THE CEREBRAL HEMISPHERES. EXTERNAL CONFORMATION.' The cerebral hemispheres together form an ovoid mass, irregularly flattened on its under side, and placed in the cranium with its smaller end forwards, its greatest width being opposite to the parietal eminences. The hemispheres are separated in a large part of their extent by a deep fissure, the great longitudinal fissure. On opening this fissure by drawing asunder the two hemispheres, it is seen, both before and behind, to pass quite through to the base of the cerebrum; but for 10 cm. in the middle. it is interrupted, at a depth of about 35 mm. at the front to 50 mm. at the back, by a large transverse mass of white substance, named the corpus callosum, which connects the two hemispheres together. While the brain is within the skull, the greater part of the longitudinal fissure is occupied by a vertical process of the dura mater-the falx cerebri— which dips down between the two hemispheres, not quite reaching to the corpus callosum in front, but touching it behind. Where the fissure is unoccupied by the falx, the inner surfaces of the two hemispheres lie in close contact with one another. Each cerebral hemisphere has an outer, convex surface, in contact with the vault of the cranium; an inner or mesial, flat surface, which forms one side of the longitudinal fissure; and an irregular under surface, in which is a deep cleft, the vallecula Sylvii. In front of this cleft the under surface (orbital division), which looks somewhat outwards as well as downwards, and is slightly concave, rests in the anterior fossa of the base of the skull. Behind the cleft the under surface lies in the middle fossa, and, farther back still, on the tentorium cerebelli. These two parts of the under surface are concave from before backwards, and look inwards and downwards. The three surfaces of the hemisphere pass into one another at the borders, which are also three in number-viz. two mesial (superior and inferior) and one lateral. The superior mesial border is the most distinct of the three. It separates the mesial and external surfaces, and lies at the side of the superior sagittal sinus. The inferior mesial border is well marked in its anterior third, where it separates the orbital part of the under surface from the mesial surface; in the middle it is interrupted by the various structures uniting the two hemispheres; posteriorly it inclines obliquely backwards and downwards from the splenium to the posterior extremity of the hemisphere, and occupies the angle at the attachment of the falx cerebri to the tentorium cerebelli. The lateral border anteriorly separates the orbital and external surfaces of the frontal lobe, and this part is sometimes termed the superciliary border. Behind the stem of the Sylvian fissure it forms the lower edge of the temporal and occipital lobes. The anterior extremity of each hemisphere is known as the frontal pole, the posterior as the occipital pole, while the free rounded anterior extremity of the temporal lobe is sometimes termed the temporal pole. The surface of the hemispheres is composed of grey matter, and is moulded into numerous smooth and tortuous eminences, named convolutions or gyri, which are marked off from each other by fissures or sulci of varying depth. The convolutions are covered closely throughout by the vascular investing membrane, the pia mater, which sends processes down to the bottom of the sulci, while the arachnoid membrane passes from one convolution to another, without dipping between them. In general, the breadth of a convolution is some 1 The most important work on the naked-eye anatomy of the human cerebral convolutions that has appeared since the last edition of this text-book is Retzius's Das Menschenhirn, 1896, which has been freely consulted in the preparation of this edition. This and the next three figures are from the same cerebral hemisphere. FIG. 255.-LEFT CEREBRAL HEMISPHERE FROM THE LATERAL ASPECT. Natural size. (J. Symington.) Ramus ant. horizontalis Ramus ant. ascendens S. occipitalis lateralis S. occipitalis transversus what greater at the surface than at the base. The convolutions present considerable variations of position, direction, and complexity in the brains of different individuals, and even in the two sides of the same brain, but they are usually recognisable without much difficulty. It is far easier, however, to trace the more important fissures and convolutions on the brain of a monkey, where they are much less obscured by tortuosities and secondary sulci. Since the external grey or cortical substance is continuous over the whole surface of the cerebral hemispheres, being found alike within the sulci and upon the gyri, it is obvious that a far greater extent of grey matter is thus provided for with a given size of the brain than could have been the case had the hemispheres been plain and destitute of convolutions; indeed, it is FIG. 257.-LEFT CEREBRAL HEMISPHERE FROM BEHIND. Natural size. (J. Symington.) calculated that the extent of 'sunken' or 'concealed' surface is twice that of 'exposed' surface. The sulci between the convolutions vary greatly in depth, being in some parts quite shallow, in others as much as 25 mm. (1 inch) or more deep. The average depth is about 10 mm. or 12 mm. In the examination of the surface of the hemispheres the student should first ascertain the position of those fissures which by their depth, length, or regularity are most easily recognised, since these will serve as landmarks to assist in the determination of the smaller and less regular The relations of many of the fissures cannot be completely studied without opening them up, as sulci which on a surface view are continuous will often be found to be separated by wellmarked deep, or concealed, annectent gyri. ones. VOL. III. S |