BONES OF THE HEAD 65 



with the, cavity of the tympanum. The petrous por- 

 tion is a pyramidal process of bone wedged in at the 

 base of the skull, between the sphenoid and occipital 

 bones. It has a foramen internal auditory for the 

 passage of the facial and auditory nerves. 



THE TYMPANIC PORTION. This is placed in front 

 of the anterior surface of the petrous portion. It 

 bounds the external auditory meatus in front, below 

 and behind, and lodges the tympanic membrane (ear- 

 drum) . 



The glenoid fossa is a depression formed by the squa- 

 mous part of the temporal and behind by the tym- 

 panic portion. It is covered with cartilage (squamous 

 portion) and articulates with the condyle of the 

 mandible. 



The Sphenoid Bone. This wedge-like bone is placed 

 across the base of the skull near its middle, and binds 

 the other cranial bones together. It assists to form 

 the cavities at the base of the cranium, orbits, and nasal 

 fossa, and has foramina and fissures for the passage 

 of six pairs of cranial nerves. It consists of a solid 

 body of bone, with a thin pair of lesser and greater 

 wings. It articulates with twelve bones, all those of 

 the cranium and five of the face; posteriorly, with the 

 occipital and temporals; anteriorly, with the ethmoid, 

 palatals, frontal, and malars; laterally, with the tem- 

 porals, frontal, and parietals ; inferiorly, with the vomer 

 and palatals. The upper surface of the body of the 

 sphenoid supports the pituitary body, and the circular 

 and cavernous sinuses, the latter enclosing the internal 

 carotid artery. 



The Ethmoid Bone (sieve-like). This projects down 

 between the orbital plates of the frontal, and enters 

 into the formation of the floor of the anterior cranial 

 fossa, the orbital, and nasal cavities. It is a very 

 small, frail bone. Its upper surface lodges the olfactory 

 bulb and portions of the olfactory tract. The fila- 

 ments of the olfactory nerve (sense of smell) pass 

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