A Method for Accurately Locating Points in the Interior of the Brain.

With the instrument devised by Horsley and Clark 1 it is possible to stimulate or destroy any desired point in the interior of the brain. When properly adjusted to the head of the animal by grasping devices inserted in the external auditory openings and the orbits, the base line of the instrument passes through the center of the external auditory meatus and through the lower border of the orbit. The zero horizontal plane is parallel to the base line but above it onethird of the distance from the interaural line to the vertex. The zero point is at the intersection of this plane and the midsagittal plane with one at right angles to them passing through the interaural line. The instrument is so constructed that it is possible to place the tip of a fine electrode at any desired position with reference to this zero point, for example, at a point 7 mm. to the right, 4 mm. rostral and 2 mm. dorsal to the zero point.

In order to use the instrument it is necessary to know the location of the point which is to be stimulated or to be destroyed by electrolysis, with reference to the rectilinear coordinates of the instrument. For this purpose we have prepared cats' brains in the following manner:

With the instrument in position on the head the cat was injected with 10% formalin. After the brain was thoroughly hardened in situ the skull was removed in certain small restricted areas. Through these openings perfectly straight 18 gauge copper wires were inserted with the aid of the instrument. Three wires were inserted horizontally from behind through the cerebellum and tentorial notch to the rostral extremity of the brain.