186 NORTH SEA INVESTIGATIONS. 
branous sacculi. The condition first described, actually met with in 
the plaice, is sufficiently applicable to the other species mentioned. 
The recessus of the upper eye is in all cases a definite sac, 
always larger than its fellow of the lower eye, and always situate on 
the blind side of the skull, and to some extent overlaid by the 
superficial jaw muscles of that side. It communicates with the 
upper orbital cavity by a narrow neck, which passes through the 
large foramen between the pseudo-mesial process of Traquair and 
the parasphenoid. It is essentially similar in internal structure to 
the fully developed organ of the lower eye. Its shape depends 
merely upon its topographical relationships. 
In the fresh condition the recessus of either eye, when sac-like in 
form, is colourless and quite translucent, except for a slight milky 
opacity where the muscular bands are most numerous. It is, as may 
be supposed from the description of its internal structure, highly 
elastic, and is filled in life by a colourless fluid also present in the 
orbital cavity. It is rather richly suppled with blood-vessels, 
distributed on the inner face of the sac when such is present, and 
we may assume, as in the case of peritoneal and synovial cavities, 
that the fluid is deposited by mechanical filtration through the blood- 
vessels. In any case I have been unable to detect any special 
secretory apparatus in the epithelium of its walls. The only source 
of innervation which I have discovered is from the V. cranial, 
whether from sensory or motor roots I am as yet unaware. 
The fluid coagulates after a time into a milky white plasma, 
finely granular in microscopic preparations, and taking on a faint 
pink stain from borax carmine, but the recessus retains its elasticity 
for some considerable time after death. Thus, if the eye is pushed 
inwards, the fluid is forced into the recessus, which thus becomes 
enormously distended ; but as soon as the pressure is relaxed the 
recessus contracts, and the eye rises to its former level. 
It will be familiar to those who have observed the habits of flat- 
fish in aquaria that the eyes are normally protracted to a con- 
siderable degree. If the fish is frightened by placing some object 
near the eyes, the latter are immediately withdrawn into their 
sockets, while a corresponding inflation may be observed in the 
region of the recessus of the lower eye—the only one which can be 
seen. As soon as the object of terror is withdrawn the eyes rise 
again. Now there is no protracting apparatus in the muscles of the 
eye, and it is evident that the mere relaxation of the oblique and 
recti muscles would be quite insufficient to protract so heavy an 
organ as the eye to the degree actually attained. It is therefore 
evident that this result is attained by the pressure of the fluid 
contents of the orbital cavity, and that the recessus is functional in 
