THE JAWS AND THEIR MUSCLES. 579 



myxines, and even, in a few cases, on the symphysis of the jaw, a position not 

 observed in any other Vertebrata. In the lampreys, most of the teeth are 

 placed on the lips. 



The teeth of Fishes are usually anchylosed to the bone on which they rest, 

 the dental and osseus tissues being blended ; occasionally it is the side of the 

 tooth, and not the base, which is thus fixed. In certain Cartilaginous fishes, 

 some of the teeth are divided at their base, and are so attached by ligaments, 

 as to allow the teeth to be bent backward, in the mouth, by casual pressure ; 

 but when this is removed, the teeth spring up again. Even the anchylosed 

 teeth are first attached by ligament only. A few examples are met with of 

 the teeth being embedded in sockets ; but then also anchylosis exists. The 

 short strong teeth, which almost pave the mouth of the wolf-fish, are anchy- 

 losed to special eminences. 



The teeth of Fishes are almost invariably composed of some kind of dentine 

 only, the enamel and cement being absent. In certain cases, as in the carp, 

 the tooth-substance is brown and semi-transparent ; in the Cyclostomata, it 

 has been differently described as dense, albuminoid, or horny ; the labial teeth 

 of certain goniodonts and chsetodonts are flexible and elastic. The true den- 

 tine of fishes' teeth is very compact, especially on the surface of the tooth, 

 where it occupies the place of enamel ; this superficial layer has been called 

 vitro-dentine. Another modification of dentine commonly found in fishes' 

 teeth, is named osteo-dentine, because it contains vascular canals, resembling 

 the Haversian canals of bone, between which are dentinal tubuli, no longer 

 minute and parallel, but large, divided, and ramified. The so-called vaso- 

 dentine is also found in the teeth of fishes, and, though more rarely, the plici- 

 dentine, labyrintho-dentine, and dendro-dentine, so called from the folded, wavy, 

 or dendritic appearances seen in them on section. Although teeth consisting 

 of dentine alone, are only found in fishes, yet the most complex known teeth 

 are met with in this Class. Thus, in the wolf-fishes, and diodons, the teeth 

 contain dentine, osteo-dentine, enamel, and cement ; and in the parrot-fishes, 

 each pharyngeal tooth is composed of non-vascular dentine, covered by an 

 enamel, anchylosed to the bone by vaso- or osteo-dentine, and fixed to the 

 neighboring teeth, in the same row, by intermediate cement. 



The teeth of Fishes, besides being liable to be accidentally torn off at their 

 bases, are shed not merely once, as in Mammalia, but many times during life. 

 In the pike and other common fishes, and in the Cartilaginous fishes, as the 

 sharks, the formidable teeth are renewed and continually advance into place 

 from behind, as the old ones break, or fall out. A few quite exceptional ex- 

 amples of strictly permanent teeth are met with, as in the lepidosiren and 

 chimera.* 



The Jaws and their Muscles. 



The oral aperture of the Vertebrata, is a transverse opening, provided with 

 jaws, one or both of which move vertically. In the Cyclostome fishes, the 

 mouth, however, is circular; and, in the amphioxus, oval and longitudinal. 



The form and strength of the jaws, the mode of articulation and motions of 

 the lower jaw, or of both jaws, and the corresponding muscular apparatus, 

 vary with the habits and food of the different Vertebrata. 



In all the Mammalia, as in Man, the upper jaw is fixed to the bones above 

 and behind it, and has no independent motion ; whilst the lower jaw is mov- 

 ably articulated to the under side of the temporal bones, and is raised, moved 

 horizontally, or depressed, by muscles exactly similar to those found in the 

 human body. 



In the large Carnivorous mammalia, however, the glenoid fossae are not 

 shallow, as in Man, but deep, narrow, and form long channels running from 

 side to side, and inclining backwards and inwards towards each other. As 

 the condyles of the lower jaw are equally narrow and elongated transversely, 

 the motions of the jaw are limited to an up and down motion, in which not 



* See the article "Teeth," by Professor Owen, Cyclop. Anat. and Phys., from 

 which the preceding account of the teeth in the Vertebrata is chiefly derived. 



