2 26 NERVE COMPONENTS OF BONY FISHES. 



formed medianly by the parasphenoid, laterally mainly by 

 the pro-otics, while the dorsal wall is for the most part 

 formed by the pro-otics and by a projection from the basi- 

 occipital bone. In the region of the figure just referred 

 to the dorsal wall is membranous, a condition maintained 

 from this point cephalad except at the extreme cephalic 

 end, where a basisphenoid appears, just behind the optic 

 chiasm. This bone is Y-shaped in cross-section, the 

 lateral wings forming the floor of the brain between the 

 hypophysis and the optic chiasm, and the vertical limb 

 articulating below with the parasphenoid in the median 

 line, thus forming the cephalic boundary of the sub- 

 cranial canal. 



Except at its most caudal end the canal is not filled by 

 the eye-muscles, but these are packed densely with fat and 

 connective tissue. 



The m. rectus externus originates in the extreme caudal 

 end of the sub-cranial canal, which at first it fills com- 

 pletely (Fig. I, r. e). As the canal enlarges, this muscle 

 rises to its dorsal side, while the m. rectus internus 

 appears below it. With the lateral expansion of the canal 

 still farther forward the externus occupies its dorso- 

 lateral angle and here under the lobi inferiores a separate 

 slip of the muscle takes origin from the fascia on the 

 dorsal surface of the m. rectus internus in the median 

 line, the fibres interdigitating with a similar slip from 

 those of the opposite side (Fig. 22, r. /). There is thus 

 formed a broad, thin sheet of muscle which runs dorso- 

 laterally under a mass of fat to join the ventro-lateral 

 side of the main muscular belly near the point where it 

 leaves the sub-cranial canal (Fig. 2), turning abruptly 

 laterad toward its insertion on the eye-ball. The fibres of 

 this separate slip are very much smaller than the other 



