— G7 — 



cnd of the curved ventral edge of tlie arm (tf the ectethmoid. The articular ridge accordiiigly fonns 

 the posterior ethmoid articular surface of tlie palatiiic. Its summit, which is wholly cartilaginous, 

 gives insertion to the strong ethmo-palatine ligament, whieh ligament is usually double and has 

 its origin on the posterior, orbital surface of the ectethmoid. 



H Y M A N D I B U I. A R. 



The hyomandibular is an irregulär cross of primary bone, with the cross-piece placed ob- 

 li(|uely across the shank, and with the four angles between the cross-piece and the shank filled by 

 tliiii webbing laniinae of what is apparently membrane bone. 



The dorsal end of the shank of the cross forms the posterior articular head of the bone, this 

 head articulatirg with the pterotic. The cross-piece has articular heads at either end, the posterior 

 one articulating with the opercular, and the anterior one with the articular facet on the sphenotic 

 and proötic. The thin web of bone that fills the angle between the two cranial articular heads of the 

 bone is frequently perforated by a large foramen, due to the wear, against its inner surface, of that 

 process of the proötic that gives origin to certain of the levator muscles of the branchial arches. A 

 relatively tall ridge of bone begins at the point where the cross-piece crosses the shank of the bone, 

 and runs downward and backward on the external surface of the shank. The dorsal end of the pre- 

 opercular fits against the hind surface of this ridge, aml also against the outer surface of the hyo- 

 mandibular posterior to the ridge, the dorsal end of the preopercular projecting dorsally across the 

 opercular arm of the hyomandibular, and there leaving a space between itself and that bone. Through 

 this little Space, that small superficial portion of the dilatator operculi muscle that arises in the 

 dilatator fossa passes, the remaining and larger portion of the muscle having its origin from the 

 preopercular and from the external surface of the hyomandibular internal to and posterior to that bone. 



The ventro-anterior edge of the web of bone that fills the space between the anterior articular 

 arm and the shank of the hyomandibular is bound by a wide but strong band of fibrous tissue to the 

 dorsal portion of the internal one of the two posterior, membrane flanges of the metapterygoid. 

 Ventral to this latter flange, and ventral also to the related web of bone on the hyomandibular, the 

 ventral half of the external one of the two metapterygoid flanges abuts against and is firmly bound 

 by tissue to the anterior edge of the shank of the hyomandibular. At the ventral edge of this latter 

 flange, between the metapterygoid, the hyomandibular and the hyomandibulo-symplectic interspace 

 of cartilage, there is an oval space which transmits the arteria hyoidea. 



The ventral end of the shank of the hyomandibular is in synchondrosis with the symplectic, 

 the two bones being separated by a relatively large interspace of cartilage which gives articulation, 

 on its postero-internal surface, to the small and rod-like interhyal. The interhyal lies, in its position 

 of rest, in the line produced of the shank of the hyomandibular; lying internal to the preopercular 

 and interopercular, and being bound by fibrous tissues to both those bones, the attachment to the 

 interopercular being particularly strong. 



The facialis canal through tbe hyomandibular enters the bone by a large pit-like opening 

 on its internal surface, this opening lying in the endosteal part of the bone, close to the angle between 

 the anterior articular arm and the shank of the bone. From this pit two canals arise. One runs 

 downward in the shank of the bone, opens on its outer surface, anterior to the ridge that gives Sup- 

 port to the preopercular, and transmits the truncus hyoideo-mandibularis facialis. The other runs 

 downward and backward and separates into two parts, one of which opens on the outer surface of 



