202 THE SWIM-BLADDER OE FISHES 



foniidation for an idea somewhat prevalent, that air or gas is 

 forced from the gullet of the fish through the pneumatic duct 

 into the swim-bladder. In those fish without a duct (Physoclisti) 

 that is impossible, and in them as well as in Physostomes, the 

 gas is evidently secreted by the vascular walls, the retia 

 mirahilia of the organ. The varying proportions of the gaseous 

 elements named, seem to show that no very important function 

 is subserved by them. Nitrogen, which in the animal organism 

 is excreted largely as urinary and f8ecal waste, has been found 

 to be absorbed under two peculiar conditions, viz. : when an 

 animal is in a state of inanition, and when an animal changes 

 its food and is accustoming its system to new forms of nutriment, 

 a fact of singular interest, to which further allusion will be 

 made on a subsequent page. The secretion, on the one liand, 

 of oxygen or, on the other hand, of nitrogen may depend upon 

 the special chemical conditions prevailing in the water being 

 breathed by fish. Of the purely chemical causes which control 

 the appearance and movements of fishes, one of the principal 

 has been found to be the abundance or scarcity of oxygen 

 minp-led with the sea water. The absence of herrings from the 

 Arctic seas has been frequently commented upon. The minute 

 crustacean life which is so attractive, and so essential, it may 

 be added, to the vast schools of herring, is extremely rich in 

 the cold northern waters, yet herring do not appear to resort 

 to those regions, whereas on both sides of the Atlantic the 

 waters, adjacent to this continent and to the British Islands 

 and the European continent, abound with herring. The Atlantic 

 is more richly oxygenated than the Arctic seas, and tliis com- 

 parative lack of oxygen is no doubt the main factor in deterring 

 the herring from migrating thither. Experiment has clearly 

 demonstrated the dependence upon temperature of the absorbtive 

 power of sea water. Barometric pressure too is important in 

 determining the amount of atmospheric air absorbed, and as 

 this air loses its oxygen far more rapidly than its nitrogen in 

 its descending passage to deeper strata of water, these deeper 



