146 



OSTEOLOGY 



the tympanic part, four for the petrous and mastoid parts, and two for the styloid process. Jus 

 before the close of fetal life (Fig. 142) the temporal bone consists of three principal parts: 1. 

 The squama is ossified in membrane from a single nucleus, which appears near the root of the 

 zygomatic process about the second month. 2. The petromasloid part is developed from four 

 centers, which make their appearance in the cartilaginous ear capsule about the fifth or sixoh 

 month. One (prodtic) appears in the neighborhood of the eminentia arcuata, spreads in front 

 and above the internal acoustic meatus and extends to the apex of the bone; it forms part of the 



Septum canalis musculotubar i i 



Fenestra vestibuli 

 Tympanic antrum} \ Sulcus tympanicus 



Bristle in facial 

 canal 



Lateral wall of 

 tympanic antrum 



FIG. 142. The three principal parts of the tempora bone at birth. 1. Outer surface of petro mastoid part. 

 2. Outer surface of tympanic ring. 3. Inner surface of squama. 



cochlea, vestibule, superior semicircular canal, and medial wall of the tympanic cavity. A second 

 (opisthotic) appears at the promontory on the medial wall of the tympanic cavity and surrounds 

 the fenestra cochleae; it forms the floor of the tympanic cavity and vestibule, surrounds the carotid 

 canal, invests the lateral and lower part of the cochlea, and spreads medially below the internal 

 acoustic meatus. A third (pterotic) roofs in the tympanic cavity and antrum; while the fourth 



Squama 



Squama 



Petrosquamous 

 suture 



Petrosquamous suture 

 Eminentia arcuata 



Tympanic ring 



Petromastoid portion 



Fia. 143. Temporal bone at birth. 

 Outer aspect. 



Fossa subarcuata 



Internal acoustic meatus 



FIG. 144. Temporal bone at birth. Inner 

 aspect. 



(epiotic) appears near the posterior semicircular canal and extends to form the mastoid process 

 (Vrolik). 3. The tympanic ring is an incomplete circle, in the concavity of which is a groove, 

 the tympanic sulcus, for the attachment of the circumference of the tympanic membrane. This 

 ring expands to form the tympanic part, and is ossified in membrane from a single center which 

 appears about the third month. The styloid process is developed from the proximal part of the 

 cartilage of the second branchial or hyoid arch by two centers: one for the proximal part, the 

 tympanohyal, appears before birth; the other, comprising the rest of the process, is named the 



