The central olfactory connexions.

Following the studies of Adrian (1942), Rose & Woolsey (1943) and Fox, McKinley & Magoun (1944), all of whom used the evoked potential method, the central connexions of the olfactory bulb were investigated neurohistologically by several workers using the Glees method (Le Gros Clark & Meyer, 1947; Meyer & Allison, 1949; Allison, 1953a; Adey, 1953). Perhaps the most important contribution of these studies was to limit the projection of the olfactory bulb to the anterior olfactory nucleus, part of the olfactory tubercle, the pyriform cortex and certain of the amygdaloid nuclei, and to confirm that the entorhinal area and hippocampus are not in receipt of direct afferents from the olfactory bulb. More recent work, using the Nauta technique, has in general, substantiated these findings, but at the same time has raised several questions regarding the relationship of the anterior limb of the anterior commissure to this projection. For example, Lohman (1963) has presented considerable evidence to indicate that this bundle arises in the anterior olfactory nucleus rather than in the main or accessory olfactory bulbs. Other workers, including Cragg (1961 a), Sanders-Woudstra (1961) and Powell, Cowan & Raisman (1963) have similarly questioned the existence of a projection, through the anterior limb of the anterior commissure, to the central nucleus of the amygdala and its supposed continuation, the bed nucleus of the stria terminalis. In view of the general agreement that the pyriform cortex represents the major site of termination of the lateral olfactory tract it is perhaps surprising that so little is known of the efferent connexions of this area. Apart from the work of Lundberg (1960) who included this area in his study of the projection of the cerebral cortex upon the hypothalamus and of Cragg (1961 a) who showed a projection from the pyriform cortex to the entorhinal area, there does not appear to have been any systematic study of the efferent connexions of the pyriform cortex. Such a study is clearly called for in view of the widespread interest in the influence of olfaction upon reproductive mechanisms (cf. Parkes & Bruce, 1961) and affective behaviour (Green, Clemente & De Groot, 1957). Whether these effects are mediated through the projection to the hypothalamus of the stria terminalis, which Cajal (1911) considered to be an important central olfactory projection pathway and to arise from the pyriform cortex, is not known as this opinion appears to have been largely overlooked in recent discussions of olfactory connexions. Recent electrophysiological studies of the olfactory bulb following stimulation of the anterior commissure and the lateral olfactory tract (Kerr & Hagbarth, 1955; Green, Mancia & Von Baumgarten, 1962; Yamamoto, Yamamoto & Iwama, 1963; Phillips, Powell & Shepherd, 1963) have raised many problems of anatomical interest; for example, whether the axons of mitral and tufted cells have a differential