SECRETION 63 



activity of the gas gland. The form of the gland varies in 

 different fishes. Typically it is a local proliferation of the 

 lining epithelium of the bladder, elsewhere composed of 

 squamous cells. The cells of the glandular region are 

 columnar where in contact with the endothelial lining of a 

 peculiar arrangement of blood-vessels called the rete mirabile. 

 This consists of a closely packed bunch of fine capillaries ; 

 its essential feature is the juxtaposition and intermingling of 

 capillaries carrying blood in both directions. It has been 

 shown by Bohr that the gas gland, like the salivary gland 

 of the miammal, is under nervous control ; and Dreser states 

 that oxygen secretion can be induced by the action of pilo- 

 carpine which provokes activity of salivary and skin glands. 

 In appearance the gas gland is a bright red diffuse mass typi- 

 cally situated on the ventral wall of the bladder. Little is 

 known of its intimate mechanism. Artificial activation of 

 the gland can be achieved by attaching a weight to the fish. 

 A load of about five per cent, of the body weight is convenient 

 for the purpose. In experiments upon the Pollack, Wood- 

 land found that when the weight is first attached, the fish sinks 

 immediately to the bottom of the tank. Soon it begins to swim 

 upwards by active movements. After about twelve hours 

 or rather less it regains, owing presumably to the activity of 

 the gas gland, its normal quiescence completely. It is able 

 to float easily near the surface without the aid of caudal move- 

 ments. Exposure of the viscera at this stage reveals the 

 bladder in a distended condition. If instead the weight is 

 removed and the fish allowed to resume its former state, it 

 floats at first to the surface ; and has to swim downwards 

 vigorously in order to keep away from the surface. 



Further Reading 



Swale Vincent. An Introduction to the Study of Secretion. Arnold. 



Bernstein. Elektro biologic. Vieweg. 



CusHNY. The Secretion of Urine. Longmans, Green. 



Newton Har\^y. The Nature of Animal I>ight. Lippincott. 



Calmette. Venoms. Bale & Danielsson. 



