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p652
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p653
Limbic Lobe The term Limbic Lobe was introduced by Broca in 1878, under it he included Two Convolutions, the Callosal and Hippocampal; which together arch round the Corpus Callosum and the Hippocampal Fissure. These he separated on the Morphological grounds that they are well developed in animals possessing a Keen Sense of Smell. To the Lobe thus defined the following parts must be added: The Peduncles and Longitudinal Striae of the Corpus Callosum, together with the Gyrus Dentatus, which form a Middle Arch, while the Outer Arch is constituted by the Callosal and Hippocampal Convolutions: the first two arches are separated from each other by the Corpus Callosum. (View Image) |
Convolutions of the Limbic Lobe
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The Olfactory Lobe The Olfactory Lobe is situated on the under surface of the Frontal Lobe. It is rudimentary in man and some other mammals. But in vertebrates generally it is well developed, and consists of a distinct extension of the Cerebral Hemispheres, enclosing a portion of the Anterior Horn of the Lateral Ventricles. In man it is long and slender and may be described as consisting of two parts: the Anterior and Posterior Olfactory Lobules. The Anterior Olfactory is made of: the Olfactory Bulb; the Olfactory Tract; the Trigonum Olfactorium; and the Area Of Broca.
The Posterior Olfactory Lobule or Anterior Perforated Space is marked off from the Anterior Lobule by the Fissura Prima, and is situated at the commencement of the Fissure Of Sylvius. Internally, it is bounded by the Peduncles of the Corpus Callosum, and is continuous with the Lamina Cinerea. Posteriorly it is bounded by the Optic Tracts, and it is partially concealed by the Temporal Lobe which overlaps it. It has received the name of Anterior Perforated Space from its being perforated by numerous openings. Which transmit Blood Vessels to the Interior of the Brain, and it corresponds to the Under surface of the Lenticular Nucleus and part of the Claustrum. |
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Base Of The EnCephalon The base of the Brain presents for examination the under surfaces of the Frontal and Temporal Lobes; the structures contained in the InterPeduncular Space, with the Cura Cerebri or Cerebral Peduncles; the under surfaces of the Pons, Cerebellum, and Medulla Oblongata; and the superficial Origins of the Cranial Nerves. The various objects exposed to view in the Middle Line and on either Side of the Middle Line, are here arranged in the order they are met with From Before Backward. The Longitudinal Fissure partially separates the two Hemispheres from each other. It divides completely the Anterior portions of the two Frontal Lobes. On raising the Cerebellum and Pons, it will be seen to separate completely the two Occipital Lobes; of these two portions of the Longitudinal Fissure, that which separates the Occipital Lobes is the longer. |
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The intermediate part of the Fissure is filled up by the great Transverse band of White Matter, the Corpus Callosum. In the Fissure between the two Frontal Lobes the Anterior Cerebral Arteries ascend on to the Corpus Callosum. p656The Corpus Callosum terminates at the base of the Brain by a concave margin, which is connected with the Tuber Cinereum through the intervention of a thin layer of Gray Substance, the Lamina Cinerea. This may be exposed by gently raising and drawing back the Optic Commissure. A white band may be observed on each side, passing backward from the under surface of the Corpus Callossum, across the Posterior Margin of the Anterior Perforated Space to the Hippocampal Gyrus, where each meets the Peduncles of the Corpus Callosum. They may be traced upward around the Genu to become continuous with the Striae Longitudinales on its upper surface. Laterally, this portion of the Corpus Callosum extends into the Frontal Lobe. The Lamina Cinerea is a thin layer of gray substance extending backward from the termination of the Corpus Callosum above the Optic Commissure to the Tuber Cinereum. It is continuous on each side with the Gray Matter of the Anterior Perforated Space, and forms the Anterior part of the Inferior Boundary of the Third Ventricle. The Optic Commissure is situated in the Middle Line, immediately in front of the Tuber Cinereum and below the Lamina Cinerea; that is to say, the Commissure is Superficial to the Lamina in the order of dissection when the base is uppermost. It is the point of junction between the two Optic Tracts, and will be described with the Cranial Nerves. Immediately behind the diverging Optic Tracts, and between them and the Peduncles of the Cerebrum is a lozenge-shaped interval. The InterPeduncular Space, which is bounded behind by the Pons, and in which are found the following parts: the Tuber Cinereum, Infundibulum, Pituitary Body, Corpora Albicantia and the Posterior Perforated Space The Tuber Cinereum is an eminence of Gray Matter, situated between the Optic Tracts, and extending from the Corpora Albicantia to the Optic Commissure, to which it is attached. It is connected with the surrounding parts of the Cerebrum, forms part of the floor of the Third Ventricle, and is continuous with the gray substance in that cavity. From the Middle of its Under surface a conical tubular process of Gray Matter, about two lines in length, is continued downward and forward to be attached to the Posterior Lobe of the Pituitary Body. This is the Infundibulum, and its canal, which is funnel shapped, communicates with the Third Ventricle. |
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The Pituitary Body is a small, reddish-gray, vascular mass, weighing from five to ten grams, and of an oval form, situated in the Sella Turcica, where it is retained by a process of Dura Matter, named the Diaphragma Sellae. This process covers in the Sella Turcica, and has a small hole in its center through which the Infundibulum passes. Pituitary's Structure The Pituitary Body is very Vascular and consists of two Lobes, separated from one another by a Fibrous Lamina. Of these, the Anterior is the larger, of an oblong form, and somewhat concave behind, where it receives the Posterior Lobe, which is round. The two Lobes differ both in development and structure. The Anterior Lobe, is of a dark, reddish-brown color, is developed from the Epiblast of the Buccal cavity, and resembles to a considerable extent, in microscopic structure, the Thyroid Body. p656It consists of a number of isolated vesicles and slightly convoluted tubules, lined by Epithelium and united together by a very Vascular connective tissue. The Epithelium is always columnar and occasionally Ciliated. The Alveoli sometimes contain a colloid material, similar to that found in the Thyroid Body, and their walls are surrounded by a close network of Lymphatic and Capillary Blood Vessels. The Posterior Lobe is developed as an outgrowth from the Embryonic Brain, and during Foetal life contains a cavity which communicates through the Infundibulum with the cavity of the Third Ventricle. In the adult it becomes firmer and more solid, and consistts of a sponge like connectivve tissue arranged in the form of reticulating bundles, between which are branched cells, some of them containing pigment. In the lower animals the two lobes are quite distinct, and it is only in the Mammalia that they become fused together. p657The Corpora Albicantia or Mammillaria are two small, round, white masses, each about the size of a pea, placed side by side immediately behind the Tuber Cinereum, and connected with each other across the Mesial Plane. They are mainly formed by the Anterior Crura of the Fornix, which after descending to the base of the Brain, are twisted upon themselves to form loops, and constitute the white covering of the Corpra Albicantia. A second fasciculus, the Bundle Of Vicq D'Azyr, converges from the Optic Thalamus, and enters the Anterior part of each body on its Dorso-Mesial surface. They are composed externally of White substance, and internally of Gray Matter; the nerve cells of the Gray Matter are arranged in two sets, inner and outer, the cells of the former set being the smaller. They are connected to the Tegmentum by a small bundle of fibers, the Peduncle of the Mammillary Body. At an early period of foetal life they are blended together into one large mass, but become separated about the seventh month. In most vertebrates there is only one Median Corpus Albicans. |
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The Posterior Perforated Space (Pons Tarini) corresponds to a whitish-gray Fossa placed between the Corpora Albicantia in front, the Pons Varollii behind, and the Crus Cerebri on either side. It forms the Posterior part of the floor of the Third Ventricle, and is perforated by numerous small orifices for the passage of the Postero-Median Ganglionic branches of the Posterior Cerebral and Posterior Communicating Arteries. The Pons Varolii is situated immediately behind the two Crura of the Cerebrum. It consists of a broad band of white fibers, which pass transversely from one Cerebellar Hemisphere to the other; the band becoming narrower as it enters the Cerebellum. In the Middle Line on its under surface a narrow groove runs from before backward and accomodates the Basilar Artery. The Medulla Oblongata emerges from the Posterior border of the Pons Varolii; it is Pyramidal in form, and is continuous below with the Cervical portion of the Spinal Cord. It is marked on its Ventral surface by a median fissure, continuous below with the Anterior median fissure of the Cord, and on either side by secondary fissures and columns, which will be described in the sequel. The Frontal Lobe's under surface is seen on the Anterior part of the base of the Brain on either side of the Median Line. It is sometines called the Orbital Lobe. p658The Fissure Of Sylvius at the base of the Brain separates the Frontal from the Temporal Lobe, and lodges the Middle Cerebral Artery. The Optic Tracts are well marked, flattened bands of Fibers, which run Obliquely across the Crus Cerebri on either side, and unite Anteriorly to form the Optic Commissure. The Crura Cerebr (Peduncles of the Cerebrum) are two thick cylindrical bundles of White Matter, which appear in front of the Anterior border of the Pons, and diverge as they pass forward and outward to enter the under surface of each Hemisphere. Each Crura is about three quarters of an inch in length, and is about the same in breadth Anteriorly, but somewhat less Posteriorly. They are marked upon their surface with Longitudinal Striae, and each is crossed, just before entering the Hemisphere, by the Fourth Nerve and the Optic Tract, the latter of which is adherent by its upper surface to the Peduncle. The Temporal Lobe's under surface is visible at the base of the Brain, on either side of the Crura and the structures contained in the InterPeduncular Space. It is separated Anteriorly from the Frontal Lobe by the Fissure Of Sylvius, and behind is limited by the Anterior border of the Lateral Hemispheres of the Cerebellum. The Fissures and Lobes on its surface have already been described (p652). p659The Hemispheres Of The Cerebellum are situated on either side of the Middle Line, and cover the Occipital Lobes of the Cerebrum, when viewed from the base. The Cerebellum differs much in appearance from the rest of the EnCephalon, being of a darker color, while its Convolutions are smaller and narrower,and arranged like the leaves of a book, and hence called Folia. _______________ |
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