OSTEOLOGY. 15 



In the angle between the squamous and petrous portions the 

 chorda tympani nerve passes from the tympanum through the 

 canal of Huguier. 



The internal surface of the squamous portion presents the 

 markings of the convolutions, and grooves for the ramifications 

 of the middle meningeal artery. The mastoid portion is convex, 

 rough, and nipple-like, and presents, externally, the following : 



The mastoid foramen, near the posterior border, transmits a 

 branch of the occipital artery to the dura mater and a small vein 

 to the lateral sinus; 



FIG. 8. 



1, lateral sinus; 2, mastoid foramen; 3, middle meningeal artery; 

 4, zygoma; 5, superior petrosal sinus; 6, petrous portion; 7, meatus 

 auditorius internus; 8, carotid canal; 9, aquseductus cochleae; 10, 

 aquaeductus vestibuli; 11, stylo-pharyngeus. 



The mastoid process, a prominent projection, composed of 

 the mastoid cells, and affording insertion for the splenius capitis, 

 sterno-mastoid, and trachelo-mastoid muscles. 



To its inner side is a groove, the digastric groove, for attach- 

 ment of the digastric muscle, and still more internally the occip- 

 ital groove, for the accommodation of the occipital artery. 



The internal surface is concave, and presents a groove, the 

 fossa sigmoidea, for the lateral sinus, in the bottom of which 

 is the opening of the mastoid foramen. 



The petrous portion is pyramidal in shape, intensely hard, 

 and contains the internal ear. It projects downward, forward, 

 and inward, and presents for examination a superior, posterior, 

 and inferior surface. 



