Nos. IAND2.] AXATOMY OF SCOMBER SCOMBER. 203 



At its insertion it is separated from the corresponding- muscle of 

 the opposite side of the head by the narrow median rib on the 

 ventral surface of the parasphenoid, that rib alone separating the 

 two muscles, which thus form almost a continuous sheet across 

 the under surface of the skull. 



Continuous with this anterior and ventral portion of the muscle, 

 and lying- at right angles to it, is the thick, middle portion, which 

 has its insertion on the inner surface of the thin anterior part of 

 the hyomandibular, and on the adjoining parts of the shank and 

 of the anterior articular arm of that bone. The anterior surface 

 of this part of the muscle forms part of the hind wall of the orbit. 

 The posterior surface is covered by the lining membrane of the 

 mouth cavity, bears the opercular gill, and forms the anterior 

 wall of the first gill cleft. 



The posterior portion of the muscle lies nearly at right angles 

 to the middle portion, inclining slightly upward and backward. 

 It is thin, its fibers run outward, and outward and backward, and 

 are inserted in a nearly horizontal line which extends backward 

 across the shank of the hyomandibular, ventral to the opening of 

 the facial canal, and then along the mesial surface of the opercular 

 process of the bone. The facial nerve, as it issues from the skull 

 through the facial foramen, turns slightly upward, close against 

 the lateral surface of the skull, and then runs outward along the 

 upper surface of this posterior part of the muscle, lying in a 

 depression on its dorsal surface. The muscle here is very thin, 

 but it forms, nevertheless, a continous sheet ventral to the nerve. 



The Adductor Operculi (Ao) arises (Fig. 11) from the lat- 

 eral surface of the squamosal, immediately posterior to the hind 

 edge of the dorso-posterior portion of the adductor hyomandib- 

 ularis, and from the adjoining dorsal portion of the lateral surface 

 of the intercalar. Its fibers spread outward and backward, and are 

 inserted, in a narrow line, along the ridge that runs backward, on 

 the inner surface of the operculum, from the upper edge of the 

 articular facet of the bone. This ridge, which is not distinctly 

 shown in Fig. 37, marks the ventral limit of the depressed region 

 that gives insertion to the levator operculi. It extends backward 

 about halfway across the operculum, approximately in the line of 

 the dorsal edge of the notch in the hind edge of the bone. The 



