The left hemisphere is now turned still more to the left, and with the brain-knife a broad, smooth cut is made through it downward and outward at an angle of 45°, reaching nearly to the cortical surface, in a line connecting the cut through the frontal lobe with that through the occipital and passing along the outer borders of the corpus striatum. The left hemisphere is thus separated in the form of a prism-shaped mass having a convex under surface. (See Fig. 29.) The severed hemisphere falls back by the force of its own weight and the flat cut-surface of the cerebrum is then bisected by a cut made at right angles to it, from before backward, and extending nearly to the cortical surface. (See Fig. 29.) In the case of both of these large incisions of the hemisphere the severed parts are left connected by a small portion of cortical tissue and the pia. The knife should be perfectly dry and clean while making these cuts, and the cut surfaces should not be touched with the fingers or knife-blade, or wet with water, until they have been carefully inspected. Other straight parallel cuts may be made through the brain substance toward the cortex, the severed portions being left connected by the pia so as to permit future orientation. The right lateral ventricle is now opened. The four fingers of the left hand are placed outside and beneath the right hemisphere with the thumb on the median surface, gently raising the hemisphere toward the left, taking care to see that the corpus callosum is not pulled over to the right of the median line. The knife is held in the right hand beneath the left one. The right ventricle is then opened in the same way as the left, beginning in the middle of the corpus callosum near to the gyrus cinguli, and opening first the anterior horn and then the posterior. The operation is somewhat more difficult on the right side than it is on the left, owing to the lack of tension in the cut corpus callosum, so that greater care must be taken to avoid injuring the floor of the ventricle. After the opening of the ventricle the right hemisphere is cut by long parallel incisions made in the same way as on the left side. (See Fig. 29.) Some prosectors in opening the right ventricle prefer to turn the board around so that the frontal lobe points to the operator. The right hemisphere is then held in the left hand and the right ventricle opened just as if it were the left ventricle, except that the posterior horn is opened before the anterior. The method given above can be just as easily learned, and time is saved by not turning the board around twice, as is necessary in the latter case.