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Show 250 ON THE STRUCTURE OF THE SKULL. The outer lip of the bulla and the auditory meatus ~re ancbylosed with that region of the pro-otic which correspo~ds With_the t~gmen tympani in Man. The inner lip of the tympanic ~ulla Is, ns IS the case wHh the corresponding edge of the tympaniC ~one of Man, applied against the opisthotic, but it does not anchylos~ with this bone in the Beaver; at any rate, for the greater part of Its extant. Consequently, a very narrow cleft or fissure, leading into the tyn1" panum, is opened up, if the inner lip of the bulla is gently prized away from the periotic mass in this region. I shall term this the "ty1npano-periotic fissure.'' The great difference between the tympanic bone of Man and that of the Beaver arises froin the circumstance, that, in Man, by far the greater part of the bono is occupied by the external auditory meatus; the interval between the groove for the attachment of the tympanic membrane and the inner edge of the tympanic bone-which forms the floor of the tyinpanum-being quite insignificant, except in the region of the Eustachian tube. In the Beaver, on the other hand, this part of the tympanic bone is greatly enlarged, and constitutes more than the inner half of the b~tlla tympani. The tympanic bone and the periotic being thus anchylosed together externally (though only coadjnsted internally), form one bone in the adult Beaver. But this "tympana-periotic bone'' is not anchylosed with any of the adjacent bones, even the squamosal remaining perfeetly distinct. Nor, indeed, js it fixed to them by very firmly interlocking sutures, so that in the dry skull it may be pushed out without difficulty. It is held in place, in fact, only by the descending post-auditory process of the squamosal (answering to the posterior part of the margo tympanicus ), which curves behind the external auditory passage; and by the fitting in of the "pars mastoidea '' between the ex-occipital aucl supra-occipital. Of the vast multitude of modifications undergone by the Mammalian skull, I select for comment, in this place, only a few of the most important, such as, lstly, those which are the result of unusual forms or com.binations of bones in skulls not otherwise abnormal. 2ndly. Those which are exhibited by the skulls of the higher 1.\famma]s as con1pared with the Jower. 3rdly. rl'HE BKlTJ~LS OF MAMMALIA. 251 'Those whieh are prosouted Ly what 1nay Le terrned aberrant l\lamn1alian skulls, e.g., the crania of the Monotrernatc" and Proboscidia, and of the aquatic Mammalia- the Sirenia, Phocidte, and Cetacea. I am not aware that there is any example among the Mammalia of the bones of the roof, or lateral wal1s, of the two posterior seg1nents of the skull taking a share in the formation of the floor of the cranial .cavity. On the other hand, a careful study of development will probably show that it is no uncommon circumstance for the orbito-sphenoids to unite together in the middle line, so as to exclude the presphenoid from the cranial floor, or even to supply its place entirely. A. still more remarkable deviation from the typical arrangement than this occurs in certain Mammals, and has been thus uoted by Cuvier (Le<;ons ii., p. 319) :-"La lame cribleuse de l' ethrnoide dans to us les Malcis, dans les Loris, et les Galagos, vient tou(~her com me dans l'homme, au sphenoide anterieur; tandisque, dans les Singes, elle en reste eloignee en arriere par le rapprochement des deux cotes du frontal." I find the union of the frontals to whieh Cuvier refers in this passage to take place in Cynocephalus, Macacus, Cercopithecus, and Semnopithecus. The frontals, however, do not real] y separate the presphenoid and ethmoid, but only form, above the junction of these two bones, the front part of a thick osseous bridge, the hinder part of which is contributed by the orbito-sphenoids. The Gorilla agrees with the Monkeys and Baboons in these respects. Thus, in the adult male Gorilla in the Museum of the Hoyal College of Surgeons, the distance from the anterior boundary of the sella turcica to the anterior end of the cribriform plate of the ethmoid is 1·4 in. Of this extent of the La~e of the skull, 0·35 in. is' occupied by the conjoi~ed orbito-sphenoids, 0·42 in. by the coalesced frontals, and 0·6 in. by the lamina perpendicularis of the ethmoid. But, in a vertical section, the ethmoid is seen to extend back under the basi-cranial processes of the frontals (which are not more than one-fifth of an inch thick) as far as the suture between the orbito-sphenoids and these processes, which end anteriorly in a free rounded, trans- |
