NORTH SEA INVESTIGATIONS. 187 
regulating these vertical movements,—in affording an outlet to the 
fluid of the orbital cavity when the eye is withdrawn by the con- 
traction of the recti and oblique muscles, and in protracting it when 
these muscles are relaxed by driving back the fluid into the cavity. 
Its action appears to be almost if not wholly involuntary ; but 
though one might expect to find a connection with the sympathetic 
system, I have not succeeded in doing so. 
According to my observations, limited to Pleuronectes, Solea, and 
Rhombus, the power of protraction is possessed in degrees corre- 
sponding to the order in which I have named these three genera, 
and the recessus is developed in the same degree. That the organ 
of the upper eye is always the larger is explained by the condition of 
the orbital cavity. That of the lower eye is in part bounded only by 
loose skin, which allows some play to the elasticity of the undiffer- 
entiated part of the membranous wall when the eye muscles are 
contracted, and is also sensible to the pressure of the external 
element when the muscles are relaxed. ‘The upper orbital cavity, 
however, bounded as it is by the skull and firm dorsal muscles, is 
wholly dependent on its accessory organ as an outlet for the fluid. 
The eye, in fact, could not possibly be retracted if no recessus were 
present. The great inequality in the accessory organs of the halibut, 
which exhibits the minimum development of the lower and the 
maximum of the upper recessus, is probably due to the convexity of 
the head, whereby the lower eye is set in a higher plane than the 
upper, which is also much nearer the dorsal ridge than in the other 
species studied. Hence it is evident that the pressure required to 
raise both eyes to the same level must be very unequal, but I have 
no means of saying from actual observation that the eyes are 
protracted to the same plane in life. 
A pouch-like diverticulum of the membranous wall of the orbital 
cavity was discovered many years ago by Dr. Giinther in Chorino- 
chdismus dentex, one of the Gobiesocide. So far as I can tell 
from examination of the organ in a specimen that has been 3 
very long time in alcohol, it corresponds well enough to the recessue 
of flat-fishes. Dr. Giinther conjectured that it might represent a 
saccus lacrymalis; but though I am loath to speculate on this subject 
without a knowledge of its development, I am bound to say that the 
relationships of the organ to the orbital cavity do not appear to point 
to this homology. The organ lies between the eye and the maxilla, 
but the different position of the recessus of the lower eye in a flat- 
fish is only such as would be brought about by the rotation which 
we know to take place in the eye of a metamorphosing Pleuronectid 
larva, A difference in the levels of the eye in the British Museum 
series of C. dentex suggests that the function of the organ is similar 
