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A study of the recent neuromorphological, neurophysiological and neuroethological literature, and data from the current research in our own laboratory have led us to a new classification of entities in the mammalian neuraxis. This classification comprises the core and the median and lateral paracores. The core of the neuraxis may be considered as a caudally extended limbic system. It extends throughout the central nervous system and, as its name implies, most of it is situated close to the ventricular cavity. This entity is characterized by the presence of (1) numerous diffuse grisea, (2) enormous amounts of thin, unmyelinated, varicose axons, many of which are arranged in diffuse fibre systems, (3) large numbers of different neuromediators, particularly neuropeptides, and (4) large numbers of neurons which concentrate estrogen and androgen hormones. Ethophysiological studies have shown that the core region contains numerous loci from which on stimulation quite characteristic behavioral patterns, like eating, drinking, fear, attack, reproductive behavior etc., can be elicited. The core region appears to be involved most directly in the organization of behavior and is of paramount importance for the regulation of processes aimed at the survival of the individual (organism) and of the species. The median and lateral paracores represent extensions of the core at the level of the brain stem. The median paracore includes the raphe nuclei, whereas the (bilateral) lateral paracore is constituted by a ventrolaterally extending lamella of tissue. Both paracores contain sets of monoaminergic cells giving rise to networks of fibres that pervade virtually all grisea of the neuraxis, i.e. the serotoninergic neurons in the median paracore and the catecholaminergic cells in the lateral paracore. The lateral paracore contains a series of grisea, including the substantia nigra, the ventral tegmental area, the nucleus reticularis parvocellularis, the tegmental pedunculopontine nucleus and the catecholaminergic cell groups A1, A2, A5, A7 and C1 and C2. It harbours a large bundle of loosely arranged, thin fibres, which forms a direct caudal continuation of the hypothalamic medial forebrain bundle. This lateral paracore bundle contains numerous catecholaminergic and peptidergic fibres. Three typical core centres, viz. the nucleus centralis amygdalae, the bed nucleus of the stria terminalis and the lateral hypothalamic area contribute substantially to this bundle. The lateral paracore contains, just like the core region, a large number of functionally defined centres related to integrated somatomotor and visceromotor responses. It is postulated that non-synaptic interneuron