THE CEREBRUM.
Separated from the rest of the mass of the Central Ner vous System and viewed, the cerebrum is much the largest part, being about eight times the weight of the cerebellum, and thirty times that of the spinal cord. It averages seven inches in a longitudinal and five inches in a transverse diame ter. It is convex upon its upper surface, and its sides seen from above, present an oval contour.
Turned over, its under surface in a general aspect is oval and flat. This surface presents three lobes and two surfaces, and is divided into lateral halves by a continuation of fissures and depressions.
The anterior or frontal portion is a flattened surface divided in the middle by the anterior median fissure. The marginal convolutions, which form the sides of the fissure, are elevated above the rest of the surface, and the parts upon each side of the elevation are triangular and slightly concave, corres• poudiug to the upper surfaces of the orbital plates of the frontal bone, upon which they rest in their normal, position in the cranial cavity. On the sides of the median fissure, and removed half an inch from it, are two other fissures, one on either side, which run parallel to it and lodge the olfactory bulbs and nerves.
The temporal lobes project above this surface behind like large tubercles, one on each side, and are separated from each other by a considerable interval. They are divided from the frontal surface, or lobes, by the fissures of Sylvius which arch around and constrict their bases.
The posterior, or occipital lobes, are directed backwards, are pointed posteriorly, and are separated from each other by the posterior median fissure, which in this situation, longitudin ally, is about three inches in depth, running forward as far as the posterior border of the corpus callosum.
The contour presented by a view of the under surfaces of the temporal and occipital lobes is oval, concave from before backwards and from side to side, forming a shallow cup, in the center of which is an oval opening. This opening is the hilus of the cerebrum which transmits the cerebral peduncles in front, and behind lodges the optic lobes, or corpora quadrigemina. In front of this opening, between the
temporal lobes, is an interspace, the floor of which contains the optic commissure, infundibulum, corpora albicantia and posterior perforated space. These parts rest upon the body of the sphenoid bone. The posterior median fissure extend ing backward completes the division of the surface into lateral hal ves.
The anterior portions of this surface, or the temporal lobes, are lodged in the temporal fosste of the skull, and the occipital lobes rest upon the tentorium eerebelli. This sur face behind forms the upper boundary of the extra ventricular portion of the great transverse fissure or fissure of liichat.
The cerebrum is divided into lateral halves, called hemis pheres, by the great longitudinal fissure, which includes the anterior and posterior median fissures that separate the frontal and the occipital lobes. The hemispheres are united at the base of the cerebrum, and by several commissures the largest of which is the corpus collosum or the great transverse commis sure of the cerebrum.
In general contour and markings the lateral halves of the cerebrum are alike, but upon closer inspection, the convolu tions and fissures of one hemisphere do not correspond with those of the other, and in a comparison of any number of brains, there cannot be found any exact resemblance in the hemispheres of the same side.
There are, however, certain distinct fissures and convolu tions which enable the anatomist to map out the surface of the brain, and to define localities which may be com pared with those of other brains. The whole surface of the brain is divided into general regions which correspond to and are named after the bones of the skull which cover them. Thus we speak of the frontal, parietal, occipital, temporal, temporo-sphenoidal, orbital and insular regions of the brain, and that which lies in apposition with its fellow within the longitudinal fissure, as the inner or fissural region.
The convolutions upon the surface of the cerebrum will be described later on with the hemispherical ganglion.