The effect of absence of the corpus callosum on the position of the hippocampus and on the formation of Probst's bundle.

Complete or partial agenesis of the corpus callosum results in an alteration in anatomical developmcnt of the midline structures of the cerebral hemisphere. The influence of such maldevelopment on the position of the hippocampus and its connections will be considered in this study. In addition, agenesis of the corpus cal-losum may allow structures existing in normal brains to become more evident because they are not masked by the abundant fiber distribution of the corpus cal-losum. Various other cerebral anomalies may often be seen in agenesis of the corpus callosum but they will not be stressed in this study for good reviews of the general subject are available (Baker and The embryologic and phylogenetic development of the midline anatomy of the human brain will first be reviewed. In a study of marsupials, Tilney ('39) concluded , ".. . in spite of its extreme primi-tiveness it (the marsupial) follows, stage for stage, the fundamental principles which underlie the growth of the brain in all mammals." Tilney noted a precal-losal stage in all the mammals (including man) which he examined, that eventually develop a corpus callosum. In the marsupial, the hippocampal formation extends from the temporal pole to the olfactory peduncle and forms the me-dial margin of the cortex (Ariens Kappers, Huber and Crosby, '36, pp. 1412). This midline arc of hippocampus has the same cortical makeup and general consistency throughout its extent. Along its inner margin runs a longitudinal bundle of nerve fibers, the fimbria. Elliot Smith ('10) stressed that the position of the hippo-campus as the medial edge of the cerebral cortex is an important feature of the brain in the ancestors of mammals and is probably a fundamental feature in the development of all vertebrates. The dorsal com-missure of the marsupial contains only fibers of hippocampal origin (fig. 1). In the bat, Nycotophilus, which represents a transition from the acallosal to the callosal brain, the anterior part of the dorsal com-missure becomes invaded by pallial com-missural fibers thus forming an anterior extension of the dorsal commissure (Smith 1897a). These pallial decussating fibers do not develop to so great an extent as in higher animals but begin to influence the position of the hippocampus. As in these lower forms, studies of higher mammals have demonstrated that the fibers which eventually form the corpus callosum are first seen as a bundle known as the tape-turn covering the neocortical inner wall of the …