GAS DISEASE IN FISHES. 365 



quently to be seen within the sac. Even in the Wood.s Hole water 

 some few fishes, and many in the spring- water of a lesser supersatura- 

 tion, died without evidence of sufficient internal ^as 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, though in a number of brains examined no gas had 

 reached their 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 necessaril}" alwa3's 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-ej'e" 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 s^niiptom, 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 e3^eball outward from its 

 position. Specimens of this sort are not very common in shallow 

 natural waters. One specimen, a butter-fish {PeprUiis 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 

 e3'e was, in part at least, the immediate means of the displacement, 

 the primary cause can not be given. Externall}' the condition simu- 

 lated strong!}" that caused b}^ 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 ej^eball (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 plainl}^ 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 



