OSTEOLOGY. 17 



The stylo-mastoid foramen, for exit of facial nerve; 



The auricular fissure, for the exit of Arnold's nerve, the 

 auricular hranch of the pneumogastric. 



The base of the petrous portion presents 



The external auditory meatus, the canal leading into the 

 tympanum, and 



The auditory process, a lamina of bone forming the greater 

 part of its walls. 



Borders. The superior border affords attachment to the 

 tentorium cerebelli, and has a groove for the superior petrosal 

 sinus. The posterior border forms part of the jugular fossa. 

 The anterior border articulates with the spinous process of the 

 sphenoid. Between the squamous and petrous portions are two 

 canals, separated by the processus cochleariformis, for the tensor 

 tympani muscle above and the Eustachian tube below. 



Muscular attachments are fifteen: 



To the squamous portion, 2 the temporal and massetcr; 

 to the mastoid portion, 6 sterno-mastoid, occipito-frontalis, 

 splenius capitis, trachelo-mastoid, retrahens aurem, and digas- 

 tricus; to the styloid portion, 3 stylo-hyoideus, stylo-pha^n- 

 geus, and stylo-glossus ; and to the petrous portion, 4 levator 

 palati, tensor palati, tensor tympani, and stapedius. 



Its ossific centres are ten (10), 1 for the squamous portion. 

 including the zygoma, 1 .for auditory process, 6 for petrous and 

 mastoid portions, and 2 for the styloid process. 



Tin-: si'iiKXOin is wedged in between the bones at the base 

 of the skull anteriorly and resembles a bat with extended wings. 

 It consists of 



A body, 2 Lesser wings, and 



2 Greater wings, 2 Pterygoid processes. 



It presents 



12 Foramina, 12 Pairs of muscles, and 



12 Articulations, 14 Centres of ossification, 



and is in every respect the most important bone of the cranium, 

 entering into the formation of 



5 Ca cities 2 Zygomatic, 



1 Anterior cci clu ;tl. 3 Spheno-maxillary, 



2 Middle cerebral, 4 Nasal, and 



3 Orbital, 3 Fissures 



4 Pluiryngeal, 1 Spheno-maxilhiry, 



5 N;i>;il. 2 Pterygoid-maxillary, and 



4 Fossw 3 Sphenoidal or foramen la- 



1 Temporal, cerum anterior. 



