The Ferrier Lecture - Inquiries into the anatomical basis of olfactory discrimination

Abstract

Sir David Ferrier, in whose memory I have the honour of giving this lecture, was particularly distinguished for his pioneer experimental studies on cerebral localization. This aspect of his work was stressed by Sherrington, who, in an obituary notice in 1928, wrote that ‘Ferrier placed the question of cerebral localization of function on an absolutely certain basis of experimental fact’. Much of Ferrier’s work in this field was summarized by him in the Croonian Lectures delivered before the Royal College of Physicians in 1890. In the fifth of these lectures he dealt with the evidence for the localization of the ‘olfactory centre’, and it is interesting to note that, while he adduced good evidence for the olfactory functions of those limited areas of the brain which, by modem technical methods, have indeed been established to be receptive areas for olfactory impulses, he expressed considerable doubt on the validity of the evidence adduced by comparative neurologists for including in the so-called ‘rhinencephalon' (i. e. ‘olfactory brain’) such widely distributed parts of the brain as the hippocampus and the cingulate convolution. He pointed out, for example, that the size of the hippocampus in different species of mammal bears no relation to the degree of elaboration of the sense of smell, and he quoted Sir William Turner’s opinion that, in spite of the trivial development of the olfactory apparatus in man, ‘the human hippocampus is absolutely larger than in mammals generally’. However, because of their relationships and apparent connexions (so far as the latter were inferred from the somewhat crude techniques then available), the hippocampus and cingulate convolution, together with quite a number of other related structures, came to be included by comparative anatomists in the comprehensive term ‘rhinencephalon’, and for many years these were accepted as essential components of the cerebral mechanism of olfaction. It is only in recent years that this error has come to be recognized, and it is interesting to consider why the detailed study and elucidation of olfactory pathways by experimental studies have lagged so far behind those of other sensory mechanisms in the brain. One reason, no doubt, is the relative inaccessibility to experimental investigation of the olfactory pathways, for, being situated in the basal region of the cerebral hemisphere, they are not very easily approached by ordinary surgical procedures. Another reason is the difficulty (which Ferrier himself found) of determining in experimental animals the effects of local lesions on the ability to discriminate olfactory stimuli. But during the last fifteen years or so, attention has become increasingly focused on the nervous mechanism of olfaction, and as a result we now have a good deal more information about it. Undoubtedly this has been due in large part to Adrian’s application of electrophysiological methods to the study of the sense of smell, and, in particular, his analysis of the electrical activity of the olfactory bulb evoked by various kinds of odour. But the renewal of anatomical studies of the olfactory pathways has also been partly stimulated by the growing realization that the evidence for the essentially olfactory nature of the greater part of the ‘rhinencephalon’ was in fact very unsatisfactory indeed. Indeed, it was this which made my colleagues and myself recognize the importance of clarifying the precise details of the olfactory pathways, if only because of a suspicion that the assumed olfactory function of the hippocampus and related structures may have long been responsible for their neglect in the study of cerebral functions in general. The results of our experimental investigations of the olfactory pathways made it clear that, in fact, there is no anatomical evidence that the hippocampus and related structures are primarily concerned with olfactory functions, at any rate in the mammalian brain, and this has been confirmed by a number of subsequent studies. But, as so commonly happens in an investigation undertaken with one particular objective in view, our studies led to other observations which seem to be of considerable interest in themselves, particularly because of their relevance to the problem of olfactory discrimination.