Effects of unilateral section of the brain stem after mesial cerebral incision.

Recently, seven young, healthy chimpanzees were subjected to an experimental surgical procedure known as mesial cerebral incision.! This procedure consists of incision and separation of corpus callosum, lamina terminalis, floor of third ventricle, pineal region, pons, cerebellum, and medulla oblongata. Actually, it is staged in two parts. The first, a parasagittal craniotomy, provides access to corpus callosum, the third ventricle and its floor, as well as the pineal regions. In this first stage, the diaphragma sellae can be visualized if care is taken during the incision of the third ventricular floor (FIGURE 1). Some time after recovery, a suboccipital craniectomy is performed (FIGURE 2). This is the second stage of mesial cerebral incision and provides exposure of vermis and medulla, both of which are incised under direct vision (FIGURE 1). The pons must be separated indirectly through the rostral apex of the suboccipital craniectomy on the caudal part of the exposure provided in stage 1. The basilar artery can be felt pulsating at the lowest extremity of this incision. In all stages and parts of mesial cerebral incision, gentle blunt separation of white fibers with the Sachs dissector permits almost bloodless access to the various anatomical levels and structural areas. For example, the dissector is introduced into the full depth of the callosum through a small incision in the pia. Then it is drawn slowly backward (or forward) with pressure sufficient to separate the white fibers but not to tear blood vessels. The resistance of the latter is greater, and, with some practice, vessels can be spared in a series of fenestrations while the surrounding fibers are clearly and cleanly cut. Five more chimpanzees have undergone this procedure since the original report, bringing the total to twelve. The techniques of mesial cerebral incision have been extended by microdissection in the macaque so as to expose the contents of the third ventricle and thus expose the anterior and mesial parts of the thalamus.2 Thereafter, the rostral thalamus was sucked out by micropipette. This local unilateral microexcision resulted in a chronic epileptic animal, which has been described in detail by Milhorat and Baldwin.2,3 The primates subjected to mesial cerebral incision recover promptly and can return to usual cage or run activity within ten days after either operative stage. These chimpanzees have exhibited normal bilateral synergistic motor activity except in two cases, in which a monoparesis of right or left upper extremity occurred. Five such animals that did not show any neurological deficit after mesial cerebral incision were also subjected to unilateral incisions of the brain stem on right or left sides. The incision was made just below the inferior colliculus and calculated so as to extend to, but not across, the midline, while sparing the cerebral peduncle. In order to accomplish this brain-stem section, the animal was carefully anesthetized with endotracheal halothane and, after shaving of the head, was postured for a left (or right) occipitoparietal craniotomy. Following aseptic preparation, the scalp was infiltrated with 111,500 nupercaine solution containing