Structures lying on the surface of the brain are easily reached and can be stimulated or destroyed without damage to other parts. But difficulty of access has retarded investigation of the functional significance of deeply placed nuclear masses and fiber tracts. To introduce precision into the study of these hidden parts of the brain Horsley and Clarke developed their stereotaxic instrument in 1908.
This instrument consists of a rigid frame which can be firmly fixed to the animal's head by clamps resting on the lower margins of the orbits and plugs in the external auditory meatuses, with bars graduated in millimeters and so arranged that a needle carrier can be brought to any desired point over the brain and the needle then inserted to any desired depth, the position of its point being accurately determined in terms of three rectilinear coordinates with reference to a fixed zero point. For stimulation
RANSON SW, KABAT H, MAGOUN HW. AUTONOMIC RESPONSES TO ELECTRICAL STIMULATION OF HYPOTHALAMUS, PREOPTIC REGION AND SEPTUM. Arch NeurPsych. 1935;33(3):467-477. doi:10.1001/archneurpsyc.1935.02250150017002