THE STRUCTURE OF THE PIKE'S SKULL. 177 



arch stretches forwards, parallel with the vomer, to the pre- 

 maxilla; its posterior eras extends backwards, and, spreading 

 out, ends in an upper (Mpt.) and a lower (Qu.) prolongation. 



Five bones enter into the composition of the arch — two 

 median, two posterior, and one anterior. The median bones are 

 so disposed that their anterior ends embrace the lower part of 

 the cartilaginous pedicle (b), the one lying more external, and 

 the other internal, to the latter. The more external has been 

 called " ecto pterygoid. ," '* It exhibits a short, ascending process, 

 running up towards the pre-frontal, and strengthening the carti- 

 laginous pedicle ; an anterior process which articulates with the 

 bone PL ; and a posterior arched prolongation, which descends in 

 front of the bone Qu., and articulates with its anterior edge. 

 The internal bone, called " entopterygoid ' (EjiL), is a nearly 

 straight, flat bone, the anterior half of which is applied, like a 

 splint, to the inner face of the ectopterygoid, while its broader 

 posterior face is similarly adjusted, above, to the bone Mpt, and, 

 below, to the bone Qu. The two last-mentioned " posterior ' : 

 bones of the arch are termed respectively the " metapterygoid ' 

 and the " quadrate " bones. The former is a broad, four-sided 

 bone, convex upon its inner surface, which presents a raised, 

 curved ridge, beneath which the entopterygoid is received 

 anteriorly and the hyomandibular posteriorly. It is connected 

 below with the quadrate bone, and, behind, it overlaps the hyo- 

 mandibular (H.M.) and the symplectic (Sy.). 



The os quadratum {Qu.), so termed, not on account of its 

 form, which is triangular, but by reason of its identity with a 

 bone called by the same name in Birds and Keptiles, presents 

 inferiorly an articular head, with an elongated articular surface, 

 convex from before backwards, for the lower jaw. On the 

 posterior part of its inner surface it has a deep groove, directed 

 from above downwards, very nearly parallel to its posterior edge. 

 Into this the symplectic is received. 



The anterior bone (Pa.), lastly, is elongated and flattened, 

 and bears teeth upon its lower surface. It is received ante- 



* Not to multiply names unnecessarily, T adopt this term, which involves no 

 theoretical implications. It must be carefully borne in mind, however, that this 

 "ectopterygoid " has nothing to do with the " external pterygoid" process of Man. 



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