THE ENDOSKELETON 159 



rate the responsible role of being the only suspensory piece 

 for the mandible. As a cartilage bone it persists in am- 

 phibians, reptiles, and birds. The several elements of the 

 mandible remain distinct in amphibians and reptiles, but con- 

 solidate in birds, the proximal end, which articulates with 'the 

 quadrate, being in all cases the free posterior end of Meckel's 

 cartilage (=articulare). In mammals a great change takes 

 place in these parts, the history of which is repeated in the de- 

 veloping embryo, through which the facts first came to light. 

 Here both quadrate and articulare, external at first, as in am- 

 phibians and reptiles, become drawn into the tympanic cavity 

 (middle ear) where, still retaining approximately their original 

 shape, though proportionately reduced in size, they become the 

 incus and malleus, respectively, while the mandible, each half 

 consolidated into a single bone, forms a new articulation di- 

 rectly with the skull in the petrosal region. The old articula- 

 tion of the mandible, that between quadrate and articulare, now 

 incus and malleus, after having served so long and well in the 

 mastication of food, emancipated from this- coarse work and 

 remaining almost embryonic in point of size, becomes attuned 

 to sound waves and assists in their transmission! The third 

 and innermost bone of the tympanic cavity, the stapes, has 

 been for a long time a true bone of contention, in spite of its 

 small size. Some authorities have attempted to identify it 

 with the missing hyomandibular, the dorsal half of the second 

 visceral arch, which disappears above the fish. It appears, 

 however, to have had a double origin, one for the loop, the 

 other for the base. The first seems to have been originally de- 

 rived in amphibians from the cartilaginous wall of the otic 

 capsule, and to be thus a part of the chondrocranium, and not 

 an element of the visceral system. The oval base is an ossified 

 membrane, secondarily fused with the other piece. The fora- 

 men in this minute bone, to which it owes its stirrup-like shape 

 in man and in some other mammals, transmits an artery which 

 in man disappears in the embryo, but in Insectivora and rodents 

 is retained throughout life, the arteria stapedialis. 



The remaining arches subserve in part the original function 

 of gill-bearers so long as there is opportunity, which occurs 



