The hemispheres are now separated until the corpus callosum comes into view. The left hemisphere is then held by the left hand, with the thumb on the median surface and the fingers on the outer and under sides, so that the hemisphere is turned outward and yet raised slightly at the same time, thus stretching the corpus callosum over the cavity of the left lateral ventricle. The point of the narrow brain-knife (Fig. 3) with cutting edge upward is then introduced with great care through the corpus callosum about midway between the genu and splenium and close to the gyrus cinguli (gyr. forn., Fig. 28). The corpus callosum at this point is about 2 to 3 mm. thick and it is gently nicked with the point of the knife until an opening is made into the cavity of the ventricle. The knife-point must not be allowed to slip through to damage the basal ganglia beneath. Into the small opening thus made the brain-knife, held nearly horizontal, with cutting edge upward, is introduced and the corpus callosum cut forward until the anterior horn of the ventricle is reached. The point of the knife is then passed into the horn and the knife-handle raised and turned over forward, cutting slightly outward through the frontal lobe to its apex and disclosing the anterior horn. The knife is then reversed, held horizontally, with cutting edge upward, and the corpus callosum cut posteriorly from the beginning of the first cut, until the posterior horn is reached, when the point of the knife is inserted into the horn and the knife turned over toward the operator, cutting backward and somewhat outward through the occipital lobe to its apex and opening up the posterior horn. (See Fig. 28.) By this method the lateral ventricle is opened first at the highest point of its cavity, and the fluid contents collect in the anterior and posterior horns so that the amount and character can be easily noted. [Illustration: FIG. 29.—Section of brain. Ventricles opened. Lines show direction of large longitudinal incisions through brain-substance. (After Nauwerck.)]