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Show 148 ON THE STRUCTURE OF THE SKULl, en d , the squama temrnoris. The periotic n1a s, the styloid ele- T ~ d. . ment and the auditory ossiclos are all pre1orm In cartilage. The manner in which the cartilaginous capsule, whieh has the form of the subsequent periotic bones and lodges the membranous labyrinth, becomes ossified, has been much misunderstood; and as it is a point of vital importance in comparing the sknll of man with that of the lower Vertebrata, I hall entf.lr into some detail regarding it, as a matter of fac.t and as a matter of anatomical history. Nearly two hundred years ago, Kerckringiu , jn his excellent "Osteogenia Frotuurn" (1670), laid the foundation for a proper understanding of this process:- "Quarto mense mirum visu, guam cito et quanta perfectione os squamosum magnam partmn factun1 it o enm. Os petrosum jam rubicunda cartilagine signavit cavitatis sure formam organorum auditils capacem, nihil tamen adhuc prre se fert o seum, prreterquam unam in longitudinem proten urn crassiusculam et inrequalem lineam, annulo seu circulo, antea nominato, subjectam, et paulo longius protensam. Os itaque temporum hoc mense tribus constat ossiculis; annulo scilicet, osse squamoso, et illo jam commemorato. "Quinto mense os squamo urn ita adauctum st ut os syncipitis fere, os autem cuneiforme ornnino attigerit. Ossis petro,'i pars ilia qure processum mammillar m constituit, terna de novo acquisivit ossicula: unum pyrifonna, acutiore, sui parte sqnamoso annectitur; alterum, scutum ovale referens, magnitndine priori vix cedens, media rartilagine ab eo s paratur ; uti ot tertium ab utroque, quamvis hoc magnitudine n utri it roquiparandum, vix aciculre majoris a put adreq uans ; sunt an tern eo situ et ordine collocata, quem t abula footus v. m nsium, nsurpata oculis facilius ad mentem quam verba transmittet." ... . "Constat ergo os temporum hoc quinto mense sox distinctis ossiculis ; osse videlicet squamo o, annulo, osse internam oavitatem efformante et tribu notabilibus qure hoc mense exorta esse diximus."-L. c., pp. 222, 223. The explanation of the third figure in tho thirty-fifth plato, r eferred to in this pnssage, runs thus :-"Tria in os e p tros ossicula ostondit, e, r, c. Tria pctrosi ossis distincta ossicula." I 1.; THE DEVELOPMENT OF THE HUMAN SKULL. 149 "Sexto mense pyriforme et ovale scutiforme coaluerunt 1n unum , tertium nonnihil auctum est magnitudine."-L. c., p. 224. The third :figure of the thirty-sixth plate exllibits tho condition thus described and the explanation is:-" Bina in osse petroso ossicula ostendit. D, o~sis p~trosi pars qure jam ex duobus coaluit; e, tertium ossis petrosi ossiculum." " Septimo mense jam tertium illud ossiculum duo bus mense superiore inter se coalitis accessit ..... "Nihil ergo de mense octavo nonoque addendum, nisi quod ne tum quidem footus ullum habeat processum mamillarem, et quod adhuc insigni cartilagine distet os petrosum ab occipitis et syncipitis ossibus."-L. c., p. 224. The tempornl bone of a seven months' fretus is represented in Plate xxxvii., Fig. 2, with the explanation :-" Quro primo tria, deinde bina, fuerunt in petroso ossicula, jam in unum coaluisse, ostendit. 0, ossis petrosi substantia, ex tribus jam srepe dictis in unum coalita." Cassebohm (" Tractatus quatuor de Aure Humana,'' 1734, pp. 19 and 4f>; "Tractatus Quintus," 1735, p. 15) discovered that the little linear ossification mentioned in the :first extract from Kerckringius is developed in the ilnmediate vi{'inity of the fenestra rotunda, eventually surrounds it, and extends upon the base of the pars petrosa. But the first definite light thrown upon the signification of Kerckringius' "Tria ossa" is in the following extract from Meckel's '' fiandbuch der Vergleichenden Anatomie" (1820. Bd. iv., p. 49), though Meckel does not take the trouble to refer to and explain the older observer's statements:- " 4. Bony labyrinth.-In investigating the formation of the bony labyrinth, the origin of the bony substance of the petrous bone is very carefully to be distinguished from that of the labyrinth itself. The former begins earlier than the latter, according to the ordinary type of ossification, by the development of a loose, soft, reticulated tissue in the previously existing homogeneous cartilage, and extends from before backwards. "The first part to ossify, about the end of the third 1nonth, is the circumference of the fenestra rotunda, which is rmnarkable by reason of the analogy of the fenestra rotunda to the tympanic |
