DISSECTION.
By separating the sides of the longitudinal fissure between the hemispheres of the cerebrum, there is brought into view a broad band of transverse fibres, the corpus callosum, which connects the hemispheres. Enclosing the corpus callosum behind, above and in front is a long continuous con volution, the gyres fornicates, which overlaps it on each side as far as the middle line. The fissure between the corpus callosum and the gyres fornicates is about five-eights of an inch in depth, extending in a lateral direction from the bot tom of the longitudinal fissure, and is sometimes called the ventricle of the corpus callosum.
If the convolutions above the corpus callosum are gently lifted and torn outward, they will break externally from an elevated crest, one on each side, which is the line of decussa tion of the corpus callosum with the internal capsule as it ascends from the crus cerebri. These crests extend longi tudinally along the middle of the hemispheres to near their extremities, are more elevated in the center than at each end, and are more widely separated behind than in front. The crests project upwards as longitudinal ridges that form the outer boundaries of the corpus callosum. The upper surface of the corpus callosum is concave from side to side forming a broad longitudinal groove, or trough about four inches in length and two inches wide, situated in the middle of the cerebrum.
External to the corpus callosum on each side is the white substance of the brain, the corona radiata, divided in front and behind into two parts by the anterior aud the posterior median fissures. It is fringed by a thin layer of grey matter upon its outer border, and the broken surface shows the ends of torn blood vessels, the puncta vasculosa.
The whole surface exposed by breaking the upper part of the cerebrum to a level with the corpus callosum is oval in shape, and convoluted on its margin by fissures. One OD each side is deep, and situated behind the middle of the hemisphere, the posterior extremity of the fissure of Sylvius. Behind the corpus eallosum is the exposed upper surface of the miens lobe of the brain, the center of vision, and the convolution immediately behind the fissure of Sylvius is the center of word memories.
To illustrate the structure of the corona radiata a triangle has been figured upon the right side of figure 3, which serves to elucidate the phenomena of word blindness, word deafness, amnesia and aphasia, also another triangle in front, to illustrate cerebral automatic action. The external side of
the triangle represents tbe outermost layer of the corona radiata which is composed of fibres passing between adjacent lobes and convolutions. These are called the internuntial fibres, and in the illustration the tract is repre sented through which the name is suggested by the object, and vice versa, and the fingers are set in motion automatically' by aural and visial impressions.
The internal and anterior sides of the triangle represent the fibres of the external longitudinal commissure which associate the memory centres behind with the seat of the intellect in the anterior lobes, and with the motor centres of the central region of the brain. The former for the produc tion of conscious and rational, and the latter for subconscious or automatic action. It is easy to determine the results pro duced upon memory, mind and action by the destruction of a nerve centre, or the interruption of any of these tracts by accident or disease.
The internal layer of the corona radiata is that next to and including the crest or decussation of the corpus callosum and internal ca.psnle. This layer of the corona lies close to the walls of the lateral ventricle previous to its distribution. External to this layer, the fibres of the several layers are mingled into a network, the formatio reticularis of the cere brum. The internal layer is composed wholly of fibres from the corpus callo.sum and the internal capsule, and as they pass toward the surface of the brain are disposed in horizontal lamium between which the fibres of the longitudinal commis sures are inserted in their pasmge from before backwards. The outermost layer of the corona racliata is a close network of fibres passing in all directions and therefore in dissection breaks into short pieces immediately beneath the convolutions. The three layers of the corona may with propriety be called, from within outwards, the capsular, commissural and inter nuntial layers. These layers are exhibited in plate No. 25 of the vertical sections.