GAS DISEASE IN" FISHES. 367 



logic and parti}' pathologic. Likewise among a brood of fry suffer- 

 ing constant losses from supersaturated water man}^ of the dead will 

 be found with a greater or less, sometimes an extreme, exophthalmia 

 without the presence of gas. It is a post-mortem occurrence, })ut the 

 previous gas disease process seems to favor its development. All 

 these cases, however, are to be carefully distinguished from the gase- 

 ous exophthalmia, directl}' a symptom of the gas disease. 



The source of the gas behind the eye must be taken to be the blood. 

 Its position appears to make it impossible that it be derived directly 

 from the water. The blisters of gas which form upon the exterior of 

 the body and fins seem explainable as derived from either source, and 

 whether this gas has realh^ passed through the blood of the lish or 

 come through the permeable integument directly from the supersatu- 

 rated water can not at present be stated, but the evidence is somewhat 

 in favor of the latter view. It is probal)ly chiefl}" in the large veins 

 that the precipitation of the embolic gas from the blood occurs. The 

 supersaturating gas is acquired at the gills, subsequent to which 

 there is a fall of blood pressure. These facts make it probable that 

 the peripheral circulation is supersaturated, and that an essential con- 

 dition for the precipitation of gas at the periphery is supplied, though 

 all the causes which combine to bring the dissolved gas in the blood 

 of the capillaries free within the tissues are not clear. On the other 

 hand, the presence of supersaturated water on one side of the very 

 membranous covering of the fins, and on the other side tissues bathed 

 in a lymph, which at the beg-inning is not supersaturated, suggests a 

 more immediate reaction by the ordinary laws of osmosis. 



THE CAISSON DISEASE ANALOGY. 



The gas disease of fishes is paralleled in man b}- an affection in which, 

 so far as it holds, the analogy is striking. The compressed-air disease — 

 caisson illness, diver's palsy, etc. — is caused by an increase of air pres- 

 sure; with divers, by the weight of the water above; in the caisson, 

 b}'^ the compression necessary to keep the water out. In so far as the 

 subject sustains an extraordinary pressure the analog}^ does not hold, 

 for the gas disease involves no necessary increase of pressure upon the 

 fishes themselves. But the osmotic process of gases passing into the 

 blood through the lung membranes, under compression, must be inten- 

 sified according to the height of the pressure, as it is through the gill 

 membranes, in supersaturation, according- to the degree of the excess. 

 In this and in the results the two cases are much alike. The caisson 

 disease has long been known and has a considerable medical literature, 

 but some uncertaint}' seems to have existed as to the immediate cause 

 of the sj'mptoms and of death. The mechanical effect of the compres- 

 sion was supposed to bo important, luit recentlv the influence of this 

 factor has been pronounced nil. Bubbles of gas in the Mood vessels 



