148 ON THE STRUCTUKE OF THE SKULL. 



end, the squama temjioris. The periotic mass, the styloid ele- 

 ment, and the auditory ossicles are all preformed in cartilage. 



The manner in which the cartilaginous capsule, which has 

 the form of the subsequent periotic bones and lodges the mem- 

 branous labyrinth, becomes ossified, has been much misunder- 

 stood ; and as it is a point of vital importance in comparing 

 the skull of man with that of the lower Vertebrata, I shall enter 

 into some detail regarding it, as a matter of fact and as a matter 

 of anatomical history. 



Nearly two hundred years ago, Kerckringius, in his excellent 

 " Osteogenia Fcetuum" (1670), laid the foundation for a proper 

 understanding of this process : — 



" Quarto mense mirum visu, quaru cito et quanta perfectione 

 os squamosum magnani partem factum sit osseum. Os petrosum 

 jam rubicunda cartilagine signavit cavitatis suae formam organo- 

 rum auditus capacem, nihil tamen adhuc prae se fert osseum, 

 praeterquaru unam in longitudinem protensum crassiusculam et 

 inaequalein lineam, annulo seu circulo, antea nominato, sub- 

 jectam, et paulo longius protensam. Os itaque temporum hoc 

 mense tribus constat ossiculis ; annulo scilicet, osse squamoso, 

 et illo jam commemorato. 



" Quinto mense os squamosum ita adauctum est ut os synci- 

 pitis fere, os autem cuneiform e omnino attigerit. Ossis petrosi 

 pars ilia quae processum mammillarem constituit, terna de novo 

 acquisivit ossicula : unum pyriforma, acutiore, sui parte squa- 

 moso annectitur ; alterum, scutum ovale referens, magnitudine 

 priori vix cedens, media oartilagme ab eo separatur ; uti et 

 tertium ab utroque, quamvis hoc magnitudine neutri sit aequipa- 

 randum, vix aciculae majoris caput adaequans; sunt autem eo 

 situ et ordine collocata, quern tabula foetus v. mensium, usurpata 

 oculis facilius ad mentern quam verba transmittet." .... 



" Constat ergo os temporum hoc quinto mense sex clistinctis 

 ossiculis ; osse videlicet squamoso, annulo, osse internam cavi- 

 tatem efformante et tribus notabilibus quae hoc mense exorta 

 esse diximus." — L. c, pp. 222, 223. 



The explanation of the third figure in the thirty-fifth plate, referred to in this pas- 

 sage, runs thus : — " Tria in osse petroso ossicula ostendit, e, e, e. Tria petrosi 

 ossis distinct;! ossic-ula." 



