OCR Text |
Show 252 ON THE STRUCTURE OF THE SKUJ,J;. versely concave ridge, constituting the posterior boundary of the olfactory fossa. Laterally, the basi-cranial processes of the frontals arch downwards for a short distance and unite with the lateral masses of the ethmoid. In the Gorilla, the frontal bridge is nuwh smaller than in the lower Catarhines. The Chimpanzee approaches lVIan still 1nore nearly ; a triangular process of the presphenoid interposing itself between the frontals and joining the ethmoid. Sometimes, however, very small processes of the frontals just unite over this junction. In the Orang, the frontals are widely separated, as in Man. The epiotic, pro-otic, and opisthotic bones are always anchylosed into a single periotic bone in the Mam·malia; but thev unite with the other elements of the temporal bone, and with th~ adjacent cranial bones, in very various modes, and the tympanic cavity presents very different boundary walls in different Mammals. In the Beaver, as we have seen, the tyn1panic and periotic bones are anchylosed into a single "tympano-periotic," which remains unanchylosed with the squamosal, and is easily detached. In the Sirenia and in Cetacea (sooner or later) the same anchylosis takes place, but the tympano-r eriotic is still less firmly fixed in its place, and, in some Cetacea, does not appear at all in the interior of the skull, in consequence of the growth of the adjacent bones towards one another over it. rrhe tympano-periotic of the Rhinoceros, Horse, and Sheep, long remains unanchylosed to the surrounding bones, but is so wedged in between them as to be practically fixed within the walls of the skull. In Echidna and in Orycteropus the periotic, the squmnosal, and the tympanic remain perfectly distinct for a long time, if not throughout life. The squamosal and tympanic of the Pig anchylose into a single "squamoso-tympanic," which is firmly fixed to the adjacent bones ; but the periotic remains free, and consequently readily falls not out of, but into the skull. In the nine-banded Armadillo (Praopus) it is the periotic and squamosal which arc anchylosed, the tympanic re1naining 'rHE SKULLS 0.1!~ MAMMAL fA. 253 rud. im. entary and free '· and the Opossums and the T api· r ex h1'b I' t a simllar arrangement. Other Mam1nalia, such as the Carnivora and Primates h 1 ] . , ave ~ 1e squamosa , tympaniC, and periotic all anchylosed together Into one "temporal bone.'' Even in one and the same order the constitution of the tympanic cavity exhibits the most remarkable differences. To take the Edentata as an example:- In the Orycte:~pus the walls of the tympanic cavity have a wonder~ull y reptihan arrangmnent ; the periotic is very large in proportw~ to th_e oth~r b_one~ of the skull, and its plane presents comparatively httle Inclination, so that its exterior face looks n1ore outw~rds than downwards. A large part of its posterior and outer face Is seen, as a pars mastoidea, upon the exterior of the skull, between t~1e sup_ra:occipital, the ex-occipital, and the squam? sal, but ~here Is no d1st1nct "mastoid process;" Lelow, the peri? tiC comes. Into con:act with the basi-occipital and basi-sphenoid; In front, With the alisphenoid. The latter bone is strongly convex outwards, so at:3 to present a posterior, as well as an external, face· t~e. posterior fac.e forms the front wall of the tympanum, and ex~ h1bits a somewhat deep excavation, or alisphenoidal air-cell. The s.qua~osal, a very large bone, is divided by a wellmarked ndge Into an upper face, which constitutes part of the roof, and an outer face, which forms a portion of the lateral waH, of the skull. The latter enters into the outer and upper wall of the tympanum ; the former, very thin, constitutes the roof of ~hat cavity, abutting internally upon the supra-occipital an~ panetal. The Fallopian canal is open for the greater part ?fIts extent, and a hook-like osseous process, which overhangs Its outer and posterior part, gives attachment to the hyoid. The tympanic is a strong hoop of bone, incomplete above, and I~uch shorter anteriorly than posteriorly. By its expanded anterior end it articulates by an interlocking suture with the squamosal. The thin posterior end is free. In Myrmecophaga tetradactyla (and essentially the same ~trrangen1ent obtains in the great Ant-eater), the squamosal, as ln Orycteropus, enters largely into the wall of the cranial eavity; but the tympanie, which is large and bullate, is anchylosed with |