§ 2.123 ENDOCRINE GLANDS FROM ECTODERMAL EPITHELIUM 41 



vary greatly in different animals (Fig. 2-11). The pars distalis 

 becomes increasingly important in land animals, and especially in 

 mammals, whereas the pars intermedia decreases in size and may 

 even be wholly lacking, as in the chick and whale. 



The blood supply of the pituitary also shows important differ- 

 ences in different classes of vertebrates (Fig. 2-11). In fish, 

 hypophysial arteries from the internal carotids break up into a 

 vascular plexus, which penetrates the whole organ and eventually 

 drains into venous sinuses under the skull. (The saccus vasculosus 

 may have a separate supply; but it is not part of the endocrine 

 gland.) In Urodela, the arterioles pass first to the pars tuberalis 

 and thence to the capillary plexus of the pars distalis and pars 

 nervosa. The pars intermedia hardly has any share in the blood 

 supply. 



In the Anura and amniotes, the plexus in the pars tuberalis 

 penetrates into the median eminence and then the vessels join up 

 to form a number of portal veins which break up into a secondary 

 venous plexus in the pars distalis (Fig. 2-11 g and h). It might be 

 thought that this portal system was less important in mammals 

 than in lower forms, since in them alone the pars distalis acquires 

 a direct arterial blood supply from the internal carotids (Fig. 

 2-12); but it may allow the passage of CRF (§ 4.323). In all land 

 animals the newly developed neural lobe of their neurohypophysis 

 also acquires an independent arterial supply from the same source. 

 These vessels from the neural lobe, together with all those from 

 the pars distalis, drain into the same venous sinus and eventually 

 join the internal jugular veins. 



Nerve axons to the pituitary fall into two distinct categories: 

 those of neurosecretory cells, and those of sympathetic nerves. 

 The neurosecretory cells are confined to the neurohypophysis, and 

 most of them end in the neural lobe; but some of them make 

 contact with the primary venous plexus in the median eminence. 

 In this way, secretions from the latter can pass into the portal 

 circulation and so to the pars distalis. How much they do so, and 

 whether they affect the rates of hormone secretion from the adeno- 

 hypophysis, is still a matter for discussion. 



The vasomotor fibres of the sympathetic nerves follow the 

 course of the hypophysial arteries (Fig. 2-12); they can therefore 



