igs 
external and inferior recti enter the orbit, and where the 
cavity of the latter is prolonged backwards for a littie 
distance round the recti. 
The recessus of the upper or left orbit has similar 
relations, but on account of the almost complete enclosure 
of the latter by bony structures it has a somewhat different 
position. It les on the eyeless side of the head external 
to the fenestra bounded by the left frontal, left prefrontal, 
parasphenoid and alisphenoid. It must therefore be dis- 
sected from that side and is easily exposed by simply 
removing the skin and a little surrounding connective 
tissue. It is (in a preserved fish of 22 inches long) a 
round flattened sac of about 2cm. in diameter, of similar 
appearance and structure to the organ of the right side. 
It also opens into the orbit, and on removing the left eye 
the large aperture is easily seen immediately posterior and 
slightly above the place of origin of the inferior oblique 
muscle. It is situated beneath the pseudomesial ridge 
and pierces the soft wall of the fenestra mentioned. 
We have stated that the eyeball can be protruded 
from the general surface of the head to a remarkable 
extent. Now while the eye muscles provide an effective 
apparatus for the retraction of the eye, there is apparently 
no muscular arrangement which can bring about protru- 
sion. This appears to be the function of the recessus. In 
life the cavities of the orbit and that of the recessus with 
which the latter is in free communication are filled by 
fluid which apparently originates by an infiltration of 
lymph from the capillaries outside the orbital wall. The 
wall of the recessus being markedly muscular, it follows 
that its contraction will expel the contained fluid into the 
orbit and press on the internal surface of the eye-ball. 
Since the skin of the head round the eye is loose it yields, 
and the eye is accordingly protruded. Conversely the 
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