176 



HUMAN ANATOMY. 



be the homologue of the interparietal bo7ie. This interpretation is inconsistent with 

 the history of ossification. Kerkring has described an occasional triangular minute 

 piece of bone which appears during the fifth month in the notch at the back of the 

 foramen magnum, and is fused before birth. We have specimens which imply that 

 it is, or may be, originally double. Improved methods of investigation will prob- 

 ably' show that this "bone is not uncommon. The cerebral side of the basilar is 

 fused with the sphenoid by seventeen ; the lower side unites later, probably before 

 twenty. 



THE TEMPORAL BONE. 



The plan of the organ of hearing must be known to understand the temporal 



bone.' The external ear, besides the auricle, consists of a cartilaginous and bony 



tube, the external auditory meatus,^ leading to the membrane of the tympanum which 



closes it. The middle ear, the cavity of the tympamim. is a space internal to the 



Fig. 195. 



squamous portion 



Supramastoid crest 



Occipitalis 



Spina suprameatum 



Mastoid foramen- 



Auricularis posterior 



Splenitis capitis ^^r^r — '" ' i'^>^& 



Squamo-mastoid suture " ^ i A^;. \ -„■ "\ '"V^^^ 



Sterno-mastoid i &Mk^ - ''M^^^M 



Zygoma 



Masseter 



Anterior root of zygoma 

 Glenoid fossa 



APEX OF PETROUS PORTION 

 Glaserian fissure 



Trachelo-mastoid 



MASTOID PORTION 



Tympano-mastoid fissure / 



Mastoid process / / 



External auditory meatus /_ 



TYMPANIC PORTION /' 



Stylo-glossus 

 Stylo-hyoid 



Styloid process 



Vaginal process 

 Right temporal bone, external aspect. 



membrane, opening through the Eustachian tube into the throat, and communicating 

 behind with cavities in the bone. It is lined with mucous membrane and is crossed 

 by a chain of small bones, the ear ossicles, the embryological importance of which is 

 explained elsewhere. The internal ear is a complicated system of cavities in the 

 substance of the bone containing the organ of hearing connected with the brain by 

 the auditory nerve, which leaves the bone through a canal, the iyiternal auditory 

 meatus. 



Development shows that the bone consists of the following three parts, f i) The 

 petro-mastoid, the petrous part^ of which is first found surrounding the special 

 apparatus of the organ of hearing, constituting the internal ear. while the mastoid 

 process is a much later outgrowth. (2) The tympayiic portion, which at birth is a 

 ring, incomplete above, encloses the membrane of the tympanum as a frame holds 

 a glass. This ring grows out later into a cylinder, still open above, which forms the 

 external auditory meatus. Not all its growth, however, is outward, since a part 



' Os temporale. * Meatus acusticus externus. 



