314 ANATOMT OF VERTEBRATES. 



physial ' portions : and these are more equal in length in Birds and 

 Hoimatocrya than in Mammals ; for the embryo of a highly modi- 

 fied and advanced class early shows the characters of its class, 

 which become deceptive when exclusively used as a light to 

 general vertebral homologies. The true guide to the homology 

 of 2S is its articular connections to one or more cranial diapo- 

 I)hyses: in Fishes and Crocodiles, e.g., to the post-frontal and 

 mastoid, in Lizards and Snakes to the mastoid, in Birds to the 

 mastoid and paroccipital, in Mammals to the mastoid. The con- 

 nection witli the squamosal is later and supplementary in the 

 Vertebrate series. 



The tympanic pedicle undergoes various and extreme modifi- 

 cations in relation to the functions, as various, allotted to the 

 second ha3mal arch (counting backward) in the head. In Fishes, 

 much of the mechanical part of the respiratory functions is performed 

 by the ' tympano-mandibular ' arch : hence the length, subdivision, 

 and resultant elasticity of the suspensory piers or pedicles. In 

 air-breathing HcBmatocrya the branchial duty ceases; but a special 

 organ of sense, claiming more direct relation Avith the air, presses 

 the tympanic pedicle into a service unknown to it in the water- 

 breathers. In Chelonia, fig. 91 (Yol. I.), the tympanic, 28, is 

 developed to form a frame for the ear-drum, and it contributes 

 more or less of that frame in Crocodiles, Lizards, and Birds : it 

 has least concern with the tympanum in Serpents ; and, as these 

 are exclusively air-breathers, 28 is restricted to its function of 

 suspending the mandible, and retains most of its simple rib-like 

 form as it descends from the lengthened diapophysis, 8, fig. 97 

 (Vol. I.) to the dentigerous hajmapophysis, si. The proximal arti- 

 cular end of the tympanic may have a double condyle, as in some 

 Fishes and Birds, a single condyle, as in Lizards and Serpents, or a 

 sutural margin for fixed junction, as in Chelonia, fig. 91, 28, and 

 Crocodilia, fig. 95, 28. Such is its mode of articulation in all Mam- 

 mals, in which class it manifests its extreme simplicity of function 

 and reduction of size. To the ear-drum, which it sustains, is 

 articulated, in Birds, a columelliform ^ stapes,' by the intermedium 

 of a cartilage ; and in Monotremes and Marsupials, fig. 197, f/, by 

 the intermedium of a bone, c. This ossicle in higher Mammals is 

 divided into ^ incus ' and ' malleus,' Avhich, like the columelliform 

 * stapes' in Birds and Reptiles, is developed, as in fig. 444, B, e 

 (Vol. I.), in connection with, but not like the tympanic (r/) and 

 mandible in and from, the peripliery of the primary ' visceral ' 

 or haemal cartilaginous arch, called, from its discoverer, ^ Meckel's 

 cartilage.' 



