BONES OF THE HEAD. 



27 



this surface is the zygomatic process,^ which forms a part of the zygo- 

 matic arch. The base of this process is triangular, and has beneath 

 it a cavity called glenoid for the articulation of the lower jaw. This 

 cavity is separated by the Glaserian fissure from another behind it, 

 containing a portion of the parotid gland. Through this fissure pass 

 the chorda tympani nerve and the laxator tympani muscle, attached 

 to the processus gracilis of the malleus. The internal surface has 

 a distinct groove* for the middle artery of the dura mater. The 

 edge is thin and cutting and has flat serrations^ for the parietal bone. 



The mastoid? portion is behind, and nipple-like. It is thick and 

 cellular ; the cells being lined 

 by a thin membrane. Ex- 

 ternally it is rough, and has 

 a large process called mastoid^ 

 into which are inserted the 

 sterno-cleido- mastoid and tra- 

 chelo-mastoid muscles. Be- 

 neaih this process is the di- 

 gastric fossa ^'^ which gives ori- 

 gin to the digastric niliscle. 

 Internally is a deep sulcus^ 

 containing a part of the lateral 

 sinus. The edges are thick 

 and dentate. Near the upper 

 edge is the mastoid foramen, which transmits a vein, one of the 

 emissaries of Santorini. 



The petrous^ portion is pyramidal and directed obliquely forwards 

 and inwards. Its structure is extremely dense and brittle. 



The base has a large oval opening externally, betweeen the zygo- 

 matic and mastoid, called the external meatus, which is surrounded 

 by a rim called the auditory process, to which is attached the carti- 

 laginous portion of the ear. In the foetus it is a ring. The apeo: is 

 obtuse and is perforated by the carotid canal and foramen lacerum. In 

 the angle between the apex and squamous portion, is the bony por- 

 tion of the Eustachian tube, and above it a canal for the tensor 

 tympani muscle. 



The inferior surface presents a large process, styloid,'^* projecting 

 from a ridge called vaginal process. Behind it, is the stylo-mastoid 

 foramen^^^ the orifice of the Fallopian canal, which transmits the 

 facial nerve and stylo-mastoid artery. Inwards from the styloid 

 process is the jugular fossa, which is converted into the posterior 

 foramen lacerum by the articulation of the occipital bone; this foramen 

 transmits the internal jugular vein and the eighth pair of nerves. 

 The tympanic canal., containing Jacobson's nerve, opens in this 

 fossa ; its other orifice is near the Eustachian tube. Before the 

 jugular fossa is the opening of the carotid canal^^^ which contains 



