The Pituitary Stalk and Ovulation 67 



It is clear then that a normal level of function of the anterior pituitary 

 depends on its vascularization by the hypophysial portal vessels and that the 

 central nervous system through the hypothalamus exerts an influence over 

 the gland through the mediation of this vascular system. It is probable that 

 nerve fibers from the hypothalamus liberate some humoral substance(s) into 

 the capillaries of the primary plexus in the median eminence and that this 

 substance is carried by the portal vessels to excite or inhibit the cells of the 

 adenohypophysis. 



Many important investigations have now been undertaken in an attempt to 

 identify such humoral agents. Most of this work has however been concerned 

 with the regulation of secretion of the adrenocorticotropic hormone (ACTH); 

 a fact which has added to the difficulties of the problem, since the discharge of 

 ACTH is evoked so easily by so many and varied stimuli. The identification 

 of any particular substance as a physiological humoral agent involved in 

 anterior pituitary control, and indeed the neurohumoral view as a whole, 

 will only be established if it is possible to ". . . firstly identify a particular 

 substance which exerts a direct action on anterior pituitary cells ; secondly, 

 to show this substance is present in the blood in the hypophysial portal vessels 

 in greater amount than in systemic blood; thirdly, to show that the concen- 

 tration of this substance in the blood of the hypophysial portal vessels varies 

 according to electrical or reflex activation of hypothalamic nerve tracts ; and 

 fourthly, to demonstrate that the activity of the adenohypophysis is correlated 

 with this varying concentration". (Harris, 47.) Data such as this are not yet 

 available for any of the substances postulated for the role of a transmitter 

 agent. Various reports have suggested that the substance involved in gonaiio- 

 tropin release may be (a) adrenergic in nature (64, see, however, 19), {b) 

 intermedin (60), (c) oxytocic hormone (6, 72), {d) posterior pituitary poly- 

 peptides (70). 



Work at the Institute of Psychiatry, London, has recently been undertaken 

 to see the effect of infusing various brain extracts into the adenohypophysis 

 of rabbits on the release of thyrotropic hormone (TSH) and LH (H. J. 

 Campbell, G. Feuer, J. Garcia and G. W. Harris, unpublished). Careful 

 consideration was first paid to two points — the anatomical limits of the 

 region which might be expected to contain active material and the methods 

 to be used for applying this material with minimal concurrent trauma to 

 anterior pituitary tissue. 



The median eminence of the tuber cinereum is the region of the infundi- 

 bulum which contains the primary plexus of the portal vessels, surrounded by 

 a wealth of nerve fibers (Fig. 6). Extracts of the median eminence might then 

 be expected to contain a greater concentration of any humoral transmitter 

 agent than surrounding structures. In the brain of the freshly-killed animal 

 the median eminence may be identified from adjacent hypothalamic tissue 

 as the pink and bulbous upper end of the pituitary stalk. If the brain is 



