586 STRUCTURE OF VERTEBRATES 



addition a prearticular and one or more splenials. Modern tetra- 

 pods show various degrees of reduction from this condition, 

 culminating in the mammals which have a single bone, the 

 dentary, which has also taken over from the articular the jointing 

 with the upper jaw ; it meets, however, not the quadrate but the 

 squamosal. The so-called * angulosplenial ' of the frog is probably 

 either the prearticular, or that bone fused with the angular. 



The relationship of the two pieces which make up the hyoid 

 arch to the elements of the branchial skeleton is not clear, but it 

 is generally assumed that the dorsal element, the hyomandibula, 

 corresponds to an epibranchial element, the ventral ceratohyal 

 to a ceratobranchial, and the median basihyal to the basibranchial. 

 The hyomandibula in fishes generally, as we have seen, suspends 

 the upper jaws. In Amphibia, Reptilia, and Aves it has moved 

 into the middle ear, and become the columella auris which trans- 

 mits sound waves from the tympanum to the membranous 

 labyrinth. The evidence for this identification depends largely on 

 the relations of the columella to nerves and blood vessels. In 

 mammals the hyomandibula is also in the middle ear, this time 

 as the stapes. The remainder of the hyoid forms, in fishes, an 

 attachment for the muscles which raise and lower the floor 

 of the buccal cavity in breathing, while the branchial arches 

 give strength to the part of the body weakened by the gill slits. 

 In embryonic tetrapods there is a cartilaginous basketwork 

 below the mouth, and in some, such as monotremes and mar- 

 supials, some connection with the arches can be made out. Most 

 of the pieces of cartilage coalesce to form the hyoid apparatus 

 which supports the tongue ; branchial as well as hyoid elements 

 are included in this, but the exact limits cannot be made out. Its 

 form varies in different groups, but it generally has a flat plate 

 or body, with two or more processes, as we have seen it in the 

 frog, bird, and mammal. There is usually some ossification. 

 The posterior parts persist only as the cartilages of the respiratory 

 system, notably the thyroid, cricoid, and arytenoid cartilages 

 of mammals and perhaps the epiglottal cartilage and tracheal 

 rings. 



There remain in the mammalian skull a few bones in the ear 

 region for which we have not accounted ; these are the tympanic, 

 which is typically inflated to form a bulla enclosing the middle 

 ear, but which in marsupials and insectivores is a ring, and the 

 first and second of the ossicles (the malleus and incus) ; the third. 



