. Cunningham's Text-book of anatomy. Anatomy. 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 e
. Cunningham's Text-book of anatomy. Anatomy. 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;. g f A B C The squamo-zygomatic part is coloured blue ; the petro-mastoid red. The tympanic ring is left uncoloured. Fig. 143.—A. The Parietal Surface op the Right Temporal Boxe at Birth. B. The same with the Squamo-zygomatic Portion removed., (The lettering is the same in both A and B.) a, Tympanic ring, b, Medial wall of tympanum, c, Fenestra cochlear, d, Fenestra vestibuli. e, Tympanic antrum, f, 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, Pe
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