THE CRANIUM. 131 



serrated. Below, is the mastoid process, a large conical pro- 

 jection eight lines long, into which are inserted the sterno-mas- 

 toid, and trachelo-mastoid muscles. At the inner side of its 

 base is a fossa affording origin to the digastric muscle. The 

 inner face of the mastoid portion is marked by a deep large fossa 

 for the lateral sinus of the dura mater. In the posterior part of 

 the suture, uniting the mastoid portion and the occipital hone, 

 or in the former bone near the suture, is the mastoid foramen, 

 for conducting a vein from the integuments into the lateral 

 sinus. 



The cells in the mastoid bone are large and numerous, and 

 obtain the name of sinuses; they communicate with the tym- 

 panum by one large orifice. On the outer side of these sinuses 

 a thin diploic structure is observable in some heads. 



The petrous portion of the temporal bone is a triangular py- 

 ramid, arising by a broad base from the inner side of the mas- 

 toid and squamous portions. It is fixed obliquely forwards, be- 

 tween the sphenoid and occipital bones. Its anterior surface is 

 marked by the convolutions of the brain. Near the centre of 

 this surface, and having a little superficial furrow leading to 

 it, is a small foramen called the Hiatus Fallopii, through which 

 passes the Vidian nerve. The posterior surface of the petrous 

 portion presents a large foramen, the meatus auditorius internus, 

 through which pass the seventh or the auditory and the facial 

 nerve. Half an inch behind this orifice, is a very small one, 

 overhung by a flat shelf of bone; this is said to be the aqueduct 

 of the vestibule. Just above the meatus auditorius internus is 

 a foramen more patulous than the aqueduct, for transmitting 

 small blood vessels. 



In the base of the petrous portion, between the mastoid and 

 zygomatic processes, is the meatus auditorius externus, a large 

 opening conducting to the tympanum. It is oval, about half an 

 inch deep, and varies much in its size in different subjects: its 

 margin is called the auditory process, the lower part of which 

 is very rough, for attaching the cartilage of the external ear. 



The lower surface of the petrous bone is exceedingly irregu- 

 lar. Immediately below the meatus externus, is a depression 



