578 SPECIAL PHYSIOLOGY. 



the dentine is singularly inflected on its surface. In the poison-fang of the 

 serpent the dentine is folded on itself, in front of the pulp-cavity, so as to form 

 the poison-groove or canal ; a longitudinal section of the tooth shows the 

 tapering pulp-cavity behind the poison-canal ; whilst a transverse section 

 shows this canal surrounded by the dentine, coalescing in front of it, the pulp- 

 cavity forming a crescentic fissure behind it. 



As the teeth of Reptiles wear out and fall away, an almost unlimited suc- 

 cession of new ones replaces them throughout life, a process entirely different 

 from the simple succession of temporary and permanent teeth in the Mam- 

 malia. The new tooth usually appears at the inner side of the base of the old 

 one ; but the poison-fangs of the serpents are replaced by new teeth, formed 

 behind the old ones. 



When, as is usual, the teeth are anchylosed to the jaw, the new tooth sim- 

 ply grows up on a papilla, and replaces the falling one ; but in the alligators 

 and crocodiles, in which the teeth are lodged in sockets, the new tooth, also 

 formed on a papilla, gains access, by a process of absorption, to the interior 

 of the old one, penetrates its pulp, grows up within it, raises it, and finally 

 throws it off from its own summit. By this time, as seen in the gavial, an- 

 other rudimentary tooth is formed, and proceeds to grow, in like manner, 

 into its predecessor. The process of absorption resembles that of the fangs of 

 the milk-teeth in Mammalia ; and like that it has been incorrectly attributed 

 to mechanical pressure ; for the growing tooth is softer than the old one, which 

 is being absorbed. 



Amphibia. Fine prehensile teeth are found on the upper jaw and palate 

 bones of the frogs and salamanders, more seldom on the lower jaw also. In 

 the toads, only palatal teeth are present. Teeth are absent in the proteus and 

 siren. 



Fishes. The teeth of Fishes present extraordinary varieties, greater than 

 those of any other Class. Their number is almost countless in the silurus, its 

 allies, and in the pike ; but they become fewer or wholly absent in the lower 

 orders of fishes. The chimaerse have two teeth in the lower, and four in the 

 upper jaw ; the lepidosiren has only a single dental plate in each jaw, and two 

 small teeth on the nasal bones ; the tench has one tooth on the occiput, besides 

 some on the pharyngeal bones ; whilst the myxine and myxinoid fishes have a 

 single tooth on the palate, meeting two dental plates upon the tongue. Lastly, 

 in the syngnathus or pipe-fish, in the hippocampus, in the Lophobranchiate 

 fishes, in the sturgeon, ammocete, and amphioxus, no teeth exist. 



The shape of the teeth in fishes differs much. They are usually simple and 

 conical ; they are minute, numerous, and villiform in the perch ; longer, cilii- 

 form or setiform, often bifid or trifid ; and rasp-like or raduliform on the back 

 of the vomer in the pike. They are commonly cylindrical, but sometimes 

 flattened into a lancet-like blade, either straight, curved, bent sideways, or 

 even barbed. The base may be broad, as in the larger teeth of the pike, the 

 lophius, and certain sharks ; the edge is sometimes finely serrated, as in the 

 sharks generally, or is notched, so as to divide the tooth into from two to five 

 lobes. In other less-known fishes, they are short and blunt, cubical, or pris- 

 matic, with from four to six sides, and closely arranged in a sort of mosaic 

 work, showing their convex or flattened summits over broad surfaces. These 

 surfaces are well calculated for grinding seaweeds, and crushing shell-fish or 

 corallines, as seen in the teeth of the scarus or wolf-fish, and the cuneiform 

 dental plates of the parrot-fish, which truly masticate their food. 



The teeth of Fishes, as already indicated,, are by no means limited to the 

 premaxillary and premandibular bones of the upper and lower jaws. In the 

 sharks and rays, they are thus confined to the fore-part of the mouth ; but in 

 the carp, all the teeth are at the back of the mouth, supported on the pharyn- 

 geal and basi-occipital bones. The parrot-fish has teeth, both at the front and 

 back of the mouth, i. e., on the premaxillary and premandibular bones, and on 

 the upper and lower pharyngeals. In most fishes, there are teeth, not only on 

 the above-named bones, but also on other bones around the middle part ol the 

 mouth, as on the palate bones, vomer, hyoid bones, and branchial arches, some- 

 times on the pterygoid, sphenoid, and nasal bones, and, though rarely, on the 

 true superior maxillse. Teeth are found in the median line, on the palate of the 



