The fibres of the white central matter are partly centrifugal, the neuraxons of the cells of Purkinje, and partly centripetal. The position of the cells of these latter fibres is not known. The fibres give rise to an abundant plexus of fibrils in the granular layer, and many reaching into the molecular layer ramify there, especially in the immediate neighbourhood of the dendrites of Purkinje’s cells. From the appearance of their plexus of fibrils these are sometimes called moss fibres.
The Fourth Ventricle is the dilated upper end of the central canal of the medulla oblongata. Its shape is like an heraldic lozenge. Its floor is formed by the grey matter of the posterior surfaces of the medulla oblongata and pons, already described (see figs. 3 and 6); its roof partly by the inferior vermis of the cerebellum, the nodule of which projects into its cavity, and partly by a thin layer, called valve of Vieussens, or superior medullary velum; its lower lateral boundaries by the divergent clavae and restiform bodies; its upper lateral boundaries by the superior peduncles of the cerebellum. The inferior medullary velum, a reflection of the pia mater and epithelium from the back of the medulla to the inferior vermis, closes it in below. Above, it communicates with the aqueduct of Sylvius, which is tunnelled below the substance of the corpora quadrigemina. Along the centre of the floor is the median furrow, which terminates below in a pen-shaped form, the so-called calamus scriptorius. Situated on its floor are the fasciculi teretes, striae acusticae, and deposits of grey matter described in connexion with the medulla oblongata. Its epithelial lining is continuous with that of the central canal.
The Cerebrum.
The Cerebrum or Great Brain lies above the plane of the tentorium, and forms much the largest division of the encephalon. It is customary in human anatomy to include under the name of cerebrum, not only the convolutions, the corpora striata, and the optic thalami, developed in the anterior cerebral vesicle, but also the corpora quadrigemina and crura cerebri developed in the mesencephalon or middle cerebral vesicle. The cerebrum is ovoid in shape, and presents superiorly, anteriorly and posteriorly a deep median longitudinal fissure, which subdivides it into two hemispheres. Inferiorly there is a continuity of structure between the two hemispheres across the mesial plane, and if the two hemispheres be drawn asunder by opening out the longitudinal fissure, a broad white band, the corpus callosum, may be seen at the bottom of the fissure passing across the mesial plane from one hemisphere to the other. The outer surface of each hemisphere is convex, and adapted in shape to the concavity of the inner table of the cranial bones; its inner surface, which bounds the longitudinal fissure, is flat and is separated from the opposite hemisphere by the falx cerebri; its under surface, where it rests on the tentorium, is concave, and is separated by that membrane from the cerebellum and pons. From the front of the pons two strong white bands, the crura cerebri or cerebral peduncles, pass forward and upward (see fig. 2). Winding round the outer side of each crus is a flat white band, the optic tract. These tracts converge in front, and join to form the optic commissure, from which the two optic nerves arise. The crura cerebri, optic tracts, and optic commissure enclose a lozenge-shaped space, which includes—(a) a grey layer, which, from being perforated by several small arteries, is called locus perforatus posticus; (b) two white mammillae, the corpora albicantia; (c) a grey nodule, the tuber cinereum, from which (d) the infundibulum projects to join the pituitary body. Immediately in front of the optic commissure is a grey layer, the lamina cinerea of the third ventricle; and between the optic commissure and the inner end of each Sylvian fissure is a grey spot perforated by small arteries, the locus perforatus anticus.
| From Cunningham, Text-book of Anatomy. Fig. 8.—Transverse Section through the Human Mesencephalon at the level of the superior Quadrigeminal Body. |
If a transverse section is made at right angles to the surface of the crura cerebri it will pass right through the mesencephalon and come out on the dorsal side through the corpora quadrigemina (see fig. 8). The ventral part of each crus forms the crusta, which is the continuation forward of the anterior pyramidal fibres of the medulla and pons, and is the great motor path from the brain to the cord. Dorsal to this is a layer of pigmented grey matter, called the substantia nigra, and dorsal to this again is the tegmentum, which is a continuation upward of the formatio reticularis of the medulla, and passing through it are seen three important nerve bundles. The superior cerebellar peduncle is the most internal of these and decussates with its fellow of the opposite side so that the two tegmenta are continuous across the middle line. More externally the mesial fillet is seen, while dorsal to the cerebellar peduncle is the posterior longitudinal bundle. If the section happens to pass through the superior corpus quadrigeminum a characteristic circular area appears between the cerebellar peduncle and the fillet, which, from its tint, is called the red nucleus. More dorsally still the section will pass through the Sylvian aqueduct or passage from the third to the fourth ventricle, and this is surrounded by a mass of grey matter in the ventral part of which are the nuclei of the third and fourth nerves. The third nerve is seen at the level of the superior corpus quadrigeminum running from its nucleus of origin, through the red nucleus, to a groove on the inner side of the crus called the oculo-motor groove, which marks the separation between the crusta and tegmentum. Dorsal to the Sylvian aqueduct is a layer called the lamina quadrigemina and on this the corpora quadrigemina rest. The superior pair of these bodies is overlapped by the pineal body and forms part of the lower visual centres. Connexions can be traced to the optic tract, the higher visual centre on the mesial surface of the occipital lobe, the deep origin of the third or oculo-motor nerve as well as to the mesial and lateral fillet. The inferior pair of quadrigeminal bodies are more closely in touch with the organs of hearing, and are connected by the lateral fillet with the cochlear nucleus of the auditory nerve.
Surface of the Brain.
The peripheral part of each hemisphere, which consists of grey matter, exhibits a characteristic folded appearance, known as gyri (or convolutions) of the cerebrum. These gyri are separated from each other by fissures and sulci, some of which are considered to subdivide the hemisphere into lobes, whilst others separate the gyri in each lobe from each other. In each hemisphere of the human brain five lobes are recognized: the temporo-sphenoidal, frontal, parietal, occipital, and the central lobe or Island of Reil; it should, however, be realized that these lobes do not exactly correspond to the outlines of the bones after which they are named. Passing obliquely on the outer face of the hemisphere from before, upward and backward, is the well marked Sylvian fissure (fig. 9, s), which is the first to appear in the development of the hemisphere. Below it lies the temporo-sphenoidal lobe, and above and in front of it, the parietal and frontal lobes. As soon as it appears on the external surface of the brain the fissure divides into three limbs, anterior horizontal (s1), ascending (s2), and posterior horizontal (s3), the latter being by far the longest. The place whence these diverge is the Sylvian point and corresponds to the pterion on the surface of the skull (see [Anatomy]: Superficial and Artistic). Between these three limbs and the vallecula or main stem of the fissure are four triangular tongues or opercula; these are named, according to their position, orbital (fig. 9, C), frontal (pars triangularis) (B), fronto-parietal (pars basilaris) (A) and temporal. The frontal lobe is separated from the parietal by the fissure of Rolando (fig. 9, r) which extends on the outer face of the hemisphere from the longitudinal fissure obliquely downward and forward towards the Sylvian fissure. About 2 in. from the hinder end of the hemisphere is the parieto-occipital fissure, which, commencing at the longitudinal fissure, passes down the inner surface of the hemisphere, and transversely outwards for a short distance on the outer surface of the hemisphere; it separates the parietal and occipital lobes from each other.
 | |
| From Cunningham, Text-book of Anatomy. Fig. 9.—Gyri and Sulci, on the outer surface of the Cerebral Hemisphere. | |
f1, Sulcus frontalis superior. f2, Sulcus frontalis inferior. f.m, Sulcus frontalis medius. p.m, Sulcus paramedialis. A, Pars basilaris. B, Pars triangularis. C, Pars orbitalis. S, Sylvian fissure. s1, Anterior horizontal limb (Sylvian fissure). s2, Ascending limb (Sylvian fissure). s3. Posterior horizontal limb (Sylvian fissure). s.asc, Ascending terminal part of the posterior horizontal limb of the Sylvianfissure. p.c.i, Inferior praecentral sulcus. p.c.s, Superior praecentral sulcus. | r, Fissure of Rolando. g.s, Superior genu. g.i, Inferior genu. d, Sulcus diagonalis. t1, Superior temporal sulcus (parallel sulcus). t2, Inferior temporal sulcus. p1, Inferior postcentral sulcus. p2, Superior postcentral sulcus. p3, Ramus horizontalis. p4, Ramus occipitalis. s.o.t, Sulcus occipitalis transversus. occ. lat, Sulcus occipitalis lateralis (the sulcus lunatus of Elliot Smith). c.m, Calloso-marginal sulcus. c.t.r, Inferior transverse furrow. |
| Fig. 10.—Orbital surface of the left
frontal lobe and the island of Reil; the
tip of the temporo-sphenoidal lobe has
been removed to display the latter. |
| 17. Convolution of the margin of the longitudinal fissure. O. Olfactory fissure, over which the olfactory peduncle and lobe are situated. TR. Orbital sulcus. 1″ 1″′. Convolutions on the orbital suface. 1,1,1,1. Under surface of infero-frontal convolution. 4. Under surface of ascending frontal; and 5, of ascending parietal convolutions. C. Central lobe or insula. |