397 



its moveable connexion with the quadratum (or representative of the 

 incus), receives the name of the articular piece of the jaw. The 

 rest of Meckel's cartilage disappears. 





Fig. 4. Dissection of the cranium and face of a foetal lamb 2 in. long. 

 The letters have the same signification as elsewhere, except N. Nasal capsules, 

 a. b. c. Septum narium. L. Lacrymal. PI. Palatine. Eu. Arrow indicating 

 the course of the Eustachian tube. i. Incus, m. Malleus. M. Meckel's cartilage. 

 II. Hyoid. Ps. Petrosal. Ty. Tympanic. 



Thus the primitive composition of the mandibular cartilaginous 

 arch is the same in the bird as in the mammal ; in each, the arch 

 becomes subdivided into an incudal and a Meckelian portion ; 

 in each, the incudal and the adjacent extremity of the Meckelian 

 cartilage, ossify, while the rest of the cartilaginous arch disappears 

 and is replaced by a bony ramus deposited round it. But from this 

 point the mammal and the bird diverge. In the former, the in- 

 cudal and Meckelian elements are so completely applied to the pur- 

 poses of the organ of hearing, that they are no longer capable of 

 supporting the ramus, which eventually comes into contact with the 

 squamosal bone. In the latter, they only subserve audition so far 

 as they help to support the tympanic membrane, their predominant 

 function being the support of the jaw. 



The tympanic bone of every mammal is, at first, a flat, thin, 

 VOL. ix. 2 E 



