GAS DISEASE IN FISHES. 367 
logic and partly pathologic. Likewise among a brood of fry suffer- 
ing constant losses from supersaturated water many 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, but the 
previous gas disease process seems to favor its development. All 
these cases, however, are to be carefully distinguished from the gase- 
ous exophthalmia, directly 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 really passed through the blood of the fish 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 probably chiefly 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 beginning is not supersaturated 
more immediate reaction by the ordinary laws of osmosis. 
, Suggests a 
THE CAISSON DISEASE ANALOGY. 
The gas disease of fishes is paralleled in man by an affection in which, 
so far as it holds, the analogy is striking. The compressed-air disease 
‘caisson illness, diver’s palsy, ete.—is caused by an increase of air pres- 
sure; with divers, by the weight of the water above; in the caisson, 
by the compression necessary to keep the water out. In so far as the 
subject sustains an extraordinary pressure the analogy 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 m the results the two cases are much alike. The caisson 
disease has long been known and has a considerable medical literature, 
but some uncertainty seems to have existed as to the immediate cause 
of the symptoms and of death. The mechanical effect of the compres- 
sion was supposed to be important, but recently the influence of this 
factor has been pronounced nil. Bubbles of gas in the blood vessels 
