STRUCTURE OF THE CEREBRUM.
In a general description of the anatomy of the cerebrum, it may be said to arise by two peduncles, the parts of which are derived in front from the pons Variolii, and behind from the cerebellum. Each peduncle is about five-eighths of an inch in length, about one inch in breadth from before back wards, and about three-fourths of an inch in thickness in a lateral direction. The peduncles are separated from each other in the median lines by a thin layer of antero posterior fibres called the raphe.
One•third of the distance from the posterior median line, on the outer surface of each erns, is a vertical depression leading upwards from the pons to a tubercle, the internal geniculate body. In front of this depression is a broad baud of coarse fibres, the crusts, which is directed upwards and back wards to the lower margin of the optic tract that marks the upper boundary of the cerebral peduncle. Behind the vertical depression is au oblique tract of finer fibres hav ing the same direction as those of the crust% and extending upwards to the testes, and to a horizontal tract which is directed outwards from the testes to the internal geniculate body, the brachium posterius The oblique tract behind the depression arises apparently from the pons Varolii, but sub sequently will be traced to the anterior column of the spinal cord. This tract is depressed below the level of the crusta and overlaps a large band of horizontal anterio posterior fibres derived from the cerebellum, the pro cesses e cerebello ad testes (a misnomer).
The firocessus (a name which we will continue to use in place of the former) of one side is connected with that of the other along their internal margins by the valve of Vieussens which, together with the processes, forms the roof of tlie fourth ventri cle. The crustze diverge as they pass upwards from the pons and are separated in front by a wide interval, the iaterpeduncular space. This interval between the crustau in front is triangular in shape, pointed below, and is about a half an inch iu depth above the polls. TWO round white eminences, the corpora albicautia, are situated between the peduncles about half an inch in front of the pons Varolii. The third nerves spring from the inner margin of the cruste immedi ately above the upper border of the pons. The crustffl are about a quarter of an inch in thickness, aud the nervous niass enclosed and supported by them, composed of numerous tracts and masses of white and gray matter, is called the tegmentutu. The cruste are the motor tracts and expand above bodily into the internal capsules of the cerebral hemis pheres, passing directly through them to reach the motor areas above upon the sides of the surface of the brain. The tegmentum contains tlie sensory- tract, and, with one excep tion, the fillet, its fibrous structures are interrupted by masses of grey- matter before entering the internal capsule to be dis tributed to the surface of the brain. In a section of the
peduncles the tegmentum consists of all that portion of the surface except, the divided tracts of the crustee, which are situated on the sides and the front of the peduncles.
The anatomy of the peduncle will be examined more closely later, but at present it is necessary- to mark the distinction between the crusta and the tegmeutum, -because these divisions of the crus cerebri or peduncle are frequently referred to in works upon anatomy.
The internal capsule of each hemisphere is derived from the expansion of a crus or peduncle of the cerebrum. The fibres of the crus cerebri pass upwards and outward to about the middle of the hemisphere, where tliey decussate with the fibres of tlie corpus callosum, external to and above the outer border of the lateral ventricle. Beyond the decus sation of these tracts, which extends from before backwards throughout the length of the outer margin of lateral ventricle, the fibres of the corpus callosum and internal capsule inter mingle with those of the longitudinal commissures of the hem isphere and constitute its white substance, or the corona radiata.
The corpus callosmn is the great transverse commissure of the brain. It forms the roof the lateral ventricles, and uniting upon its sides with the fibres of the internal capsules, a triangular space is enclosed containing the ventricles and the internal basal ganglia.
A vertical section across the middle of the brain shows the internal capsule below its junction with the corpus cal.
losum to be in relation with internal and external masses of grey matter, the basal ganglia. Those on the inside, are the caudate nucleus, and below it the thalamus opticus; llpou the outside of the capsule are the three divisions of the lenticular body, the corpus lenticularis which is bounded externally by the external capsule, claustrum and insula suc cessively. Above the decussation of the internal capsule and the corpus callosum is the white substance of the brain, the corona radiata, composed of three layers ; an ex ternal or sub-convolutional, a middle or the layer of the longitudinal commissures, and an internal or capsular layer which immediately surrounds the ventricle.
The mass of white matter constituting the corona radiate is surrounded by a sheet of grey matter which extends along the surface of the convolutions and into the depths of the sulci upon the surface of the cerebrum. This lamina of grey matter covering the hemispheres constitutes the hemis pherical ganglia and is the seat of consciousness, memory and mind.
The foregoing general description has prepared us to con sider each part of the cerebrum more particularly as it appears in the order of dissection.