THE BONES. 55 



superior') upon the upper edge, margo petrosus ; at the apex a semilunar 

 depression (for the gangl. Gassert). 



e. Posterior surface. This is the anterior wall of the posterior cranial fossa. 

 Here, we see the Meatus auditariusinternus ; it is short, three lines long, directed 

 obliquely forwards and outwards; at the base divided by two cross lines into 

 four portions; in the superior anterior, apertura intern, canal. Fallopice (for nerv. 

 facialis)- above and behind, the funnel-shaped, fossa vestibuli superior; below 

 and behind, the fossa vestib. inferior; below and before, fossa cochlea; these 

 three, with their lamina cribrosee (for the nerv. acusticus and vasa auditoria 

 internd). Immediately above the meatus a fissure or blind foramen for the 

 reception of the Dura mater; an eminence of the inferior semicircular canal 

 of the Labyrinth ; half an inch posterior, the aquaductus vestibuli concealed 

 behind an osseous lamella. 



f. Borders. The superior (vide d.) ; the inferior and internal extends from 

 the apex to the Incisura parietalis, where it blends with the superior, unites 

 with the pars basilaris of the sphenoid and with the os ocdpitis. The inferior 

 external lies close to and unites with the great wing of the Sphenoid. The 

 anterior and free border forms the posterior half of the foram. lacerum anlerius 

 (which is an interval between the Petrous, Sphenoid, and Occipital bones 

 filled up in the fresh state with fibre-cartilage). 



Canalis Fallopiee (for the nerv. facialis}, leads from the internal auditory 

 meatus over the Vestibule transversely outwards, turns (geniculum) at the 

 Hiatus canalis Fallopii backwards, passes horizontally over the fenestra ovalis 

 beneath the horizontal semicircular canal to the posterior wall of the tym- 

 panum, curves itself once more, and descending vertically, makes its exit at 

 the foram. stylomastoideum. With the tympanum it is connected by the canal 

 chord, tympani, by the canal of the eminentia papillaris which, like the canalic. 

 mastoideus, passes off from the vertical portion. 



Canaliculus Mastoideus (for ram. auricularus nerv. Vagi} commences with two 

 small openings in the fissure between the external auditory meatus and mas- 

 toid process, and terminates, singly, close to the canal, chordce tymp. in the canal. 

 Fallopue, 



Canalis tympanicus ( for the nerv. tympanicus of the gangl. petrosum} runs from, 

 the vallecula upon the floor of the tympanum, continues as a groove upon the 

 promontory, and again opens as a canal between can. Fallopii and can. tensor 

 tympani, close to the hiatus upon the anterior surface of the Petrous bone. 



Development. Five points of ossification ; the first in the squamous portion 

 in the second month of embryonic existence ; the second in the pyramid ; the 

 third is the ring around the tympanum, annulus tympani, which in many 

 animals continues as a distinct bone during life, in the embryo lies at the com- 

 mencement almost horizontally, and is open above ; the fourth in the mastoid 

 portion; the fifth in the Styloid process. The zygoma, at birth, is little, the 

 external auditory canal not all developed, the articular cavity therefore almost 

 flat; the mastoid process also is not developed until late; the Labyrinth and 

 ossicula auditus are completely formed. 



53. Of the Cranium in general. 



By the union of the seven bones, now described, with one another 

 by means of peculiar sutures, a tolerably closed shell is formed 

 of an egg shape above, but somewhat flattened below and upon 

 the sides. In it the Brain is contained. 



