l6o -MORPHOLOGY OF THE ORGAXS OF VERTEBRATES. 



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

 €mbr\-o 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 t}'mpanum and 

 the external auditor}- me- 

 atus, /. e., 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- 



FiG. 169. Diagram of auditory ossicles and ^ 



related parts in the mammalia, based on the with the skull, On the 



embryo rat. a, external auditory meatus; ct, under Surface of the Zy- 



chorda tympani; / facial nerve ;^, hyoid; h,n, ^j^ ^^^^^ ^j^jj^ ^^ 



Jiyomandibular nerve ; hn, hyoid branch of faci- ° ^ 



alis; /«, mallear portion of Meckel's cartilage, its the Same time the prox- 



process extending down between tympanum and imal end of ^leckel's car- 



meatus; mn, mandibular branch of facialis; tilage bccOmCS Segmented 



^ \i), quadrate, later mcus; s, stapes; sm, sta- ° 



pedial muscle; /, tympanum. off from the rest, and 



gives rise to the malleus. 

 The quadrate, having no longer to sene 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., 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. 



