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relaxation of the wall of the recessus and the contraction 
of the eye muscles effect the retraction of the eye-ball. If 
the eye in a fresh or living fish be gently pressed fluid 
passes into the recessus from the orbit and the former can 
be seen to swell. 
The eye muscles (text-fig. 5) are the usual six pairs, 
two pairs of oblique muscles, and four pairs of straight 
muscles or recti. ; 
The superior oblique muscles (O6/. Swp.).—Both right 
and left muscles take origin on the left side of the head. 
It will be remembered that the left prefrontal sends back- 
wards a long process which fits into a groove on the dorsal 
surface of the parasphenoid. At the beginning of this 
process the prefrontal is deeply grooved for the reception 
of the ethmoid cartilage, the groove being formed by two 
horizontal bony lamine, and on the upper of these laminze 
and on the adjacent anterior concave surface of the pre- 
frontal the oblique muscles take origin. The left superior 
muscle passes upwards and slightly backwards along the 
left side of the interorbital septum to its insertion on the 
eye-ball. The right muscle, however, immediately passes 
through the ethmoidal fenestra and so through the inter- 
orbital septum to the right orbit. It then passes upwards 
and backwards, diverging slightly from the left muscle, 
along the right side of the interorbital septum to its inser- 
tion in the right eye-ball. 
The insertion of these muscles is peculiar. Hach 
splits up near its distal extremity into anterior and 
posterior slips. The anterior slips (0b/. sup.), which are 
the larger of the two, have wide insertions on the superior 
and anterior surfaces of the eye-balls. The posterior slips 
(o. s. a.) curve round the internal and superior surfaces of 
the eye-balls, crossing over the insertions of the superior 
