58 ATLAS AND TEXT-BOOK OF HUMAN ANATOMY. 



Fig. 64. — The outer surface of the right parietal bone (f). 

 Fig. 65. — The inner surface of the right parietal bone (i). 



While the facial canal is the longest of the canals of the petrous portion, the carotid canal 

 (Figs. 61 to 63) is the widest. It is also bent at a right angle. It commences at the external 

 carotid foramen upon the external surface of the petrous portion of the temporal and passes 

 at first almost vertically upward, in contact with the anterior wall of the tympanum; in the 

 immediate vicinity of the cochlea it bends at a right angle and then runs almost horizontally 

 in the axis of the pyramid to reach its irregular and frequently incomplete superior orifice at the 

 apex of the petrous portion. Fine canals, the caroticotympanic canaliculi, lead from the carotid 

 canal into the tympanic cavity. 



The musculotubar canal (Fig. 63) runs parallel to the carotid canal and in such immediate 

 proximity to it that portions of both canals have a common wall. It begins at the inner side 

 of the apex of the petrous portion of the temporal bone upon the anterior border of the pyramid, 

 between the petrous portion and the horizontal part of the squamous portion, and runs ap- 

 proximately in the axis of the pyramid. An incomplete horizontal septum which proceeds from 

 the inner and posterior wall (bordering upon the carotid canal) separates an upper smaller 

 compartment, the semicanal for the tensor tympani, from a larger inferior one, the semicanal 

 for the Eustachian tube; both compartments have their exits in the anterior wall of the tym- 

 panum. The canal contains the tensor tympani muscle and the Eustachian tube. 



The tympanic canaliculus, for the nerve of the same name, proceeds from the petrosal 

 fossa, and at first passes vertically upward into the inferior wall of the tympanum to be con- 

 tinued as a groove, the groove 0} the promontory, upon the inner wall of the tympanic cavity. 

 It then leaves the tympanum as a canal passing from its upper wall to the superior aperture 

 of the tympanic canaliculus on the anterior surface of the petrous portion, where it communicates 

 with the cranial cavity. 



The mastoid canaliculus begins in the jugular fossa as a groove, crosses the lower portion 

 of the facial canal at a right angle, and terminates in the tympanomastoid fissure. It transmits 

 the auricular branch of the pneumogastric nerve. 



In addition to the air-cells of the mastoid process, the temporal bone also contains a larger cavity, the tympanic 

 antrum (Fig. 63). (For a detailed description see the special sense organs.) This is in connection with the mastoid cells 

 behind and with the musculotubar canal in front. It is not separated from the external auditory meatus by osseous 

 tissue but only by membrane. 



In the development of the temporal bone, the petrous portion and the mastoid process are developed in common 

 from the cartilaginous auditory vesicle, and the squamous and tympanic portions are added later as supplementary 

 bones. The styloid process has no connection whatever with the remaining portions of the temporal bone and arises 

 from a portion of the branchial skeleton. 



Commencing at the third fetal month, several centers appear in the cartilaginous auditor}.' vesicle and subsequently 

 unite to form a common origin for the petrous and mastoid portions. The latter is not distinctly developed even at birth; 

 a mastoid process gradually forms, but it is not distinct until late in childhood. The first center of ossification for the 

 squamous portion appears toward the end of the second fetal month, and the tympanic portion ossifies at about the same 

 time as the pyramid. 



In the temporal bone of the new-born (Fig. 59) the tympanic portion has the shape of a ring open above, and is 

 known as the anmdus tympanicus; it is at first merely attached to the remaining bones. A very distinct suture, the 



