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the vicinity of the parts which lie subjacent to the parietal tuberosities of the cranium. The massive rounded character of the anterior or frontal end of each cerebral hemisphere constitutes a leading human characteristic; but the posterior or occipital end is narrow and pointed, and is directed somewhat downwards. The two cerebral hemispheres are separated from each other by a deep median cleft, termed the longitudinal fissure.
The cerebral hemisphere is formed from a small area of the extreme anterior end of the alar lamina, in the angle between the foremost part of the roof and the upper end of the lamina terminalis (Fig. 548, L), which becomes continuous with the roof (at the point marked X).
The rapid expansion of this area leads to the development of a lateral bulging containing a diverticulum of the third ventricle, which is known as the ventriculus lateralis. This at first communicates with the third ventricle by
FORE - BRAIN
means of a wide opening (F.M.), the foramen interventriculare [Monroi], cor
responding in size to the extent of the area of the side wall that was bulged outwards to form the hemisphere vesicle. The thin epithelial roof of the telencephalon takes no share in the formation of the two cerebral hemispheres, but serves with the lamina terminalis (L) as a bond of union between them. At a somewhat later stage in development two folds become invaginated from this epithelial roof in the whole extent of the prosencephalon, both its telencephalic and diencephalic parts. In the greater part of their length these folds project into the third ventricle, and form its chorioid plexus (Fig. 549); but the anterior parts of the two chorioidal folds, namely, those parts formed from the roof of the interventricular foramina (F.M.), become greatly enlarged and project each into the corresponding lateral ventricle. The furrow corresponding to this invagination of the roof is called the fissura chorioidea. When the hemisphere vesicle first begins to expand, the thinner part of the hemisphere wall, which is called the pallium, is freely continuous around the vertical caudal margin of the foramen interventriculare (Fig. 548, Y) with the thalamus (TH).
FIG. 548.-Two DRAWINGS OF THE EMBRYONIC BRAIN (by His). A, Reconstruction of the fore-brain and mid-brain of His's embryo KO; profile view. B, Same brain as A, divided along the median plane and viewed upon its inner aspect.
Mamillary eminence; Tc, Tuber cinereum; Hp, Hypophysis (hypophyseal diverticulum from buccal cavity); Opt, Optic stalk; TH, Thalamus; Tg, Tegmental part of mesencephalon; Ps, Pars hypothalamica; Cs, Corpus striatum; FM, Foramen interventriculare; L, Lamina terminalis; RO, Recessus opticus; Ri, Recessus infundibuli, Met, Metathalamus.
But as development proceeds the wall of the prosencephalon becomes attenuated along the line of this pallio-thalamic junction, and eventually the edge of the pallium fringing this attachment to the thalamus becomes reduced to a thin layer of epithelium which is continuous at its superior end with the lamina chorioidea of
the roof. Into this secondarily formed caudal extension of the chorioid lamina the invagination that commenced in the roof of the foramen interventriculare extends until it reaches the inferior extremity of the deep cleft separating the cerebral hemisphere from the thalamus (Fig. 548, A). Below this point the thalamus remains in uninterrupted continuity with the floor of the cerebral hemisphere (Cs), which is becoming thickened to form the corpus striatum.
At a very early stage in the development of the embryo, long before there is any sign of the hemisphere vesicles, the ectoderm upon each side of the anterior neuro
pore (see p. 500) becomes thickened to form the area olfactoria (see Fig. 440, D, p. 501). Certain of the epithelial cells in this area become converted into bipolar sensory cells, which become specially adapted to be affected by certain kinds of air-borne chemical stimuli that awaken a consciousness of smell. These cells always remain in situ in the olfactory epithelium, just as the most primitive sensory cells do in Hydra (Fig. 439, p. 497). But other nerve - cells seem to be derived from the area olfactoria which do not remain in the parent epithelium,
but become attached to the adjoining part of the neural tube. These cells form the olfactory ganglion, which acts as the receptive organ for the impressions brought into it by the processes of the sensory cells in the olfactory epithelium; and the
FIG. 549.-DIAGRAM OF A TRANSVERSE SECTION THROUGH A
Lamina chorioidea ventriculi tertii. Corpus striatum.
P.M.H. Pars mamillaris hypothalami.
M. Mamillary region.
F. Frontal lobe.
FIG. 550.-TWO DRAWINGS BY HIS, ILLUSTRATING THE DEVELOPMENT OF THE HUMAN BRAIN.
B, Schema showing the directions in which the cerebral hemisphere expands during its growth
O. Occipital lobe.
area of the neural tube to which it becomes attached is destined to become part of the cerebral hemisphere. At the end of the first month this portion of the hemisphere becomes drawn out as a hollow protrusion, the distal end of which is coated with a layer of olfactory ganglion and is known as the bulbus olfactorius; the rest forms a peduncle. In the course of its subsequent development in the human brain (though not in those of most mammals) the cavity in the bulb and peduncle becomes completely obliterated. The peduncle becomes so greatly elongated and attenuated that, to the unaided eye, it appears to be wholly formed
of white nerve-fibres passing to and fro between the olfactory bulb and the hemisphere; hence it is called the tractus olfactorius.
The cerebral hemisphere first appears in the form of a slight bulging upon each side of the fore-brain, but it soon assumes large dimensions. At first it grows forwards and upwards (Fig. 550), and a distinct cleft, the floor of which is the roof-plate and lamina terminalis, appears between the two hemispheres: this is known as the fissura longitudinalis cerebri. The separation of the two cerebral vesicles. by the longitudinal fissure begins at the end of the first month. This fissure becomes occupied by mesodermic tissue, which later on forms the falx cerebri. The cerebral hemisphere, in its further growth, is carried progressively backwards over the posterior parts of the developing brain. At the end of the third month it. has covered the thalamus. A month later it reaches the corpora quadrigemina, and by the seventh month it has not only covered these, but also the entire upper surface of the cerebellum.
In the earlier stages of its development the cerebral hemisphere is a thin-walled vesicle with a relatively large cavity, which represents the primitive condition of the lateral ventricle. At first the vesicle is bean-shaped and the cavity is curved. As development proceeds the posterior portion of the hemisphere grows backwards over the cerebellum in the shape of a hollow protrusion, and a distinct occipital lobe enclosing the posterior horn of the lateral ventricle is the result. This developmental stage begins about the fourth month.
THE CONNEXIONS OF THE OLFACTORY NERVES.
The olfactory nerves are the axons of the spindle-shaped bipolar cells situated in the olfactory mucous membrane (Fig. 551). These axons collect in the submucous layer to form small bundles, which enter the cranial cavity through the foramina in the lamina cribrosa of the ethmoid bone. They at once enter the inferior surface of the bulbus olfactorius, and each fibre breaks up into a tuft of terminal filaments. Towards these tufts dendrites proceed from large mitral cells placed in a deeper plane within the bulb, and each dendrite also. breaks up into numerous terminal branches intertwined with those of the olfactory nerves. In this way are formed a large number of globular bodies, each consisting of the arborescent terminations of a mitral dendrite and of certain olfactory nervefibres. These are the olfactory glomeruli of the bulb. Each mitral cell gives off several dendrites and one axon. Only one dendrite enters into the formation of a glomerulus, but several nerve-fibres may be connected with such a body. It thus happens that, through its dendrite, a mitral cell may stand in connexion with several olfactory nerve-fibres. The axon of the mitral cell passes upwards to the white matter of the bulb, enters this, and, bending backwards, is conducted through the tract towards the FIG. 551.-DIAGRAM OF THE MINUTE cerebral cortex.
STRUCTURE OF THE OLFACTORY BULB.
OLFACTORY MUCOUS mw! MEMBRANE
The olfactory bulb is a small, flattened, elliptical mass of gray substance placed upon the upper surface of the lamina cribrosa of the ethmoid. Its posterior extremity is attached to the rest of the cerebral hemisphere by the long tractus. olfactorius (Fig. 476), a prismatic band of white substance placed in a furrow (sulcus olfactorius) on the under surface of the frontal region of the cerebral hemisphere. A short distance in front of the optic chiasma each olfactory tract. becomes inserted into the hemisphere (Fig. 552). The swollen pyramidal-shaped
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-
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
Piriform area (posterior part)
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).