82 



SPECIAL ANATOMY OF THE SKELETON 



alternately at the expense of the inner and outer surfaces, for articulation with 

 the greater wing of the sphenoid. 



The Petromastoid Portion (paries petrosa et mastoidea}. The petromastoid 

 portion consists of (a) a mastoid portion, the thick conical posterior part behind 

 the external auditory meatus, and (6) a pyramidal portion named the petrous 

 portion, which contains the internal ear and forms part of the floor of the cranial 

 cavity- 



Eminentia 

 arcuata 



Foramen mastoideum I 



Aquaeductus vestibuli 



Aquaeductus cochleae 



Meatus acusticus internus 



FIG. 48. Left temporal bone. Inner surface. 



The Mastoid Portion (pars mastoidea). The mastoid portion is situated atj 

 the posterior part of the bone (Figs. 47 and 49). 



Surfaces. Outer Surface. The outer surface of the mastoid is rough, and 

 gives attachment to the Occipitofrontalis and Retrahens aurem (M. auricularis 

 posterior) muscles. It is perforated by numerous foramina; one of these, of large; 

 size, situated at the posterior border of the bone, is termed the mastoid foramen 

 (foramen mastoideum); jt transmits a vein (o the lateral sinus and a small artery; 

 from the occipital to supply the dura. The position and size'of this foramen are 

 very variable. It is not always present; sometimes it is situated in the occipital 

 bone or in the suture between the temporal and the occipital. The mastoid portion 

 is continued below into a conical projection, the mastoid process (processus mas- 

 toideus), the size and form of which vary somewhat. On the inner side of the 

 mastoid process is a deep groove, the digastric fossa (incisura mastoidea), for the ! 

 attachment of the Digastric muscle; and, running parallel with it, but more' 

 internal, the occipital groove (sulcus a. occipitalis), wjiich lodges the occipital 

 artery (fossa mastoidea). 



