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GAS DISEASE IN FISHES. 365 
quently to be seen within the sac. Even in the Woods Hole water 
some few fishes, and many in the spring water of a lesser supersatura- 
tion, died without evidence of sufficient internal gas to produce an 
effectual embolism or enough apparent mechanical disturbance to 
account for death. Yet these fishes no doubt died of the excess of 
air. It is possible that in these cases there were internal lesions that 
escaped observation, minute emboli of gas, for instance, in the vessels 
of the brain, onan in a number of brains eed no gas had 
reached Tee vessels. It seems probable that the metabolic func- 
tional disturbance due to the abnormal osmosis is itself suflicient to 
cause death without apparent gas symptoms. 
EXOPHTHALMIA OR “POP-EYE.” 
Though not necessarily always occurring in all cases of supersat- 
urated water, this affection is so prominent among symptoms of gas 
disease as to deserve special consideration. It is not an infallible 
sign of supersaturation. As ‘‘pop-eye” or ‘‘frog-eye” it has long 
been familiar to fish culturists, and these terms are vernacular for any 
protrusion of the eye from its orbit, whatever may be the essential 
cause. It is not a disease, but a symptom, the expression of any one 
of a variety of causes or underlying conditions. Inflammation, from 
a wound or other irritation within the orbital cavity, may cause a 
swelling of the tissues which pushes the eyeball outward from its 
position. Specimens of this sort are not very common in shallow 
natural waters. One specimen, a butter-fish (Peprilus triacanthus), 
which apparently falls in this class, was taken from the trap nets at 
Woods Hole August 3, 1903, and examined immediately by the writers. 
It showed a moderate exophthalmia on each side. The globus was 
still lenticular in shape, and on dissection under water no sign of gas 
was detected. The brain and optic nerves appeared normal. If there 
had been a traumatic injury evidence of it had disappeared. The 
inflammation was not pronounced, and while an exudate behind the 
eye was, in part at least, the immediate means of the displacement, 
the primary cause can not be given. Externally the condition simu- 
lated strongly that. caused by supersaturation, to which in this case it 
could not possibly have been due. 
Mechanical injuries alone, as a sudden blow upon the head, may 
produce an immediate protrusion of the eyeball (Hofer, 1904, p. 292). 
Among the menhaden which died from the epidemic prevailing dur- 
ing the summer of 1904 in Narragansett Bay there were many cases 
of pop-eye, due, no doubt, to the injuries received during the peculiar 
death struggles characteristic of the disease. In some cases of pop- 
eye, where gas is plainly present and responsible for the displacement, 
it is possible that some other cause than supersaturation with air may 
be concerned, though none such is definitely known to the writers. 
In the great majority of cases where gas is present the cause will be 
