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Show 248 ON THE STRUCTURE OF THE SKUJ..~L. In the roof of the skull (Fig. 98) a large inter-parietal, SOl, which corresponds with the upper part of the squama occipitis of l\fan, occupies an interval left, posteriorly, between the two Fig. 98. 0 Fig. 98.-Side view of the skull of a Beaver. Ty, tympanic bone; M, pars mastoidea ,· Pm, the downward process of the ex-occipital, called" paramastoid." parietals; otherwise, the bones correspond with those found in the roof of the skull of the Bird. The ethmoid, the vomer, the nasal bones, the premaxillre, maxillre, lachrymal, jugal, and squamosal bones, the palatines, and the pterygoids of the Rodent, present no difficulties to the student acquainted with the structure of the Bird's and Reptile's skull; but he will miss the pre-frontals, the post-frontals, the quadrato-jugal, the transverse, and the quadrate bones, together with all the pieces of the lower jaw, save the dentary. The post-frontals, the quadrato~ugal, the transverse, and four out of the five missing pieces of each ramus of the lower jaw, appear to have no representatives in the Mammalian skull. The pre-frontals, on the other hand, are represented by the so-called" lateral masses of the ethmoid," with their developments, the superior and middle ethmoidal turbinal bones, which answer precisely to those of Man. A third turbinal, developed from the primitive cartilaginous wall of the olfactory chamber, eventually becomes united with the maxilla, and answers to the inferior or maxillary turbinal of Man; while, in the Beaver, there is a fourth turbinal, connected with the superior turbinal and with the nasal bones, which may be termed the "nasal turbinal." How far these well-defined turbinals of the Mammal answer to the cartilaginous and osseous turbinals of Birds and Reptiles, is a question which has yet to be elucidated. THE SKULLS OF MAMMAT~IA. 249 I have already endeavoured to show that the quadrate and articular bones of the oviparous Vertebrata are represented by the incus and malleus of Man, and consequently by the corresponding bones in all Mammalia; and that, as a consequence of the appropriation of two bones of the mandibular series to the uses of the- organ of hearing, the dentary bone develops its own condyle, and articulates with the squamosal. Another bone which appears to have no distinct representative in most oviparous Vertebrates* is very conspicuous in most ;Mammals, and far more so in the Beaver than in Man. This is the tympanic element, and it will be useful to study with especial attention the characters of this bone, its relations to the periotic, and the manner in which both are connected with the other bones of the skull. In a transverse section of the conjoined tympanic and periotic Lones, taken through the canal which i~ common to the anterior and posterior vertical semicircular canals (Fig. 99), the periotic mas~ is seen to be prolonged, external to and below the horizontal semicircular canal and that for the passage of tho portio dura, into a stout "mastoid process" (M), which appears upon the outer surface of the skull, between the ex-occipital, the squamosal, and the tympanic, as a production downwards and outwards of the'' pars rnastoidea," which is doubtless, as in Mau, conlposed partly of the pro-otic and partly of the epiotic and opisthotic bones. Fig. 99. 'I:J Vli. Fig. 99.-A vertical section of the conjoined tympanic and periotic bones ofthe Beaver (Castor fiber ). a, external auditory meatus ; b, groo,re for the tympanic mem· brane; c, the inner lip of the tympanic; Eu, Em:;tachian tube ; Ct, cochlea; M, pm·s mastoidea. The tympanic bone is produced, externally, into a spout-like tube, directed forwards and upwards, which is the external auditory meatus (Au, Fig. 98); below and internally the tube dilates into a thin walled hemispherical bulla (b, c, Fig. 99), open superiorly, and produced in front and auteriorly into a perforated process, which contains the osseous part of the Eustachian tube. * I learn from Mr. Parker that all Birds above the Struthionidro have a more or less perfect chain of tympanic bones, of which there are six in 0o1'vus corone. |
