REPRODUCTIVE ENDOCRINOLOGY IN BIRDS 



1099 



dibular arteries supply the i)ituitary. They 

 supply a very dense plexus of capillaries 

 which covers the median eminence and sur- 

 face of the infundibular stem. The capil- 

 laries of this primary plexus fuse in the 

 central zone of the median eminence to form 

 larger vessels, the so-called portal vessels. 

 These vessels are often embedded in the 

 epithelium of the portal zone of the pars 

 tuberalis, but some vessels run directly into 

 the pars distalis. No other arteries supply 

 the pars distalis. In the pars distalis the por- 

 tal vessels send their blood through the 

 sinusoids and then drain into the sinus cav- 

 ernosus or flattened veins connecting with 

 it. In his extensive material Wingstrand 

 found no case in which an artery or its 

 branches penetrated the hypophyseal cap- 

 sule, and in only one pigeon, did he find an 

 artery going from the neural lobe to the 

 pars distalis. The walls of the sinus caverno- 

 sus are formed by the periosteum of the sella, 

 turcica, the capsule of the pituitary, and the 

 connective tissue septa and adventitia of 

 the carotid veins. Blood from the sinus 

 drains into the carotid veins. 



The neural lobe of Pica, Corvus, Columba, 

 Apus, and others is supplied by blood from 

 the inferior hypophyseal arteries, whereas 

 in passerine birds [Regidus, Panis, Ember- 

 iza, Riparia, Phylloscopiis) and the fowl the 

 blood supply comes from the infundibular 

 arteries (Wingstrand, 1951). The blood 

 drains into the sinus cavernosus from the 

 capillaries of the neural lobe tissue. 



In addition to this main blood supply of 

 the pituitary, small blood vessel connections 

 sometimes occur between the primary plexus 

 of the median eminence and the capillary 

 bed of the neural lobe, and occasionally 

 single blood vessels are found between the 

 hypothalamic vascular bed and the primary 

 capillary bed of the median eminence. The 

 rarity of such connections makes functional 

 connection between these various capillary 

 beds unlikely. This condition is different 

 from the considerable anastomosis between 

 the vascular beds of posterior lobe and pars 

 distalis in the rat (Landsmeer, 1951). 



The lack of vascular connections between 

 pars distalis and neurohypophysis plus the 

 presence of a layer of connective tissue be- 

 tween anterior and posterior lobes of the 



pituitary (Legait, 1959) makes the transport 

 of hormones from one lobe to the other ex- 

 tremely improbable. On the basis of the 

 blood supply of the avian pituitary, Wing- 

 strand (1951) concluded that the blood flows 

 from median eminence to portal vessels. 

 This conclusion has been confirmed by di- 

 rect observation of the blood flow in the 

 duck (Assenmacher, 1958). 



Studies of the pars distalis of the avian 

 pituitary by Drager (1945) and Green 

 (1951) showed that the nerve fibers never 

 penetrate the glandular part of the pars dis- 

 talis. Metuzals (1955) claimed that a few 

 nerve fibers penetrate into the pars distalis 

 of ducks, but the origin of the fibers could 

 not be established. Wingstrand (1951) 

 wrote, "The pars distalis also contains a few 

 scattered fibres in most preparations of pi- 

 geons and also in the geese and the ducks. 

 The fibres are, however, so rare that large 

 areas in a section must be examined before 

 a single fibre is found." He continued, "It 

 has been seen several times that fibres in 

 the pars distalis are continuous with the 

 autonomic fibre bundles in the capsule of 

 the organ, and the nervous character of the 

 fibres can therefore hardly be doubted. The 

 fibres in the pars distalis are, however, so 

 few that they cannot be able to influence 

 the function of the gland." 



The posterior pituitary is innervated by 

 the tractiis hypophyseus anterior, the trac- 

 tiis supra opticohypophyseus, the tractus tu- 

 berohypophyseus, and the tractus hypophy- 

 seus posterior, which originate in the lateral 

 and inferior hypothalamic nuclei, the nu- 

 cleus tuberis (Kuhlenbeck, 1937), and the 

 nucleus subdecussationis (Wingstrand, 

 1951). According to Green (1951), the me- 

 dial forebrain bundle may contribute fibers 

 to the tractus hypophyseus, but Wingstrand 

 ( 1951 ) is not as definite on this. The tubero- 

 hypophyseal tract shows some special adap- 

 tations, in that fibers form "loops" into the 

 glandular layer of the median eminence 

 where the blood supply is most prominent. 

 In this manner the blood vessels can trans- 

 port neurosecretory material to the pars dis- 

 talis and thus affect its function (Wing- 

 strand, 1951). A similar mechanism was 

 projiosed for some fine fibers of the tractus 

 hypophyseus anterior. Because the nucleus 



