PARIETAL AND ETHMOID BONES 23 



The petrous portion is exceedingly hard, like stone, hence its 

 name. A slender point of bone, called the styloid process, is seen 

 on its lower surface; the carotid artery, on its way to the brain, 

 passes through the carotid canal, which is in this portion. 



The petrous bone contains the greater part of the ear; the 

 internal auditory canal for the auditory nerve, or nerve of hear- 

 ing, is on its posterior surface (seen within the skull). 



The tympanic portion forms the greater part of the external auditory 

 meatus, or canal. 



Parietal bones (ossa parietales). Right and left, situated at 

 the top and sides of the head, and so named because they form 

 the sides or walls of the skull. They are flat in structure, and 

 nearly square in shape; the four borders are called sagittal, 

 squamous, frontal, and occipital (Figs. 12 and 14). 



At the extremities of the -borders are the angles the frontal 

 and occipital angles above, and the sphenoid and mastoid angles 

 below. The most prominent point on the side of the skull is near 

 the center of the parietal bone and is called the parietal eminence. 



On the inner surface of this bone well-marked grooves are 

 seen for the middle meningeal artery, and depressions for the 

 convolutions of the brain. 



Ethmoid bone (os ethmoidale). Situated between the orbits 

 and, therefore, in the upper part of the nose. (For illustration 

 see pages 33, 34.) 



It consists principally of two lateral portions formed of spongy 

 bone, and containing the ethmoid cells or sinuses. These portions 

 are called ethmoid labyrinths. They are in the walls of the nasal 

 cavity and the cells open into it, therefore they contain air. 

 The labyrinths are attached to the borders of the horizontal 

 plate, which is situated in the roof of the nose and perforated for 

 the passage of the nerves of smell. 



The upper part of the nasal septum, which divides the nasal 

 cavity into two parts, is formed by the vertical plate of the ethmoid, 

 which hangs from the horizontal plate, and is, therefore, between 

 the two labyrinths (Fig. 26). 



Two of the turbinated bones (superior and middle) project from the 

 medial surface of the labyrinths (Fig. 25). (For description of inferior 

 turbinated bone see page 26.) 



