50 



A strong flat muscle on each side wliicli arises from the lower end of the clavicle 

 and is inserted into the sides of the jugular bone. 



Small muscles between the upper and lower extremities of the branchial arches. 



The eye muscles in the flat fishes are of special interest on account of the peculiar 

 distortion of the eyes which these fishes exhibit. 



The eye muscles in the sole consist, as in symmetrical fishes, of four recti muscles 

 and two oblique. The recti pass obliquely forwards from the back of the orbit to the 

 outer surface of the eyeball, to which they are attached at equal distances ; in a 

 symmetrical fish, as in every other vertebrate, one is attached at the dorsal side, one at 

 the ventral, one to the anterior, and one to the posterior : these are respectively 

 named the superior and inferior, internal and external, recti. In the sole the superior 

 rectus is uppermost in the natural position of the fish, having followed the torsion of 

 the orbits without altering its position in relation to the interorbital processes of 

 the frontal bones. Similarly the internal rectus of each eye is next to the interorbital 

 septum. The superior and inferior recti of each eye are much thicker than the 

 internal and external. All the recti of both ej'es take their origin from the internal 

 surface of the parasphenoid bone, within the cavity of the skull, but below the anterior 

 part of the brain. 



Of the two oblique muscles of each eye that which is nearest to the interorbital 

 septum is the superior, the other the inferior. Their direction is transverse to that of 

 the recti muscles. The superior passes outside the end of the internal rectus, and is 

 inserted into the eyeball between the insertion of that muscle and that of the superior 

 rectus. The inferior oblique passes outside the end of the inferior rectus, and is 

 inserted between the insertion of that muscle and that of the external rectus. The 

 origins of the oblique muscles in the sole and their direction are extremely peculiar and 

 interesting. In a sj^mmetrical fish the origins of these mv;scles are on the inner sides 

 of the orbits, that is, on the side towards the median plane of the head. The median 

 plane of the head in the sole is morphologically represented by the bony interorbital 

 septum ; the oblique muscles of the ventral eye in the sole do arise from the interorbital 

 septum, those of the dorsal or right eye do not. The superior oblique of the ventral 

 eye arises from the small left ectethmoid which is on the right edge of the interorbital 

 septum ; the inferior oblique arises from the external surface of the parasphenoid 

 below the right ectethmoid. But both oblique muscles of the left or dorsal eye arise 

 from the inner surface of the left ectethmoid, which is not part of the interorbital 

 septum. The origin of the muscles is just outside the olfactory foramen. The surface 

 from which the muscles spring looks to the right side of the sole, or, in the natural 

 position of the sole, directly upwards. Thus the direction of the oblique muscles of 

 the dorsal (left) eye of the sole is at right angles to the direction of those of the ventral 

 (right). In fact, though the right ectethmoid bone has been shifted from its original 

 position in the symmetrical fish to a very remarkable degree, the left ectethmoid is in 

 tlio same position : it has been rotated but not shifted in position. The surface of the 



