FUNCTIONS OF SWIMBLADDER OF FISHES. IO5 



The Swimbladder Gland. 



If one examines the structure of the swimbladder of certain 

 fishes, at one region or another he will see a glandular structure 

 consisting of a thickened layer of epithelial cells. Below this a 

 more or less conspicuous area, red in color, is visible. This 

 "red body" and its contiguous epithelium is known as the swim- 

 bladder gland. To Johannes Miiller (1842) may be ascribed 

 the discovery of its true glandular nature. The mass of red 

 blood vessels of the red body, which has for years attracted the 

 attention of anatomists, was discovered by Redi (1684) and 

 described by him. Since his time it has been known by the 

 name of the rete mirabile. 



Woodland (1911) made a very careful study of the structure 

 of swimbladder glands of many species of fishes. He describes 

 the gland as having two separate parts the glandular epithelium 

 or gas gland and the rete mirabile. The rete may or may not be 

 contiguous with the epithelium. The glandular epithelium may 

 be a single layer or many layered. Woodland found that the 

 arterioles passing to the gland break up into capillaries which 

 come into intimate contact with corresponding venous capil- 

 laries from the venules coming from it. There was a free 

 anastomosing of the capillaries on the arterial side with those on 

 the venous side. 



The capillaries were regarded for a long time as passive 

 endothelial filters with no muscle and therefore possessing no 

 independent contractility. Krogh (1920) has, however, proved 

 that the capillaries do possess the ability to contract. He has 

 also shown that when many substances are introduced into the 

 blood stream they have marked action on capillaries. Urethane 

 produces extreme dilatation of the capillaries and stasis. Oxygen 

 deficiency leads also to a marked capillary dilatation. Krogh 

 has adduced evidence, from an admirable series of experiments, 

 that the capillaries are normally kept tonically contracted by a 

 substance present in the blood stream. He believes that the 

 pituitary hormone is the substance that maintains the capillary 

 tone, and is present normally in concentration of about one part 

 in a hundred million or less. Krogh also maintains that oxygen, 



