66 G. W. Harris 



A primary plexus of vessels, formed by a multitude of capillary loops or 

 twisted skeins of capillaries, is situated in the median eminence of the tuber 

 cinercum. This plexus drains into wide vascular trunks which pass down the 

 pituitary stalk and break up to distribute blood into the sinusoids of the 

 anterior pituitary. The system is supplied with blood by small arterial twigs, 

 from the internal carotid arteries or Circle of Willis, which penetrate the 

 pars tuberalis and median eminence. The portal vessels form a constant link 

 between the median eminence and anterior pituitary in all vertebrates 

 investigated from amphibians to man. Analogous vessels are found in cyclo- 

 stomes and fishes. Microscopic examination in living amphibians, rats, dogs 

 and mice has established the direction of blood flow as being from the tuber 

 cinereum to the pituitary. From the anatomical point of view these vessels 

 form the only direct and constant system linking the central nervous system 

 and adenohypophysis. 



Experimental data confirm the importance of the hypophysial portal 

 vessels for normal anterior pituitary function : 



(a) As mentioned above, Markee, Sawyer and Hollinshead (65) and Harris 

 (44) found electrical stimulation of the tuber cinereum effective in evoking 

 ovulation in the rabbit, though similar stimuli applied to the pituitary gland 

 did not cause ovulation. These data are compatible with the view that the 

 hypothalamus controls the adenohypophysis by humoral means. 



(b) Pituitary stalk section is followed by very varied results so far as anterior 

 pituitary activity is concerned. The extensive literature on this topic has been 

 recently reviewed (20). It was found first in rats (45) and later in other forms — 

 the duck (4), ferret (18), rabbit (35) and Triturus cristatus (71) — that the return 

 of normal, or near normal, levels of anterior pituitary function after operation 

 occurs in those animals in which regeneration of the hypophysial portal 

 vessels takes place across the site of stalk section. If regeneration is prevented 

 by the placement of a barrier between the hypothalamus and pituitary, FSH 

 and LH secretion ceases. The results of Fortier, Harris and McDonald (35) 

 may be taken as an example. These workers cut the pituitary stalk in thirty- 

 two rabbits. In twenty-two the stalk was severed and a paper plate left in situ 

 between the cut ends. These animals all showed atrophic reproductive organs 

 when killed. In ten rabbits the stalk was cut, a paper plate inserted between 

 the cut ends but immediately removed (Fig. 5). Six of these animals showed 

 marked portal vessel regeneration and had ovarian weights not significantly 

 different from the normal (two of them accepted the male and ovulated in 

 the normal way). 



(c) Transplantation of the anterior pituitary gland to a site in the body 

 remote from the sella turcica results in a marked loss of anterior pituitary 

 function, though if the tissue is placed in the subarachnoid space beneath 

 the median eminence it becomes vascularized by the hypophysial portal 

 vessels and apparently normal anterior pituitary function returns (48, 73, 81). 



