OCR Text |
Show 246 ON THE STRUCTURE OF THE SKULL. (Fig. 88, A), aud exhibiting a corresponding extent of the cranium. The three segments of the basi-cranial axis are at once recognisable and identifiable ·with the basi-occipital, basi-sphenoid, and presphenoid of the Bird ; but the basi-sphenoid is truncated at its anterior end, as in the Crocodile, not produced into a long beak, as in Birds and many Lizards. The ex-occipital and supra-occipital bones, again, have all the relations of those of the Ostrich, and are universally ad1nitted to be the homolognes of the latter. In the Ostrich, as we have seen (Fig. S8, A), there lies in front of the ex-occipitals a large bony mass, composed of the confluent opisthotic, epiotic, and pro-otic hones. The inner face of the single periotic bone thus formed is divided into two surfaces, one anterior and one posterior, by a ridge which runs somewhat obliquely from above downwards and forwards. The anterior surface is concave, looks somewhat forwards, articulates in front with the alisphenoid, aud contains no part of the organ of hearing; the third division of the trigeminal nerve passes out in front of it. The posterior surfac~ presents, inferiorly, the apertures for the portio dura and the portio mollis; superiorly and in front, a fossa arched over by the anterior vertical semicircular canal; while, superiorly and behind, it contains the posterior vertical semicircular canaL Between the posterior edge of this division of the bone and the ex-occipital the eighth pair of nerves leaves the skull. In the Beaver (Fig. 97), there is a single mass of bone not dissimilar in form and proportional size, which has always beeu admitted to be the homologue of the pars petrosa and pars mastoidea of Man, and the general relations and characters of which may be described in exactly the same terms. The inner face is divided into two surfaces, one anterior and one posterior, by a ridge which runs, som.ewhat obliquely, from above downwards and forwards. The anterior surface is concave, and looks sligbtly forwards; it articulates in front with a bone which, as all agree, corresponds with the alisphenoid of Man, and lies behind the exit of the third division of the trigeminal. The posterior division presents, inferiorly, the apertures for the portio dura and portio mollis; superiorly and in front, a fossa arehed over by the anterior vertical semicircular canal; while superiorly and behind 'fHE SKULLS OF lVfA.l\11\'JAJ-'IA. 24.7 it contains the posterior vertical semicircular canal. Between the hinder edge of this division and the ex-occipital, the. eighth pair of nerves leaves the skull. The inferior, or internal, edge of the periotic bone in the Bird, and that of the pars petrosa in the Beaver, comes into relation with the basi-occipital and basi-sphenoid; externally, each exhibits the fenestra ovalis and rotunda, and is related, above, to the squamosal. In fact, the only noteworthy differences between the ornithic periotic, and the Mammalian pars petrosa and mastoidea, are that the fonner becomes anchylosed with the adjacent bones of the cranial wall, while the latter remain separate from thein; and that, while the periotic articulates, above, with the parietal in the .Bird, the corresponding ossification in the Mamn1al is separated from that bone by the squan1osal. On the former distinction it would of course be absurd to lay any weight; as regards the latter, it is deprived of all significance by the circumstances that in some Birds-as, e.g., the common Fowl-the squamosal interposes between the periotic and the parietal in the wall of the skull; and that in some Mammals- as, e.g., the Sheep-the squamosal is completely excluded from the skull, and the pars petrosa unites with the parietal. The simple anatomical comparison of the parts appears, then, to be amply sufficient to demonstrate, that the pars petrosa and mastoidea of the Beaver correspond in every essential respect with the periotic n1ass of the Bird, and therefore with the pro-otic, opisthotic, and epiotic bones of Reptiles and Fishes. On the other hand, no one has ever doubted that the petrosal and mastoid of the Beaver answer to the petrosal and mastoid of 1\Ian; and therefore we are led by the comparison of adult structure, merely, to exactly the same conclusion as that at which we arrived by the study of development, to wit, that the pars petrosa and pars rnas: toidea of Man answer to the periotic bones of the lower Vertebrates. In front of the periotic, the side wall of the cranium is formed by an alisphenoid, anchylosed below with the. basi-sphenoid; and, still more anteriorly, by a large orbito-sphenoid, united inferiorly with the presphenoid, which is distinct from the basi-sphenoid behind, and fro1n the ethmoid in front. |