THE REPTILIAN CEREBRAL HEMISPHERES

Abstract

Summary1. The reptilian hemisphere has been studied mainly from a morphological point of view and for the most part with techniques which are capable of providing no more than a preliminary survey of the distribution of the perikarya of neurones and of the topography of the main fibre systems.2. The hemisphere is small, and almost completely smooth. Its most characteristic feature is a strio‐amygdaloid complex of great relative size, and in particular the differentiation of the dorsal part of the striatum.3. Olfactory structures are very well developed in Chelonia which in this respect resemble amphibians and primitive mammals. They are proportionately smaller in other reptilian orders, and some lizards are microsmatic. The pattern of the olfactory connexions resembles that of amphibians and mammals. A characteristic feature of many of the Squamata is a high degree of development of the accessory olfactory bulb and of the amygdaloid nucleus which receives fibres from it.4. A cortex showing some of the characteristics of the mammalian cerebral cortex is present in all reptiles, and hippocampal, dorsal and piriform areas are always recognizable. The dorsal area occupies a region where the mammalian neocortex must have been formed. A thickening in its anterolateral border (the ‘primordium neopallii’ of some authors) may represent a phylogenetic stage in the development of the mammalian neocortex. Equally valid arguments can be produced for regarding it as a reptilian specialization and for deriving the mammalian condition from the more generalized amphibian condition.5. Embryological investigations are purely descriptive and have shown little more than that the pattern of development of the reptilian hemisphere conforms in its general features to that of other tetrapod classes.6. Functional investigations indicate that the basal parts of the hemisphere may be of importance in effecting orientation between the animal and its environment, perhaps by facilitating neural mechanisms at lower levels and by integrating the elements of behaviour patterns. The results so far as the cortex is concerned are mainly negative; nothing comparable to the mammalian ‘motor area’ is present.7. The limitations of the techniques employed are discussed, and it is concluded that the main value of the information at present available is as a basis for systematic classification, and for application of experimental techniques necessary for the investigation of morphogenetic and functional problems.