THE TEMPORAL BOXES. 



179 



tion of the aquseductus Fallopii. and transmits the facial nerve and stylo-mastoid 

 artery : 11. the auricular ri**ure. situated between the auditory and mastoid pro- 

 s, for the exit of the auricular branch of the pneumogastric nerve. 



Borders. The superior, the longest, is grooved for the superior petrosal sinus, 

 ami has attached to it the tentorium cerebelli ; at its inner extremity is a semilunar 

 notch, upon which the fifth nerve lies. The posterior border is intermediate in 

 length between the superior and the anterior. Its inner half is marked by a groove, 

 which, when completed by its articulation with the occipital, forms the channel 

 for the inferior petrosal sinus. Its outer half presents a deep excavation the 

 jnyular T\>s*a which, with a similar notch on the occipital, forms the foramen 

 lacerum posterius. A projecting eminence of bone occasionally stands out from 

 the centre of the notch, and divides the foramen into two parts. The anterior 

 border i* divided into two parts an outer joined to the squamous portion by a 

 suture, the remains of which are distinct ; an inner, free, articulating with the 

 spiuous process of the sphenoid. At the angle of junction of the petrous and 

 squamous portions are seen two canals, separated from one another by a thin plate 

 of bone, the processus cocJileariformis; they both lead into the tympanum, the 

 upper one transmitting the Tensor tympani muscle, the lower one the Eustachian 

 tube. 



Structure. The squamous portion is like that of the other cranial bones ; the 

 mastoid portion, cellular ; and the petrous portion, dense and hard. 



Development (Fig. 142). The temporal bone is developed by ten centres, 

 exclusive of those for the internal ear and the ossicula viz. one for the squamous 

 portion including the zygoma, one for the tympanic plate, six for the petrous and 

 mastoid parts, and two for the styloid process. Just before the close of foetal life 

 the temporal bone consists of four parts : 1. The squamo-zygomatic, which is ossi- 

 fied in membrane from a single nucleus, which appears at its lower part about the 

 second month. 2. The tympanic plate, an imperfect ring, which encloses the tym- 

 panic membrane. This is also ossified from a single centre, which appears rather 

 later than that for the squamous portion. 3. The petro-mastoid, which is developed 

 from six centres, which appear about the fifth or sixth month. Four of these are 

 for the petrous portion, and are placed around the labyrinth, and two for the mas- 

 toid (Vrolik). According to Huxley, the centres are more numerous, and are dis- 

 posed so as to form three portions : (1) including most of the labyrinth, with a part 

 of the petrous and mastoid. he has 

 named prootic ; (2) the rest of the 

 petrous, the opisthotic ; and (3) the 

 remainder of the mastoid, the epiotic. 

 The petro-mastoid is ossified in carti- 

 lage. 4. The styloid process is also 

 ossified in cartilage from two centres : 

 one for the base, which appears before 

 birth, and is termed the tympano-hyal; 

 the other, comprising the rest of the 

 process, is named the stylo-hyal, and 

 does not appear until after birth. 

 Shortly before birth the tympanic 

 plate joins with the squamous. The 

 petrous and mastoid join with the 

 squamous during the first year, and 

 the tympano-hyal portion of the sty- 

 loid process about the same time. The 

 stylo-hyal does not join the rest of the 

 bone until after puberty, and in some 

 skulls never becomes united. The 

 subsequent changes in this bone are. 

 that the tympanic plate extends outward, so 



If or 

 squamous 

 portion, 

 including 



1 for tympanic 

 plate. 



6 for petrous 



and mastoid 



portions. 



g for styloid process. 



FIG. 142. Development of the temporal bone, 

 ten centres. 



By 



as to form the meat us auditorius ; 



