144 BRAIN AND BODY OF FISH 



The capillaries from the acini converge to form three groups 

 from which large vessels arise, two lateral and one central, and these 

 unite towards the hilum to form one main trunk which passes 

 backwards and enters the substance of the lobus infundibuli. The 

 evidence as to the direction of the circulation in this system, which 

 appears to start from the fine capillaries, is as follows ; there can 

 be seen in the course of the branching nerve fibres of the pars 

 nervosa, as it enters the pockets of the pars intermedia, a number 

 of small roimd globules of colloid which are homogeneous and 

 refractile. It will also be noted that the hollows into which the 

 nerves dip are bordered by blood vessels, from which it seems that 

 the globules have escaped. These globules have been described 

 in the mammalian pituitary by Miss Una Fielding, who has looked 

 over my sections and given me much help in reference to the volum- 

 inous literature on the subject. She points out that the direction 

 of the blood flow can be gauged by their colloid accompaniment. 

 As colloid (whatever it may be) is formed in the pituitary, and not 

 in the brain, its association with vessels may be taken as incUcating 

 that the direction of the flow is certainly hypophysio-hypothalmic 

 (that is from the pituitary to the part of the brain known as the 

 hypothalamus). 



Sometimes the colloid globules occur in the lumen of the vessels 

 and sometimes in the inter-vascular tissue of the neural portion 

 of the stalk. Our very deflnite observation of the colloid material 

 in the network of capillaries that surround the acini seems to confirm 

 the suggestion of de Beer that products other than those of the 

 anterior lobe may be blood borne. 



It has been established by Hogben that the normal rhythm of 

 colour change in amphibia is controlled by the secretion of the 

 pituitary gland. He points out that " the pituitary gland is a speci- 

 fically vertebrate structure, from which no active substance has 

 been isolated in pure form. Extracts of the pituitary of mammals, 

 birds, reptiles, and fishes (teleostean and elasmobranch) have a 

 powerful excitatory action on the mammalian uterus and upon 

 mammary secretion. Extracts of all classes of Amniota, Amphibia, 

 and Teleostei exert a pressure diuretic action on the mammal. In 

 fact the posterior lobe of the mammal while the storehouse of 

 probably several substances of very great activity has not yet been 

 proved conclusively to have any functional significance." There is, 

 on the other hand, the clearest evidence for regarding the pituitary 

 secretion as the main factor in co-ordinating the pigmentary res- 

 ponses of amphibia to the changing conditions of its environment. 



The pigmentary changes in the eel when about to migrate are 



