. Elements of Comparative Anatomy. SKULL OF VERTEBEATA. 467. § 352. Very early iu development, the primitive cartilage of the lower jaw turns off from that line of differentiation which obtains in the rest of the Vertebrata. The part, which, in others, forms the articular bone is converted into one of the auditory ossicles, the malleus (Fig. 254,9?i.); the Meckelian cartilage (p), which is never developed beyond the cartilaginous stage, is continuous with it. ^. m ji The dentary forms an investing bone on the outer surface of this car- tilage. It abuts in the middle line on its fellow of the o
. Elements of Comparative Anatomy. SKULL OF VERTEBEATA. 467. § 352. Very early iu development, the primitive cartilage of the lower jaw turns off from that line of differentiation which obtains in the rest of the Vertebrata. The part, which, in others, forms the articular bone is converted into one of the auditory ossicles, the malleus (Fig. 254,9?i.); the Meckelian cartilage (p), which is never developed beyond the cartilaginous stage, is continuous with it. ^. m ji The dentary forms an investing bone on the outer surface of this car- tilage. It abuts in the middle line on its fellow of the opposite side, and unites with it to form the lower jaw; this is articulated to the skull on the lower surface of the jugal process of the squamosal. It repre- sents therefore a new formation, though the primitive one has not disappeared, but per- sists in other relations. MeckeFs cartilage (p) is retained for some time longer on the inner sur- face of the lower jaw, but then disappears; the only portion of it which persists is the part which is placed within the tympanic cavity, and which extends to the Glaserian fissure, where it is ossified to form the processus folianus mallei. The early differentiation, and the, at first, relatively large size of the auditory ossicles, shows that they must be regarded as skeletal parts, which in a lower stage were much more developed in size. The two halves of the lower jaw are permanently separate in a large number of Mammals ; in others they unite early (Perissodactyla, Chiroptera, Primates). Low morphological conditions are implied by the straight mandibles of the Monotremata, in which there is no distinct coronoid process; in some others, also, this process is merely indicated (Cetacea). The piece which is developed from the upper portion of the primitive hyoid arch (hyomandibular of Fishes) appears to form the rudiment of a third auditory ossicle—the stapes. Fig. 254. Lateral view of the skull of a human fcEtus, with its auditory os
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