May 11, 1883.] 



SCIENCE. 



381 



tympanic tegmeu, and its posterior border is 

 continuous with, tlie spongy substance of the 

 outer wall of the mastoid antrum. 



Fis. 1. — View from within of the outer portion of the left tem- 

 lioral bone, sawed througli the tympanum, fore and aft, 

 parallel with its inclination, a, tymp.anic margin of the ex- 

 ternal auditory meatus, formed helow and at the sides by the 

 grooved margin of the tympanic plate, and above by the 

 margin of the auditory plate; t, scute, forming the outer 

 boundary of the altie; c, legmen; <7, mastoid antrum; e, 

 prominence of the inner posterior boundary of the attic; /, 

 canal for the accommodation of the long process of the mallet ; 

 g, petro-squamosal fissure. Below e are seen the pyramid, 

 and the aperture of the tympanic cord. 



The inner surface of the mastoidea forms 

 part of the posterior cranial fossa. Contigu- 

 ous to the petrosa, it is impressed bj' the large 

 curved channel for the lateral sinus. The 

 upper border is defined from the squamosa bj^ 

 a notch, which receives the posterior inferior 

 angle of the parietal bone. Usuallj' a short 

 canal ^ pierces the posterior border, from the 

 outer surface to the groove for the lateral sinus, 

 for the transmission of a vein. 



The petrosa is a prostrate three-sided pyra- 

 mid, with its base applied outwardly against 

 the squamosa and mastoidea, and with its apex 

 directed obliquely forward in the base of the 

 cranium, between the occipital and sphenoid 

 bones. 



The posterior surface of the petrosa is the 

 smallest, and forms an uneven slope at the fore- 

 part of the posterior cranial fossa, defined out- 

 wardlj" bj' the groove for the lateral sinus. 

 Internallj', to its middle, is the internal audi- 

 tory meatus, a short, wide canal for the passage 

 of the auditory and facial nerves. Outward 

 from the meatus is an oblique cleft of variable 

 extent, sometimes large and conspicuous, and 

 sometimes nearlj' obsolete. It communicates 

 with a fine canal,- extending to the vestibule, 

 for the passage of a vein. 



1 Mastoid foramen. " Aqueduct of the vestibule. 



The upper border of the petrosa is an acute 

 ridge, which continues outwardly to the upper 

 border of the mastoidea, and gives attachment 

 to the tentorium. It is usuallj' marked along 

 part of its course hy a groove for the superior 

 petrosal sinus, and its inner extremity is im- 

 pressed bj' the trifacial nerve. 



The posterior border of the petrosa is sharp 

 and irregular, and joins the occipital bone. 

 Its middle skirts the fore-part of the jugular 

 foramen, and commonly exhibits two notches, 

 separated by an angular process, which gives 

 attachment to a partition of the dura dividing 

 the foramen. At the apex it is usually marked 

 by a groove for the inferior petrosal sinus. 



The anterior border of the petrosa is the 

 shortest ; and it forms, with the squamosa, a 

 notch, which receives the angular process of 

 the sphenoid bone. 



The anterior surface of the petrosa, broad 

 and uneven, forms the posterior boundary of 

 the middle cranial fossa. Above its centre, 

 a conspicuous prominence, together with the 

 contiguous depression internally, marks the po- 

 sition within of the labyrinth. Another depres- 

 sion in front of the apex accommodates the 

 ganglion of the trifacial nerve, and the notch 

 just below communicates with the carotid ca- 

 nal. The portion of surface below the posi- 

 tion occupied by the labyrinth is formed by a 

 wide, triangular plate, the tegmen,^ which 

 covers the tympanum, the mastoid antrum, 

 and the eustachian tube. It is commonh" more 

 or less defined bjr a fissure, remaining as part 

 of the petro-squamosal suture, which at birth 

 extends from the notch at the bottom of the 

 squamosa to that at its upper border. Fre- 

 quently, also, a vascular groove, and several 

 foramina for the transmission of vessels, mark 

 the line of separation. The inner extremity 

 of the tegmen is further somewhat defined 

 from the rest of the petrosa by a groove di- 

 rected outwardly, and ending in a small aper- 

 ture, the hiatus^ of the facial canal, for the 

 entrance of the great superficial petrosal nerve. 

 From the extremity of the tegmen, a narrow 

 bar dips into the glenoid fissure, and separates 

 the tj-mpanic plate from the squamosa. The 

 under part of the tegmen is commonly formed 

 bj' a layer of spongj- substance of variable 

 thickness. 



The inferior surface of the petrosa is very 

 uneven. At its fore-part, outwardly, is situ- 

 ated the tympanic iilate., originally' a distinct 

 bone from the rest of the petrosa. It presents 

 a broad, concave surface, directed towards the 

 glenoid fossa, and defined from this by the gle- 



