366 REPORT OF THE COMMISSIONER OF FISHERIES. 
found to be an excess of air, with spring waters usually an excess of 
the nitrogen of the air alone, and the location of the gas will be behind 
the eyeball. Some species of fishes are not susceptible to this symp- 
tom from supersaturated water, or at least it has not been observed in 
them. The anatomical structure and the degree of the excess seem to 
be the factors which control. Among marine fishes, the dog-fish 
(AMustelus canis) and other sharks, eels, puffers, sea-robins, the flat- 
fishes, and others do not develop typical cases, if any, while the scup, 
the king-fish (A/entictrrhus), the tautog, the cunner, the sea bass, and 
the butter-fish may exhibit it in various degrees. Of all these the 
scup (Stenotomus chrysops) shows it most readily and in extreme 
degree (Plate III). With a certain degree of excess not exactly 
known, but probably above 38 c¢. c. of nitrogen per liter, embolism 
becomes fatal before there is time for an accumulation of gas behind 
the eye. An excess of not over 2 or 3 c. c., and probably less, per 
liter is favorable to the development of the symptom, which may be 
taken to indicate a moderate excess of air. The eyeball is sometimes 
pushed almost completely out of the head (Gorham 1899, Plate 12). 
Without much displacement of the ball the conjunctiva may be raised 
and inflated into a balloon of gas projecting far out beyond the eye- 
ball (Plates I and II of this paper). 
Among fresh-water fishes salmonoids chiefly have been seen to be 
affected. The black sucker (Catostomus nigricans) showed a typical 
case at Erwin, while some cyprinoids (WVotropis galacturus and a 
Hybognathus) ander the same conditions died with the eyes normal. 
It is no doubt because not many fishes save the trout of artificial 
propagation have been observed in supersaturated fresh water that 
few fresh-water species are known to show the lesion. In brook and 
rainbow trout the pop-eye is seldom so extreme as that shown in the 
illustrations of the seup. The excess being slight, the symptom may 
grow very slowly and be present for months, or even years, impairing 
more or less the activities of the fish. Blindness frequently results, 
with accompanying increases of dark pigment inthe skin. The expos- 
ure of the eyeball makes it subject to injury, and it is sometimes bitten 
off by other fishes, or drops or sloughs away, leaving the socket empty. 
In trout fry past the sac stage a certain exophthalmia may develop 
after death if they remain in water, and the younger and smaller the 
fry the more quickly it appears. In general its development requires 
from twelve hours to three days. Evidently there is a physiological 
post-mortem accumulation of transudate behind the eye. There is ¢ 
pathologic exudate which occurs in trout fry suffering from anemia 
and this exudate may localize, sometimes in the abdominal cavity, 
‘ausing ascites, sometimes behind the eye, causing exophthalmia with- 
out gas. Fry having this form of anemia, though their eyes still be 
normal at death, more readily than healthy fry develop in water the 
post-mortem exophthalmia which in this case seems to be p»rtly physio- 
