In the diagnosis of lesions at the base of the brain, the history of the development of the malady, with particular reference to the chronologic sequence of the symptoms, may be of the utmost importance. The so-called brain stem includes the medulla oblongata, the pons and the midbrain—structures in which the third to the twelfth pairs of cerebral nerves have their nuclei of origin and of termination. A thorough knowledge of the anatomy (gross and microscopic) and the physiology (human and experimental) of these areas, as well as of the architecture and functions of their component mechanisms, is essential for finding the way to diagnostic conclusions that make any attempt at precision with regard either to the localization of the organic lesions or to the nature of the malady on which these lesions depend. There are now known a whole series of bulbar syndromes, pontile syndromes, peduncular syndromes and quadrigeminal