying impressions from other sense organs than those of smell make their way into the cerebral hemisphere and influence the state of its activities. In other words, the hemisphere is primarily an olfactory receptive nucleus, but is also the place where impressions of smell are brought under the modifying influences of other sensory impressions before they make their effects manifest in behaviour.
But it is only in the most. highly organised types of brain, more especially those of mammals and birds, that the non-olfactory senses acquire a representation in the hemisphere which is relatively independent of, or at any rate not wholly subservient to, the influence of the sense of smell. In the mammalian brain a definite area of pallium is set apart to receive impressions of the tactile, visual, acoustic, and other senses. This area is the neopallium. In the human brain it has grown to such an extent that it forms almost the whole of the hemispheresa mass far greater than the whole of the rest of the central nervous system.
THE CEREBRAL COMMISSURES AND THE SEPTUM PELLUCIDUM.
We have seen that certain fibres from the hippocampi cross from one hemisphere to the other, using the upper part of the lamina terminalis as a bridge across the median plane. But at an earlier stage of development other fibres can be detected at a slightly lower level in the lamina terminalis forming a bundle, of oval outline in sagittal section, called the commissura anterior. Its fibres come from the olfactory bulb, area piriformis, tuberculum olfactorium, and a small temporal area of neopallium. If the composition of the hippocampal commissure is analysed in a foetus of the third month, it will be found that there are intermingled with the truly hippocampal fibres some which come from the neopallium. During the fourth month the bulk of the neopallial element in this dorsal commissure outgrows the hippocampal element. The latter fibres become crowded into the postero-inferior corner of the commissure and the neopallial fibres come to form a flattened transverse bridge-the corpus callosum-above them. These fibres are enclosed in a neuroglial matrix derived from the lamina terminalis and the adjoining paraterminal bodies. Some nerve-cells also may make their way into this matrix. As it elongates, the corpus callosum pushes its way forwards in the upper part of the paraterminal body of each hemisphere, and as development proceeds a small area of this body becomes almost completely circumscribed by the corpus callosum and commissura hippocampi. As these commissural bands increase in size this small circumscribed patch of paraterminal body becomes greatly stretched and expanded to form a thin translucent leaf. The two leaves thus formed in the medial walls of the two hemispheres are known as the septum pellucidum; and the narrow cleft that separates them the one from the other in the median plane is called the cavum septi pellucidi.
There is still an element of uncertainty concerning the precise manner in which these changes are brought about, and especially as to the precise mode of closure of the cavum septi. As the cerebral hemisphere expands, some parts of it grow forwards, others upwards, and others again backwards. Such growth in each part will naturally tend to exert traction upon its commissural fibres that pass through the corpus callosum. Hence the anterior part of this great commissure becomes drawn forwards, its posterior part backwards, and the greater intermediate part upwards, so that it comes to assume the form shown in Fig. 557, C. As the posterior part of the corpus callosum pushes its way backwards, it exerts traction upon the fibres of the hippocampal commissure and their matrix, which becomes enormously stretched so as to form a thin lamella (the floor of the cavum septi) stretching from a point just above the anterior commissure to the under surface of the swollen posterior end of the corpus callosum, which is called the splenium (Fig. 558). The hippocampal commissural fibres are scattered throughout this lamella. The backward growth of the splenium also thrusts back the
upper end of the hippocampal formation so that it becomes removed far from the lamina terminalis. The fibres of the fimbria which are prolonged forwards under the corpus callosum and septum pellucidum to bridge this great gap form the crus fornicis on each side. As a rule in the human adult brain the crura fornicis of the two hemispheres become crowded together at the median plane so as to obscure the connecting lamella which serves as a matrix for the commissura hippocampi (Fig. 557, C); but the true arrangement can be seen in the brains of foetuses of the sixth, seventh, and eighth months, and is at once revealed in the adult if the corpus callosum is raised up by an accumulation of fluid in the lateral ventricles (hydrocephalus), so as to put a strain upon the septum pellucidum. The mass formed by the crura fornicis and their commissure is called the corpus fornicis.
The fascia dentata appears as a notched band behind and below the fimbria; its upper end passes on to the under surface of the splenium of the corpus callosum, where it tapers and ends (fasciola cinerea); but as it dwindles the upper end of the hippocampus emerges upon the surface below and behind it and passes into a thin film of gray matter-indusium griseum-which is prolonged on to the upper surface of the corpus callosum. It proceeds forwards, becoming as a rule still Vestiges of the supracallosal hippocampus
Vestiges of the supracallosal hippocampus
Olfactory bulb Vestiges of the precallosal hippocampus
FIG. 557.-THREE STAGES IN THE DEVELOPMENT OF THE CORPUS CALLOSUM.
more attenuated, and after surrounding the anterior end (genu) of the corpus callosum it passes downwards towards the trigonum olfactorium along the line that separates the corpus paraterminale from the neopallium. The indusium represents the atrophied remains of the anterior part of the hippocampal arc of the foetal brain (Fig. 555), from which the fascia dentata has entirely disappeared. It is accompanied by longitudinal fibres homologous to the fornix system: in other words, the fornix fibres of the atrophied supracallosal hippocampus; they form the stria longitudinales of the corpus callosum (Fig. 558; Fig. 564, p. 635; Fig. 559, p. 631).
The inferior (or anterior) extremity of the fascia dentata dips into a deep furrow, around which the area piriformis is bent in a hook-like manner (uncus); in this it becomes considerably reduced in diameter and then emerges (at right angles to its previous direction) to form Giacomini's "banderella," which we may call the cauda fascia dentatæ. Behind this the inferior end of the hippocampus. comes to the surface, but is turned inside out, hippocampus inversus. Just in front of the upper ending of the cauda fasciæ dentatæ a little knob of solid gray matter appears upon the surface, surrounded by area piriformis. It is the nucleus amygdala (Fig. 558).
Corpus Callosum.-The corpus callosum is the great transverse commissure which passes between the two cerebral hemispheres. It is placed nearer the anterior than the posterior aspect of the brain, and it unites the medial surfaces of the hemispheres throughout very nearly a half of their antero-posterior