26 EMBRYOLOGY. 



In the articular part of the cartilage of the lower jaw, which has 

 assumed the form of the malleus (figs. 334, 338 ha), there arises a 

 special centre of ossification, which corresponds to the articulare of 

 other Vertebrates. In its vicinity appears, as a covering bone, an 

 exceedingly small angulare, which subsequently fuses with it, pro- 

 ducing the long process of the malleus. The second covering bone, 

 the dentale (fig. 338 uk), attains, on the contrary, a great size and 

 alone becomes the subsequently functioning lower jaw, whereas the 

 remaining parts, which in the compound mandibular apparatus of 

 Teleosts, Amphibia, Reptiles, and Birds participate in the function 

 of chewing (palato-quadratum, or quadratum, articulare, angu- 

 lare, and MECKEL'S cartilage), lose their original function and are 

 employed in another manner. 



The most important motive to this profound metamorphosis is 

 to be found in the fact that in Mammals and Man there is developed 

 in place of the primary articulation of the jaw a secondary one. The 

 primary articulation, upon which the tooth-bearing dentale is moved, 

 lies, as we have seen, between palato-quadratum and articulare. 

 Inasmuch as these elements correspond respectively to the incus 

 and malleus of Mammals, the primary articulation of the jaw of 

 lower Vertebrates is to be sought in the incus-malleus articulation of 

 l/ie higher Vertebrates. In Mammals and Man the dentale is no 

 longer moved at this joint, because the dentale itself forms a direct 

 articulation with the cranial capsule by means of a bony projection, 

 the processus condyloideus (fig. 338), which it sends upward, and 

 through which it is united to the squamous portion of the temporal 

 bone at some distance in front of the primary articulation. This 

 union constitutes the secondary articulation of the jaw, in which only 

 covering bones participate. 



The natural result of the formation of a new articulation is, that 

 the primary lower-jaw apparatus has become superfluous for the 

 act of mastication, and that its development is restricted. Incus, 

 malleus, and angulare, which is united with the malleus, are con- 

 verted into parts of the auditory organ (see p. 613). The remaining 

 part of MECKEL'S cartilage (MK) begins to degenerate, in Man in 

 the sixth month. A portion of it, which is a prolongation of the 

 long process of the malleus, extending from the fissura petrotym- 

 panica as far as the entrance into the bony lower jaw at the 

 foramen alveolare, is converted into a connective-tissue cord, the 

 ligamentum laterale internum maxillae inferioris. A small portion 

 near the front end early acquires a special centre of ossification and 



