SKULL OF VEETEBKATA. 



4G7 



in vesting 



§ 352. 



Very early in 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, m); the Meckelian cartilage (p), which is never developed beyond 

 the cartilaginous stage, 

 is continuous with it. 



The dentary forms an 

 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. 

 Meckel's 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. 



ma 



Fig. 254. Lateral view of the skull of a human 

 foetus, with its auditory ossicles. Part of the upper 

 wall of the tympanic cavity and the tympanic mem- 

 brane have been removed, at Tympanic ring from 

 which a piece has been removed superiorly, m Mai- 

 leus. ma Manubrium of the malleus, p Meckel's 

 process extending along the inner side of the 

 lower jaw. i Incus, s Stapes, st Styloid process. 

 1st Stylohyoid ligament extending to the anterior 

 cornu of the hyoid. t Mastoid foramen. 



2 n 2 



