and Ungulata) dwindling, which is equally shared by the human brain, in common with those of the other Simiidae, the Cercopithecidae, and the Cebidae.  But all the parts of the rhinencephalon, which are so distinct in macrosmatic mammals, can also be recognized in the human brain.  The small ellipsoidal olfactory bulb is moored, so to speak, on the cribriform plate of the ethmoid bone by the olfactory nerves; so that, as the place of attachment of the olfactory peduncle to the expanding cerebral hemisphere becomes removed (as a result of the forward extension of the hemisphere) progressively farther and farther backward, the peduncle becomes greatly stretched and elongated.  And, as this stretching involves the gray matter without lessening the number of nerve-fibres in the olfactory tract, the peduncle becomes practically what it is usually called—­i.e., the olfactory ‘tract.’  The tuberculum olfactorium becomes greatly reduced and at the same time flattened; so that it is not easy to draw a line of demarcation between it and the anterior perforated space.  The anterior rhinal fissure, which is present in the early human foetus, vanishes (almost, if not altogether) in the adult.  Part of the posterior rhinal fissure is always present in the ‘incisura temporalis,’ and sometimes, especially in some of the non-European races, the whole of the posterior rhinal fissure is retained in that typical form which we find in the anthropoid apes.” (G.  Elliot Smith, in Descriptive and Illustrated Catalogue of the Physiological Series of Comparative Anatomy Contained in the Museum of the Royal College of Surgeons of England, second edition, vol. ii.) A full statement of Elliot Smith’s investigations, with diagrams, is given by Bullen, Journal of Mental Science, July, 1899.  It may be added that the whole subject of the olfactory centres has been thoroughly studied by Elliot Smith, as well as by Edinger, Mayer, and C.L.  Herrick.  In the Journal of Comparative Neurology, edited by the last named, numerous discussions and summaries bearing on the subject will be found from 1896 onward.  Regarding the primitive sense-organs of smell in the various invertebrate groups some information will be found in A.B.  Griffiths’s Physiology of the Invertebrata, Chapter XI.

The predominance of the olfactory area in the nervous system of the vertebrates generally has inevitably involved intimate psychic associations between olfactory stimuli and the sexual impulse.  For most mammals not only are all sexual associations mainly olfactory, but the impressions received by this sense suffice to dominate all others.  An animal not only receives adequate sexual excitement from olfactory stimuli, but those stimuli often suffice to counterbalance all the evidence of the other senses.