The Structure of the Limbic System

Abstract

The term limbic system derives from the concept of a limbic lobe presented by the French anatomist Broca, in 1878. The word limbic refers to a border, fringe, or hem. Broca used the term limbic lobe to designate the brain tissue that surrounds the brain stem and that lies beneath the neocortical mantle. In a gross fashion, this term includes the cingulate and hippocampal gyri, as well as the isthmus that connects the two. It also includes the various gyri that surround the olfactory fibers running back from the olfactory bulb and stalk. Within this inner lobe of the brain, the structures are presumed to be organized into two layers. The tissue thought to be phyletically the oldest (the allocortex) makes up the inner ring. The outer limbic ring does not resemble either neocortex or allocortex based on study of its cellular structure. It is therefore called transitional cortex, or juxtallocortex (“next to the allocortex”). This approach is of some value as a general conceptual scheme, but it fails to recognize that the inner ring is not a uniform band of tissue. In fact, it is a discrete set of structures, which can be identified more or less readily by gross dissection, and it is these structural subunits of the inner ring of the limbic lobe on which our attention will be centered during the remainder of this book.