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Show 256 ON 'l'HE S'l'RUC'l'URE OF 'l'HE SKULL. tympano-per1.o t1' c fi ssur.e is closed ' either by. the clho se apposit.i on, or by the actual anchylosis, of the inner hp of t e tympanic to the periotic. · d b 'd B t · th Sheep and Pig, this fissure Is replace y a WI e u ' In e . b ll b . elongated aperture, the inner edge of the tympanic u a e~ng rolled in like a scroll. In the Seals and Cetacea the sc:oll-hke form of the immensely thick tympanic bull~ becmn~s stillinore marked, and the tympano-periotic fissure wider; while the latt~r is converted into a great gap in the floor of the tympanum In Orycteropus and in the Sirenia, the tympanic being reduced to a mere thick hoop. In many Marsupialia the alisphenoid dilates posteriorly and inferiorly into a funnel-shaped, thi~-walled, .. bony ~han1Ler, which closes the tympanic cavity antenorly, uniting by Its edges with the tympanic bone. In certain Insectivora, s~eh as t~e Hedgehog and Tenrec,. the ty~panic cavity is partly walled In by a process of the basi-sphenoid. In Hyram, and in many Marsupialia ~nd Rodents, the jugal enters into the composition of the glenoid facet for the l_ower jaw. In the Marsupials the alisphenoid may also contnbute towards the formation of this articular surface. In almost all Marsupials the angle of the mandible is continued inwards into a horizontal plate of bone. This "inflexion of the angle of the jaw" is peculiar to these lVIammals. . The palatine and pterygoid bones present very considerable differences in their connections among Mam1nalia. Thus in the Ornithorhynchus, in the larger Myrmecophagm, and in :orne Cetacea, the pterygoids unite in the middle line below, so as to prolong the bony palate beyond the palatines, as in the Crocodiles. In the Marsupials, on the other hand, the bony palate, formed only by the maxillre and palatines, is oft~n defectively ossified, so that large open spaces are left therein on the dry skull. In order to understand the changes which the normal type of skull undergoes in the Mammalian series, it is necessary to define a few lines and planes by the help of certain well-marked organic fixed points. 1'HE Sl\ ULLS OF 1\IAJ\iMALIA. 257 A line drawn from the hinder extremity of the basi-occipital to the uppermost part of the junction between the presphenoid and the ethmoid, may be called the line of the axis of the basis cranii, or the "basi-cranial line." A second line, drawn fi·om the premaxilla to the basis cranii through the junction of the vomer with the ethmoid, traverses the axis of the facial part of the skull, and may be termed the line of the axis of the basis faciei, or " basifacial line." This line, if produced upwards and backwards, will cut the foregoing so as to form an angle open do·wn wards, which I shall term the '' cranw• :_;.f .a c~• a l ang l e. " A third line, drawn from the ond of the basi-occipital bone to the posterior edge of the supra-occipital in the median line, will give the general direction of the plane of the occipital foramen, or the occipital plane. The angle it forms with the basi-cranial line is the "occipital angle." A fourth line, drawn from the torcular Herophili, or junction of the lateral and longitudinal sinuses, through the middle of a plane joining the tentorial edges of the pro-otic bones, will give the general direction of the tentorium, or, jn other words, of the demarcation between cerebrum and cerebellum.* This line, therefore, may be taken to indicate the "tentorial plane." The angle it forms with the basi-cranialline is the "tentorial angle." A fifth line, drawn through the median junctions of the cribriform plate of the ethmoid, with the frontal above and anteriorly, and with the presphenoid below and posteriorly, will give, in the same general way, the "olfactory plane." The angle it forms with the basi-cranial line is the "olfactory angle." Lastly, the longest antero-posterior measurement of the cavity which lodges the cerebrum will give the "cerebral length.'' Having defined these lines and planes, the following general rules may be laid down :- 1. The lower Mammalia have the basi-cranial line longer in proportion to the cerebral length than the higher. Taking the length of the basi-cranial line as 100, I have observed the cerebral * Of course no straight line can give this boundary with exactness, as the coadapted surfaces of tho cerebrum and cerebellum, and couscquontly of tho interposed tentorium, are curved in all directions. s |
