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Show 180 ON THE STRUCTURE OF THE SKULL. and are not represented at all in the Fish. Hone , in the latter, we might expect to :find only manclibular and hyoidean branches of the portio dura corresponding with the chorda tympani on the one hand, and the stylo-hyoidean and digastric branches, on the other, in l\fan. And this is really the case. F or tho portio dura of the Pike, which leaves the skull by a special foramen in ·the pro-otic bone, traver es the hyomandibular bone, and then divides into two branches, one of which runs backwards to the hyoidean arch; ·while the other, dire ·tod forward and downwards, passes to the inner side of the quadrate bone, and over its articulation with tho a1·ticulare to tb inn r surface of the lower jaw, along which it runs to tho extron1ity of the ramus. This last branch is obviously the r presentatjve of tho chorda tympani, and its relations to the quadrate and articular bones are, it will be observed, v ry sin1ilar to those which the corresponding nerve has to the incus and rnalle~£s in 1\f an. Holding fast, then, by this d tenn ination of the homologies of the artic~dare and quadrat,um, what i tho natur of the other bones entering into the palato-quadrate arch ? The metapterygoid 1nay p rhaps an w r to the os orbiculare. The manuer of its connection with th cpu drate (incus) suggests this view, for which, howev r, I cannot I r 't ud to offer any positive proof. That the other three bon answer in a general way to the pterygo-palatine bones of 1\'Ian is certain. The pterygoid of Man, it is true, is in no way C'Onn ct d with the incus, while both bones Ecpt. and Ept. are united with th • quaclratun1. But this is in reality no d.iffi<'ulty, for w . hall find that, in the higher oviparous Vertebrata, the os q~tacb·aturn is very generally connected with a bone whi h is univ r ally athnitted to correspond with the pterygoid of Man. Again, both the palatine and the pt rygoid bones of l\1an are articulated with the ba. e of the skull, wl1ilo tho palato· pterygoid arch of the l i.,i h i. not directly connected with any of the basi-cranial bonos ; but, in n1any of the higher Vertebrata, the pterygo-palatine arch is almost as fre of th base of tl1e skull as in the Fish. No douut, then, the palato-pterygoid Lones of the Fish, THE S'l'RUC'l'URE OF THE PIKE'S SKULL. 181 taken together, answer to the palato-pterygoid bones of the Man; but it is a very difficult matter to identity the separate constituents of the two arches. One of the most striking features of the palatine bone, not only in Man, but in the Vertebrata generally, is its articulation with the pre·frontal, or lateral mass of the ethm.oid. If~ guided by this character, we seek for the homologue of the palatine in the Fish, the so-called " ectopterygoid " alone satisfies the conditions. But if this bone be the homologue of the true palatine, the bone Pl. rnust be regarded as a dismemberment, or subdivision of the palatine,* and the entopterygoid will take the place of the true pterygoid. The palato-quadrate arch, with the lowor jaw, is i1nmediately suspended to the skull only by the articulation of the cartilaginous pedicle b (Fig. 71) with the pre-frontal, none of the posterior elements of the arch being directly articulated with the skull. They are indirectly united with the latter, however, by two very remarkable bones, the Hyomandibular (H.M.) and the Symplectic (Sy.). The os hyornandibu.Zare is a broad flattened bone, somewhat constricted in the middle, and divided below into an anterior and a posterior process. The upper convex edge of the bone ( d, Fig. 71) :fits into an elongated, concave, glenoidal fossa bounded by the squamosal, opisthotic, and pro-otic bones, and swings freely therein, in a plane perpendieular to the longitudinal axis of the skull. The large anterior inferior proeess articulates by its anterior edge and outer face with the metapterygoid, while below it is united by a persistent synchondrosis with t~e irregular styliform bone, the Symplectic, whieh is firmly fitted 1nto the groove already described upon the inner face of the quadrate bone. The connection thus established between the hyomandibular and the symplectic, is strengthened externally by the firm apposition of a curved elongated bone, the Pre-operculum, to the hyomandibular above and to the quadrate bone below. * ~oo~ing upon Pa. and Ecpt. as one bone homologous with the palatine of Man, 1t WI~ be ~ound that in osseous Fishes the snparation between them takes pla~e s~met~es m front of the pre-frontal articulation, as in the Pike, sometimes behmd 1 t, as m the Cod and most bony fishes. |