THE TEMPORAL BONES. 



71 



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

 jugular fossa 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 is 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 

 spinous 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 cocldeariformis ; they both lead into the tympanum, the 

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

 bony part of the Enstachian tube. 



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

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



Development (Fig. 36). The temporal bone is developed by ten centres, 

 exclusive of those for the internal ear and the ossicula viz., one of 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, in the concavity of 

 which is a groove, the sulcus tympanicus, for the attachment of the circumference 

 of the tympanic membrane. This is also ossified from a single centre, which 

 appears about the third month. 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 mastoid 

 (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 and backward, so as to form the meatus 

 nuditorius. The extension of the tympanic plate, however, does not take place at 

 an equal rate all round the circumference of the ring, but occurs most rapidly 

 on its anterior and posterior portions, and these outgrowths meet and blend, and 

 thus, for a time, there exists in the floor of the meatus a foramen, the foramen of 

 Hnsehke: this foramen may persist throughout life. The glenoid cavity is at first 

 extremely shallow, and looks outward as well as downward ; it becomes deeper 

 and is ultimately directed downward. Its change in direction is accounted for 



( Unite about 

 / time of birth. 



Ifor 

 squamous 

 portion, 

 including 

 zygoma : 

 %d month. 



1 for tympanic 

 plate. 



6 for petrous 



and mastoid 



portions. 



2 for styloid process. 



FIG. 36. Development of the temporal bone, 

 ten centres. 



3-"S. 



