l6o MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



jaw of the adult is without a quadrate suspensorium. In the 

 embryo mammal, however (Fig. 169), Meckel's cartilage is seen 

 to be connected to the otic capsule by means of a quadrate, 

 from which a stapedial process extends backwards to articulate 

 with the outer end of the stapes, in a manner which strikingly 

 recalls the relations in the urodeles. The proximal end of 

 Meckel's cartilage is expanded, and, besides articulating with the 

 quadrate, sends a long process, the future manubrium, back- 

 wards and inwards, be- 

 tween the tympanum and 

 the external auditory me- 

 atus, i. e. t into the tym- 

 panic membrane. Later, 

 with the formation of 

 membrane bone (dentary) 

 around the more distal 

 portion of Meckel's car- 

 tilage, the lower jaw ac- 

 quires a new articulation 

 with the skull, on the 

 under surface of the zy- 

 gomatic process, while at 

 the same time the prox- 



FlG. 169. Diagram of auditory ossicles and 

 related parts in the mammalia, based on the 

 embryo rat. a, external auditory meatus; ct, 

 chorda tympani; f, facial nerve; h, hyoid; km, 

 hyomandibular nerve ; /;/, hyoid branch of faci- 

 alis; ;, mallear portion of Meckel's cartilage, its 



process extending down between tympanum and 

 meatus; mn, mandibular branch of facialis; 

 q (V), quadrate, later incus; s, stapes; sm, sta- 

 pedial muscle ; /, tympanum. 



imal end of Meckel's car- 

 tilage becomes segmented 

 off from the rest, and 

 gives rise to the malleus. 

 The quadrate, having no longer to serve as a suspensorium, 

 loses its connection with the otic capsule, and becomes the 

 incus. Incus and malleus extend into the tympanic cavity 

 from in front, i.e. t are prespiracular, and cannot be homologous 

 with the anuran and sauropsidan columella. Further, it will 

 be noticed that the ossicula of the mammal are on the oppo- 

 site side of the chorda tympani from what is found in the rep- 

 tilia (Fig. 168). It is an interesting fact, the bearings of which 

 will be alluded to later, that the quadrate, in both urodeles and 

 mammals, retains its articulation with the stapes throughout life. 

 In the reptiles nothing of the sort occurs. 



