Olfactory Receptors in the Rabbit *

In ordinary histological preparations the olfactory epithelium of mammals appears to have a relatively uniform structure throughout its extent, and, as in Man and the higher Primates, it may be disposed quite evenly in flat sheets which are uncomplicated by folds of turbinal processes. In spite of the apparent homogeneity of its composition, the discriminative potentiality of the epithelium is remarkable, for it permits the distinction and recognition by their odoriferous properties of a seemingly indefinite number of chemical substances (many of which have a comparatively simple molecular structure). The olfactory receptors and their connexions with the olfactory centres of the brain thus present a particularly interesting problem for the study of the morphological basis of sensory discrimination. Presumably the receptors are in some manner capable of functioning as "analysers" which are differentially sensitive to diverse odoriferous substances, for on a priori consideration this seems to be the most likely basis for the discriminatory functions of the olfactory epithelium. Such an hypothesis is also consonant with the phenomenon of selective olfactory fatigue. In recent years direct evidence that the olfactory receptors are not all equally sensitive to specific odorous substances has accrued from the electrophysiological studies of Adrian,. Beidler and Tucker,5 and Ottoson"; for example, some receptors are reported to have a higher threshold of sensitivity for a particular chemical substance and other adjacent elements, a lower threshold. According to Adrian, also, these different types of receptor are not uniformly distributed over the olfactory epithelium, though there are some regions in which they are closely intermingled. This evidence that the olfactory receptors are not functionally homogeneous naturally raises the question whether they show any corresponding differentiation in their morphology and connexions. It was partly with these