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There are two networks of primary portal capillaries arising separately from the ventral surfaces of the anterior and posterior divisions of the median eminence (Fig. 5.5). Anterior and posterior portal veins may be enclosed as a bundle by the pars tuberalis of the adenohypophysis, flowing to secondary capillary plexuses and sinuses in the rostral and caudal lobes respectively (Fig. 5.5, e.g. Vitums et al. 1964, 1966; Duvernoy et al. 1969, 1970; Singh and Dominic 1975). It is assumed that this portal system delivers neurohormones from neurons in specific areas of the hypothalamus to discrete areas in the pars distalis of the adenohypophysis (Vitums et al. 1966), although as yet there is no actual physiological evidence that supports this hypothesis (Wingfield and Farner 1993).