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Royal Society : — 



in which the petrosal bone subsequently makes its appearance. 

 Near its proximal end, the rod-like " mandibular cartilage " sends 

 off another slender cartilaginous process, which extends forwards 

 parallel with the base of the skull. "With the progress of develop- 

 ment, ossification takes place in the last-named cartilage, and con- 

 verts it, anteriorly, into the palatine, and posteriorly, into the ptery- 

 goid bone. The mandibular cartilage itself becomes divided into 

 two portions, a short, proximal, and a long, distal, by an articulation 

 which makes its appearance just below the junction of the pterygo- 

 palatine cartilage. The long distal division is termed, from the name 

 of its original discoverer, Meckel's cartilage. It lengthens, and an 

 ossific deposit takes place around, but, at first, not in it. The 

 proximal division in the mammal ossifies, but usually loses its con- 

 nexion with the pterygoid, remains very small, and becomes the incus. 

 In the bird the corresponding part enlarges, ossifies, and becomes 

 the OS quadratum, retaining its primitive connexion with the ptery- 

 goid. In the mammal, the proximal end of Meckel's cartilage ossi- 

 fies and becomes the malleus, while the rest ultimately disappears. 

 The ossific mass which is formed around Meckel's cartilage remains 

 quite distinct from the proximal end of that cartilage, or the malleus, 

 gradually acquires the form of the ramus of the lower jaw, and 



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Fig. 3. — Dissection of the cranium and face of a foetal lamb 2 inches long. 

 The letters have the same signification as elsewhere, except N. Nasal capsules. 

 a. b. c. Septum narium. L. Lacrynial. PI. Palatine. Eu. Arrow indicating 

 the course of the Eustachian tube. i. Incus, m. Malleus. M. Meckel's cartilage. 

 11. llyoid. Ps. Petrosal. Ty. Tympanic. 



