BONES OF THE CKANIUM. 189 



end of the second month; very soon afterward the petrous 

 portion begins to ossify; in the fifth month the mastoid, 

 and the last of all the styloid. 



The squamous, mastoid, and petrous portions become uni- 

 ted during the first year. The styloid process is not con- 

 nected with the petrous portion for several years after birth, 

 and sometimes remains permanently separate. Occasion- 

 ally it has been found to extend, by several pieces, to the 

 hyoid bone, thus forming the hyoid arch. The tympanic 

 ring becomes united to the squamous portion about the last 

 month of foetal life. Other changes are observed in the 

 after development of the temporal bone, as in the growth 

 of the mastoid cells, the extension of the meatus auditorius 

 externus, the enlargement of the glenoid fossa, and the 

 filling up of the irregularities of the petrous portion. At 

 birth three pieces compose the temporal bone, viz: the 

 squamous and zygomatic, the mastoid and petrous, and the 

 tympanic. 



Its articulations are with five bones. With the parietal 

 by the superior border of the squamous ; with the sphenoid 

 by the anterior, and the occipital by the posterior border; 

 also with the malar at the zygomatic suture, and the lower 

 jaw in the glenoid cavity. 



Ethmoid Bone. (E^J, a sieve.) The situation of the 

 ethmoid bone is in the large notch be- Fio 42 



tween the orbitar plates of the fron- 

 tal bone. It enters into the forma- 

 tion of the nose, the orbit of the eye, 

 and the anterior base of the cranium. 

 It receives its name from its cribri- 

 form or sieve-like appearance. Its 

 form is cuboidal. Its surfaces are 

 three, one superior or cerebral, and 

 two lateral or orbital; there is also 

 an inferior y anterior, and & posterior portion. 



FIG. 42, ^Ethmoid Bone, showing an upper and posterior view, a Nasal 

 lamella. 6 Cellular portion or body, c Crista-Galli. d Cribriform plate, 

 e Superior meatus. / Superior turbinated bone, g Middle turbinated bone. 

 & Os-planum. i Surface for olfactory nerve. 



