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attached end of the peduncle is called the trigonum olfactorium. Immediately behind the trigone a small obliquely placed ovoid area of gray matter, the tuberculum olfactorium, can sometimes be detected in the human brain; but in the brains of most mammals with a greater development of the organs of smell this swollen area is much more prominent and constant. In most human brains, however, it is difficult to distinguish it from a much more extensive area, which is situated behind it and to its lateral side, and is named the substantia perforata anterior (Fig. 552). Along the anterior margin of this perforated substance there can sometimes be detected a small, rounded, rope-like strand of gray matter, the medial end of which passes into the trigonum olfactorium. This is the anterior
FIG. 552.-PART OF THE VENTRAL SURFACE OF THE PROSENCEPHALON, SHOWING THE ATTACH
MENT OF THE OLFACTORY TRACT.
Olfactory area, dull yellow; optic, blue; motor fibres, red; acoustic fibres, bright yellow.
part of the area piriformis-the stalk of the pear-shaped lobe-and upon its surface is placed a very well-defined narrow band of nerve-fibres, the stria olfactoria lateralis, which is composed of axons of mitral cells (in the olfactory bulb) proceeding to the piriform area. Even when the anterior part of the piriform area is not distinguishable, the stria lateralis is always a prominent feature.
The piriform area extends transversely laterally in the deep valley between the orbital and temporal regions of the hemisphere (fossa cerebri lateralis); becoming slightly broader, and reaching what is known as the insula (of which it forms the limen insula), it becomes sharply bent upon itself (Figs. 552, and 553, C), It then passes medially and backwards, and emerges from the fossa as a broad area upon the under surface of the temporal region (Fig. 553, C). This greatly expanded caudal extremity of the pear is the area piriformis in the strict sense
of the term.
If the brain of almost any other mammal is examined (take the rabbit's as an example), the area piriformis will be found to constitute relatively an enormously larger proportion of the cerebral hemisphere than it does in the human brain; and it is separated from the part of the hemisphere (neopallium) that lies above it by a longitudinal furrow called the fissura rhinalis. The enormous expansion of the neopallium in the human brain accentuates the flexure of the piriform area at the point x (Fig. 553), and at the point y the exuberant growth of neopallium relegates the swollen posterior part of the piriform area on to the medial surface (Fig. 554), where the posterior part of the rhinal fissure persists to separate it from the neopallium.
The surface of the piriform area often presents numerous small wart-like
A, The lateral aspect of the left cerebral hemisphere of a rabbit. B, The inferior aspect of the right half of a rabbit's brain. C, The corresponding view of a human foetal brain at the fifth month. Olfactory areas, green; neopallium, blue.
excrescences; and it is whitened by a thin layer of fibres (substantia reticularis alba) prolonged backwards from the stria olfactoria lateralis. By these fibres olfactory impulses are poured directly from the mitral cells of the bulb into the piriform area. If we call the olfactory nerves the primary olfactory neurones, the fibres which pass from the bulb to the piriform area would then be secondary olfactory neurones.
Formatio Hippocampalis. From all parts of the area piriformis, as well as the trigonum and tuberculum olfactorium, fibres arise (tertiary olfactory neurones), and proceed on to the medial aspect of the hemisphere, where they terminate in the edge of the pallium, alongside the lamina chorioidea. In the human brain the vast majority of these tertiary neurones proceed from the posterior extremity of the piriform area, but a certain number arise in the neighbourhood of the substantia perforata anterior and proceed at once on to the medial surface of the hemisphere. The large number of small nerve-cells that collect in the medial edge of the pallium become specially modified in structure to form a receptive organ for impressions of smell, known as the fascia dentata; and the axons of these cells pass into the part of the pallium which immediately surrounds the peripheral edge of the fascia dentata and is known as the hippocampus (Fig. 556).
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
Sulcus sagittalis eun
Suleus retrocalcaria Area striata
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 dentata, where they bend into the longitudinal direction (i.e. 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
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 fœtus 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