268

COMPLEX AND COMPOUND TEETH.

crown of the tooth, but is greater at the base. All the inflected folds of cement, at the base of the tooth, have the same complicated disposition with increased extent; but, as they approach their termination towards the upper part of the tooth, they also gradually diminish in breadth, and consequently penetrate to a less distance into the substance of the tooth. Hence, in such a section as is delineated (Fig. 4), it will be observed that some of the convoluted folds, as those marked c c, extend near to the centre of the tooth; others, as those marked c', reach only about half way to the centre; and those folds, c", which, to use a geological expression, are "cropping out," penetrate to a very short distance into the dentine, and resemble, in their extent and simplicity, the converging folds of cement in the fangs of the tooth of the ichthyosaurus.

The disposition of the dentine, d, is still more complicated than that of the cement. It consists of a slender, central, conical column, excavated, by a conical pulp-cavity, a, for a certain distance from the base of the tooth; and this column sends from its circumference, radiating outwards, a series of vertical plates, which divide into two, once or twice, before they terminate at the periphery of the tooth. Each of these diverging and dichotomising plates gives off, throughout its course, smaller processes, which stand at right angles, or nearly so, to the main plate. They are generally opposite, but sometimes alternate; many of the secondary plates or processes, which are given off near the centre of the tooth, also divide into two before they terminate, as at n; and their contour is seen, in the transverse section, to partake of all the undulations of the folds of cement which invest them, and divide the dentinal plates and processes from each other.

Another kind of complication is produced by an aggregation of many simple teeth into a single mass.

The examples of these truly compound teeth are most common in the class of fishes; but the illustration here selected is from the mammalian class. Each tooth of the Cape

Fig. 5.

ant-eater (Orycteropus), presents a simple form, is deeply set in the jaw, but without dividing into fangs; its broad and flat base is porous, like the section of a common cane. The canals to which these pores lead, contain processes of a vascular pulp, and are the centres of radiation of as many independent series of dentinal tubules. Each tooth, in fact, consists of a congeries of long and slender prismatic denticles of dentine, which are cemented together by their ossified capsules, the columnar denticles slightly decreasing in diameter, and occasionally bifurcating as they approach the grinding surface of the tooth. Fig. 5 gives a magnified view of a portion of the transverse section of the fourth molar, showing c, the cement; d, the dentine; the pulp-cavity of the denticles.

and P,

In the elephant the denticles of the compound molars are in the form of plates, vertical to the grinding surface and transverse to the long diameter of the tooth. When the tooth is bisected vertically and lengthwise, the three substances, d, dentine, e, enamel, and c, cement, are seen interblended, as in Fig. 6, in which p is the common pulp-cavity, and r one of the roots of this complex tooth.

Such are some of the prominent features of a field of observation which Comparative Anatomy opens out to our view-such the varied nature, and such the gradation of

Fig. 6.

DENTAL SYSTEM OF FISHES.

269

complexity of the dental tissues, which, up to December, 1839, continued, notwithstanding successive approximations to the truth, to be described, in systematic works, as a "phaneros," or a dead part or product, exhaled from the surface of a formative bulb !"*

GRINDER OF ELEPHANT.

Dental System of Fishes.-The teeth of fishes, whether we study them in regard to their number, form, substance, structure, situation, or mode of attachment, offer a greater and more striking series of varieties than do those of any other class of animals.

As to number, they range from zero to countless quantities. The lancelet, the ammocete, the sturgeon, the paddle-fish, and the whole order of lephobranchii, are edentulous. The myxinoids have a single pointed tooth on the roof of the mouth, and two serrated dental plates on the tongue. The tench has a single grinding tooth on the occiput, opposed to two dentigerous pharyngeal jaws below. In the lepidosiren a single maxillary dental plate is opposed to a single mandibular one, and there are two small denticles on the nasal bone. In the extinct sharks with crushing teeth, called ceratodus and In the

tenodus, the jaws were armed with four teeth, two above and two below. Fig 7.

himara, two mandibular teeth are opposed to four maxillary teeth. From this low point See the Fasciculus of M. de Blainville's great work, "Ostéographie et Odontographie d'Animaux Vertébrés," which he submitted to the Academy of Sciences of the Institute of France on the same day, December 16th, 1839, on which I communicated, on the occasion of my election as corresponding member of that body, my "Theory of the development of dentine by centripetal calcification and conversion of the cells of the pulp."

270

DENTAL SYSTEM OF FISHES.

the number in different fishes is progressively multiplied, until in the pike (Fig. 7), the siluroids, and many other fishes, the mouth becomes crowded with countless teeth.

Fig. 8.

With respect to form, it may be premised that, as organized beings withdraw themselves more and more, in their ascent in the scale of life, from the influence of the general polarizing forces, so their parts progressively deviate from geometrical figures: it is only, therefore, in the lowest vertebrated class that we find teeth in the form of perfect cubes, and of prisms or plates with three sides (as in myletes), four sides (as in scarus), five or six sides (as in myliobates, Fig. 8). The cone is the most common form in fishes: such teeth may be slender, sharppointed, and so minute, numerous, and closely aggregated, as to resemble the plush or pile of velvet. These are called "villiform teeth" (dentes villiformes, Lat.; dents en velours, Fr.) All the teeth of the perch are of this kind. When the teeth are equally fine and numerous, but longer, these are called "ciliiform" (dentes ciliiformes); when the teeth are similar to but rather stronger than these, they are called "setiform" (dentes setiformes, Lat.; dents en brosse, Fr.): the teeth in the upper jaw of the pike (Fig. 7) are of this kind. Conical teeth, as close-set and sharp-pointed as the villiform teeth, but of larger size, are called "rasp-teeth" (dentes raduliformes, Lat. ; dents en rape, or en cardes, Fr.): the pike presents such teeth on the back part of the vomer. The teeth of the sheat-fish (Silurus glanis) present all the gradations between the villiform and raduliform types. Setiform teeth are common in the fishes thence called Chatodonts; in the genus Citharina they bifurcate at their free extremities; in the genus Platax they end there in three diverging points, and the cone here merges into the long and slender cylinder. Sometimes the cone is compressed into a slender trenchant blade: and this may be pointed and recurved, as in the murana; or barbed, as in trichiurus and some other Scomberoids; or it may be bent upon itself, like a tenter-hook, as in the fishes thence called Goniodonts. In the bonito may be perceived a progressive thickening of the base of the conical teeth; and this being combined in other predatory fishes with increased size and recurved direction, they then resemble the laniary or canine teeth of carnivorous quadrupeds, as we see in the large teeth of the pike (Fig. 7), in the lophius, and in certain sharks.

(Myliobates).

The anterior diverging grappling teeth of the wolf-fish (Fig. 9), i, form stronger cones; and by progressive blunting, flattening, and expansion of the apex, observable in different fishes, the cone gradually changes to the thick and short cylinder, such as is seen in the back teeth of the wolf-fish, m, and in similar grinding and crushing teeth in other genera, whether the fishes be feeders on sea-weeds, or on crustaceous and testaceous animals. The grinding surface of these short cylindrical teeth may be convex, as in the sheep's-head fish (Sargus); or flattened, as in the pharyngeal teeth of the wrasse (Labrus). Sometimes the hemispheric teeth are so numerous, and spread over so broad a surface, as to resemble a pavement, as in the pharyngeal bones of the wrasse; or they may be so small, as well as numerous, as to give a granulated surface to the part of the mouth to which they are attached, when they are called, in ichthyology, dentes graniformes.

TEETH OF THE WOLF-FISH.

271

A progressive increase of the transverse over the vertical diameter may be traced in the molar teeth of different fishes, and sometimes in those of the same individual, as in labrus, until the cylindrical form is exchanged for that of the depressed plate. Such dental plates (dentes lamelliformes) may be formed not only circular, but elliptical, oval, semilunar, sigmoid, oblong, or even square, hexagonal, pentagonal, or triangular; and the grinding surface may present various and beautiful kinds of sculpturing. The broadest and thinnest lamelliform teeth are those that form the complex grinding tubercle of the diodon.

In the sharks and rays the teeth are supported by the upper and lower jaws, as in most quadrupeds; but many other fishes have teeth growing from the roof of the mouth, from the surface of the tongue, from the bony hoops or arches supporting the gills, and some have them developed from the bone of the nose and the base of the skull. In the carp and tench the teeth are confined to this latter unusual position, and to a pair of bones, called "pharyngeal," which circumscribe the back outlet of the mouth.

Fishes exhibit, moreover, a greater range of variety in the mode of attachment of the teeth than any other class of animals. In the sharks, and the singular fish called the "angler," the teeth are moveable, their base being tied by ligaments to the jaw. In the angler the ligaments are so inserted that they do not permit the teeth to be bent outwards beyond the vertical position, but yield to pressure in the contrary direction, by which the point of the tooth may be directed towards the back of the mouth; the instant, however, that the pressure is remitted, the tooth returns through the elasticity of the bent ligaments, as by the action of a spring, to its usual erect position; the deglutition of the prey of this voracious fish is thus facilitated, and its escape prevented. The broad and generally bifurcate bony base of the teeth of sharks is attached by ligaments to the semi-ossified crust of the cartilaginous jaws; but they have no power of erecting or depressing the teeth at will.

The teeth of the sphyræna are examples of the ordinary implantation in sockets, with the addition of a slight anchylosis of the base of the fully-formed tooth with the alveolar walls; and the compressed rostral teeth of the saw-fish are deeply implanted in sockets; the hind margin of their base is grooved, and a corresponding ridge from the back part of the socket fits into the groove, and gives additional fixation to the tooth.

The singular and powerfully developed dental system of the wolf-fish (Anarrhicas lupus, Fig. 9) has been a subject of interest to many anatomists. Most of the teeth are powerful crushers; some present the laniary type, with the apices more or less recurved and blunted by use, and consist of strong cones, spread abroad, like grappling hooks, at the anterior part of the mouth, i, i.

The premaxillary teeth, 22, i, are all conical, and arranged in two rows; there are two, three, or four in the exterior row, at the mesial half of the bone, which are the largest; and from six to eight smaller teeth are irregularly arranged behind. There are three large, strong, diverging laniaries at the anterior end of each premandibular bone, and immediately behind these an irregular number of shorter and smaller conical teeth, which gradually exchange this form for that of large obtuse tubercles, m, m ; these extend backwards, in a double alternate series, along a great part of the alveolar border of the bone, and are terminated by two or three smaller teeth in a single row, the last of which again presents the conical form. Each palatine bone, 20, supports a double row of teeth, the outer ones being conical and straight, and from four to six in number;

the

272

TISSUE OF TOOTH IN FISHES.

inner ones two, three, or four in number, and tuberculate. The lower surface of the vomer, 13, is covered by a double irregularly alternate series of the same kind of large tuberculate crushing teeth as those at the middle of the promandibular bone. Thus the inside of the mouth appears to be paved with teeth, by means of which the wolf-fish can break in pieces the shells of whelks and lobsters, and effectually disengage the nutritious animal parts from them. All the teeth are anchylosed to more or less developed alveolar eminences of bones. From the enormous power of the muscles of

the jaws, and the strength of the shells which are cracked and crushed by the teeth, their fracture and displacement must obviously be no unfrequent occurrence; and most specimens of the jaws of the wolf-fish exhibit some of the teeth either separated at this line of imperfect anchylosis, or, more rarely, detached by fracture of the supporting osseous alveolar process.

Thus, with reference to the main and fundamental tissue of tooth, we find not fewer than six leading modifications in fishes.

Hard or true dentine-Sparoids, labroids, lophius, balistes, pycnodonts, prionodon, sphyræna, megalichthys, rhizodes, diodon, scarus;