106 F. G. HALL. 



carbon dioxide and probably all crystalloids pass through the 

 capillary walls by purely physical diffusion. 



It hardly seems necessary to go further into a discussion of the 

 many experiments that have been performed in this field. It is 

 apparent that when certain metabolic products are thrown into 

 the blood stream because of a heightened metabolism of a gland 

 an increased vascular dilatation ensues, producing a heightened 

 flow of blood to that gland. What application can be made of 

 this process in explaining the mechanism of gas transference in 

 the swimbladder of fishes? Before attempting to answer that 

 question it will perhaps be fitting to mention another means by 

 which the gland is able to regulate the supply of oxygen in 

 proportion to the needs. 



A study was made of the structure of the swimbladder gland 

 of the yellow perch by the writer. It is located on the ventral 

 side of the swimbladder wall near the anterior end. The rete 

 mirabile is seen as a group of small fan-shaped tufts formed by 

 an artery and a vein. A thickened glandular mass of epithelium 

 surrounds the "red body." Micro-photographs (Plate I) also 

 show the mass of capillaries and their terminations in the epithe- 

 lial cells of the gland. 



The blood supply is arterial, coming from either the aorta or 

 the coeliac axis, and in some fishes different portions receive 

 blood from both sources. In the walls of the bladder the arteries 

 break up into the networks of the rete mirabile. From the rete 

 the blood passes to the body veins postcardinal, hepatic, or 

 vertebral. It seems reasonable to conclude that this remarkable 

 distribution of the blood vessels furnishes an adequate means 

 of supplying the swimbladder with blood for the purpose of 

 secretion. 



The second provision the process by which the oxygen is 

 transferred from the capillary blood vessels to the swimbladder 

 is a more strictly physiological problem. It is evident that the 

 oxygen is first given to the cells of the rete mirabile and then 

 passes out the glandular cells into the swimbladder cavity. 

 This transference of oxygen from the red cell involves a chemical 

 breakdown of the oxy-hemoglobin in the first place, and, fol- 

 lowing this liberation of oxygen, a diffusion of gas occurs between 



