The olfactory organ in man appears in a rudimentary condition as compared with its highly developed state in certain macrosmatic animals. To comprehend the immense change that has taken place in the smell apparatus, together with the development of the higher brain, and which has attained its climax in man (of those animals still possessing a sense of smell), it is necessary to be acquainted with the anatomy of the limbic lobe in macrosmatic animals, and to follow the transitions occurring until the high mammal type of brain be reached. Moreover, in this process is to be found the only means of discerning in the human cerebrum the rudiments of an important organ and of allocating its present site of functional activity. The researches of Kölliker, and, still later, of Elliot Smith, have largely removed the confusion existing in respect to the true limbic lobe, and the definition of this and its constituent parts may now be safely attempted. The extensive and hardly yet completed work of the latter author, contained in a series of valuable contributions to the Journal of Anatomy and Physiology, and in the Transactions of the Linnean Society, N.S.W., has been used by the present writer in giving the following sketch of the constitution of the limbic lobe and the cerebral commissures.