THE SPHENOID BONE. 



133 



sion, which forms the post-auditory process ; this ultimately blends with the posterior limb 

 of the tympanic ring, being separated from it in the adult by the petro-mastoid fissure. It- 

 forms the lateral wall of the tympanic antrum, and constitutes the anterior and upper part 

 of the mastoid process in the adult. About the third month a centre appears in the outer 

 membranous wall of the tympanum : from this the tympanic ring is developed. Incom- 

 plete above, it displays two free extremities. Of these, the anterior is somewhat enlarged, 

 and unites in front with the mandibular portion of the squamo-zygomatic, being separated 

 from it by the petro-tympanic fissure and the downgrowth from the tegmen tympani ; the 

 posterior joins the post -auditory process of the squamo-zygomatic above mentioned. 

 Below, it blends medially with the portion of the petro-mastoid which forms the floor of 

 the tympanum and ensheathes the tympanohyal behind. From the medial surface of 

 the ring below there is an extension medially and forwards which forms the floor of the 

 osseous part of the auditory tube, as well as the lateral wall and half the floor of the 

 carotid canal. From the lateral side of the lower part of this ring two tubercles arise ; 



A B C 



The squamo-zygomatic part is coloured blue ; the petro-mastoid red. The tympanic ring is left uncoloured. 



143. A. THE PARIETAL SURFACE OF THE RIGHT TEMPORAL BONE AT 



IRTH. B. THE SAME WITH THE SQUAMO-ZYGOMATIC PORTION REMOVED. 



ae lettering is the same in both A and B.) a, Tympanic ring, b, Medial wall 

 of tympanum, c, Fenestra cochleae, d, Fenestra vestibuli'. e, Tympanic 

 antrum. /, Mastoid process, g. Masto - squamosal suture, with foramen 

 for transmission of vessels, h, Squamo-zygomatic, removed in figure B 

 to show how its descending process forms the lateral wall of the 

 tympanic antrum. 



C. CEREBRAL SURFACE OF THE RIGHT 

 TEMPORAL BONE AT BIRTH. 



a, Squamo-zygomatic. b, Petro- 

 squamosal suture and foramen (just 

 above the end of the lead line). 

 c, Subarcuate fossa, d, Aquaeductus 

 vestibuli. e, Aquaeductus cochleae. 

 f, Internal acoustic meatus. g, 

 Upper end of carotid canal. 



these grow laterally, and so form the floor of the external acoustic meatus. The interval 

 between them remains unossified till about the age of five or six r after which closure takes 

 place. This deficiency may, however, persist even in adult life (see Appendix B, Temporal). 

 At birth the temporal bone can usually be separated into its component parts. The 

 lateral surface of the petrous part not only forms the medial wall of the tympanum, but is 

 hollowed out behind and above to form the inner side of the tympanic antrum, the 

 outer wall of which is completed by the post-auditory process of the squamo-zygomatic. 

 As yet the mastoid process is undeveloped. It only assumes its nipple-like form about the 

 second year. Towards puberty its spongy substance becomes permeated with air spaces, 

 which are in communication with and extensions from the tympanic antrum. Occasionally 

 this pneumatic condition is met with in early childhood. The external acoustic meatus 

 is unossified in front and below, the outgrowth from the tympanic ring occurring 

 subsequent to birth. The mandibular fossa is shallow and everted; the jugular fossa is 

 ill-marked ; whilst the subarcuate fossa is represented by a deep pit, the so-called 

 floccular fossa of comparative anatomy. The hiatus of the facial canal is an open groove, 

 displaying at either end the openings of the medial and lateral portions of the facial canal. 



Os Sphenoidale. 



The sphenoid bone lies in front of the basilar part of the occipital medially, 

 and the temporals on either side. It enters into the formation of the cranial, 

 orbital, and nasal cavities, as well as the temporal, infra-temporal, and pterygo- 



96 



