becomes much diminished in size, or wholly obliterated by what may be called a secondary development of dentine. The pulp, in such cases, is formed into dentine, the new uniting or not with the previously formed tis-ue. Such dentine is usually traversed by vascular canals, around each of which the characteristic branching tubuli are araned radially, as those of the body of the tooth were arranged round the pulp cavity. Chemically, the dentine is, according to Bibra, composed of the following organic and inorganic substances.

Incisors of Adult Man.-Dentine.

Like bone, the dentine is coloured by feeding the animal on madder. After mixing a large proportion of madder in the food of a young pig about six weeks old, for a period of three weeks, the animal was destroyed, and the teeth, on examination, were found deeply coloured. A section shewed that the dentine of the pulp cavity, as well as the cement of the surface of the fang, had been equally affected with the osseous tissue of the skeleton. But I could not discover that the colour extended further in the tubuli than in the outer tubular tissue. Neither did the opaque line which marks the interior of the tubes, as seen by reflected light, seem coloured.

Structure of the enamel.

The enamel, the hardest of the dental structures, is composed of dense semi-transparent fibres, placed side by side, and closely united. Their form is an approximation to a six-sided prism, and their size tolerably uniform, being about the th of an inch in diameter. The direction taken by the enamel fibre is for the most part vertical to the surface of the dentine upon which it rests; those, therefore, which proceed from the flat surface of the crown, will rise vertically, while those

from the lateral surface of the tooth will be horizontal. Where the coronal surface of the dentine is concave, the enamel fibres of the opposite sides of the concavity form with each other angles, and meet at their external

ends. This juncture is frequently imperfect, and leaves a fissure, under which the dentine, being less protected from external influence than on the other parts of the crown of the tooth, is more frequently attacked by disease. The fissures on the crown of the molars are often subject to this defect of development. The ends of the enamel fibres are received into the shallow hexagonal depressions of the coronal surface of the dentine, from whence, in their course, they frequently describe curves. The direction taken by neighbouring fibres is not, however, at all times perfectly parallel; indeed, they often diverge, or cross each other at considerable angles. The curves also seem less regular than those formed by the dentinal tubes. Near the surface of the dentine, small linear interspaces not unfrequently exist between the enamel fibres. With these the terminal branches of the dentinal tubuli often communicate. When the interspaces exist in a great number, and extend to the surface of the enamel, they produce a pearly white appearance, and render the texture comparatively friable. This condition is indicative of imperfect development, and is followed by early decay in the affected teeth. The enamel fibre is not in all cases solid, but has running through the whole or part of its best seen in newly-developed enamel, but a length an extremely minute cavity. This is trace of the canal may be seen in that of adult teeth. Interposed between the individual fibres of the tissue under consideration are the remains of the membrane in which the development has taken place, and which, when hardened by the reception of This tissue, however, is not traceable except earthy matter, serves to connect the fibres. in imperfectly developed enamel, unless by the aid of acids. Transverse markings are sometimes observable upon the surface of the enamel fibres, but they are often indistinct, and their nature, though little understood, is probably connected with the development of this substance. An appearance resembling transverse lines is frequently seen in a specimen, which is traceable to the obliquity of the section, by which lines at equal distances are formed by the cut edges

of the enamel fibres. The enamel in the

incisors yields to chemical analysis 3·59 of organic, and 96-41 of inorganic matter.

cementum. In such teeth the cement is commonly pierced by one or more canals for blood-vessels (shown in fig. 8, d). Upon the neck of the teeth the cement exists, but in a thin layer, and is here traversed by minute tubes only, and thes, commencing on the surface, pass horizontally inwards towards the dentine; but further down the roct the layer thickens, and then the cement is hollowed by cells with branching tubes, their number being in proportion to the amount of cement. If the layer of cement be further thickened, we find it provided with canals for blood-vessels. In arrangement the tooth-bone presents the appearance of lamina concentrically placed, the centre of the tooth being their common centre; or should a vascular canal exist it is surrounded by concentric lamina-in this respect resembling, in its laminated arrangement, osseous tissue. The cells are scattered through the cement with some degree of regularity, generally, though not always, following the course as though placed between the lamina.

The majority of the radiating tubes of the cells pass, either towards the surface of the tooth, or, when such exists, towards the surface of a canal for a blood-vessel. Many branches also go towards the dentine, and anastomose with the terminal branches of the dentinal tubes, while a few follow the course of the length of the tooth, anastomosing freely with tubes pursuing a like direction. Frequently, however, a cell with its tubuli resembles a tuft of moss, the tubes taking in a mass one direction only, and that towards a surface upon which bloodvessels pass. In size the cells have very little uniformity, varying from the to the of an inch. The form is gene. rally oval, sometimes round, and occasionally fusiform. The traversing tubes are large at their commencement, but quickly assume a smaller diameter, which they retain to their termination.

Mixed with the cemented cells I have occasionally found tubes which pass across the cement towards the surface of the tooth, and present the peculiarity of having fever branchings, though equal in size to dentinal tubuli, and no doubt performing a similar function. Occasionally, however, the smaller tubuli of the cells enter them. Of the three dental tissues, the cement is softest and contains the largest amount of animal matter, analysis giving 29:42 of organic matter, and 70-58 of inorganic. In the teeth of lower animals, especially in the Edentata, in which enamel is absent, the cement forms a larger portion of the tooth. The cement when exposed is highly sensible to the touch, unless from any cause it has lost its vitality. When from recedence of the gum a tooth is unprotected, pressure with the nail upon the ex

posed surface will produce severe pain, which sometimes endures a short time after the removal of the cause.

Having deecribed especially the dental tissues, I will now give you some account of the properties they possess in common, of their relation to each other, and of their reactions to the vascular system.

It will be recollected, that, strictly speaking, all tissues are in themselves extra-vascular, that vessels do not permeate their substance, but pass only between their fibres, lamina, or granules, whatever be the structure of the tissue. Thus, in muscles, capillaries pass in the interstices between the primitive muscular fibres; in bone they pass between the lamina; and in the brain between the tubes and granules. But we regard a tissue to be highly organised, or not, in proportion to the relative frequency or absence of capillaries and vessels in the interstices. Thus we speak of the highly organised tissue of the brain, from the vast number of capillaries which traverse at short intervals its substance; while from their absence we regard the cornea as possessing a less degree of organisation.

Taking the relative frequency of vessels in a tissue as an index of the degree of its organization, teeth will be placed near the bottom of the scale, but different grades will be given to their three component tissues.

All that seems necessary for the healthy existence of a tissue is the proximity of a vascular current; more or less close to the individual elements, according to the character of the particular tissue in question. But in tissues where frequent interspaces for vascular currents would interfere with the functions, we find in the absence of vessels special arrangement providing for the due nutrition of the part. In no instance are these arrangements more beautiful than in osseous and dental structures; for in each of those their functions require that there should be great power of mechanical resistance. In the tooth we find that the centre is hollowed in the form of an arch, in which lie free from injury the dental vessels and nerves, while the tubes and fibres of the dental substance are placed vertical to the surface of the arch, thus giving to the whole and each tube or fibre the position in which their greater power of resistance exists, and at the same time providing for the nutrition of each part by the permeability of the tubes, which passing from the vascular surface, radiate, and by their branching pass to every part of the tooth, not even excluding the enamel.

To return to the consideration of the relative degrees of organisation of the dental tissues. The cement or tooth-bone when collected in any amount is possessed of vessels, as well as with cells and radiat

ing tubes in connexion with the vascular surfaces. To this element of the tooth we must, in accordance with the above plan, give the highest place. The dentine, possessing sometimes, though not constantly, vessels, has in all cases its tubes or capillary pores opening directly upon a vascular surface; this then must be considered as the second, while the enamel itself without vessels is only connected with a vascular surface, by the intervening dentinal tubes, and holds the third or lowest degree of organisation of the three dental tissues.

If the relative density of tissues be in proportion to the low degree of vitality, still the dental substances will hold the above arrangement. Again, if the relative sensibility of tissues be regarded as an index of their degree of vitality, still the same places must be accorded to the cement, dentine, and enamel.

It is a law of nature, that in passing from one form of organised matter to another, no sudden transition shall be made, but that the individual changes shall be so gradual as to be almost imperceptible.

This law we find beautifully exemplified in the gradual change of structure in passing from the cement to the dentine, and from the latter to the enamel.

The cement and dentine possess so many properties in common, and are often so like each other, that in some specimens it is difficult to determine to which of the two tissues the various parts belong. Thus, in the cementum, we find tubuli terminating in open mouths upon a vascular surface, such as the surface of the root of a tooth, while in the dentine we observe the presence of cells with radiating tubes: indeed, we see the dentinal tubuli themselves taking the form of cells with tubular branchings. In the tooth of the Kangaroo the two tissues are fairly mixed. Numerous vascular canals pass from the pulp cavity to the exterior of the tooth, and each canal has scattered round it cemental cells, the radiating tubes of which either pass into the canals or connect themselves with the dentinal tubes.

From the facts which I have laid before you, we are led to infer, that the dentine is but a modification of the cement; that the dentinal tubes are but elongated cemental cells, and that this elongation is necessary to enable the tooth to perform its allotted function.

In tracing the relations existing between the dentine and enamel we find the change in passing from one structure to another equally gradual.

highly developed present points of marked dissimilarity.

From what has already been said of the dental tissues it will be seen that the area of the tubes, of the cemental cells, and of the tubes or interspaces of the enamel, form no part of the tissues themselves, but are in fact spaces in them. These spaces we have seen are of characteristic form in each tissue, and we have considered their relations, &c. &c. It now remains to say a few words on the structural relations of the dental to other tissues. Before going to this point allow me to state in a few words the ultimate structure of osseous tissue, in order that in comparing the dental structures with each other we may understand their relations to the tissues to which they are most nearly connected.

Bone is composed of extremely minute granules closely united to each other, and so disposed as to form lamina. The lamina are concentrically arranged, the inner layer forming the parietes of a tube for the transmission of one or more vessels. Between, or in the lines of the lamina, cells of an oval, or round shape, flattened on their side, occur, from which proceed numerous minute branchings: tubes which are [directed either to a vascular surface, on which to end by an open mouth, or are directed towards other tubes, with which to anastomose, thereby connecting the cells. Thus, in bone, as in the cementum, and in dentine, and indirectly in the enamel, we find a set of capillary tubes, commencing upon a surface, bathed by vascular currents, and passing into the structure, and there establish a perfect continuous network of tnbes, so that a fluid may pass through the whole mass. That this arrangement is subservient to nutrition of the texture is sufficiently apparent, when we consider its relations to the vascular system, coupled with the fact, that these tubuli must be filled with fluid, even by atmospheric pressure, and that the only source of fluid is the blood.

That the tubes do contain fluid is proved by the following experiments. After removing several teeth, I carefully wiped the external surface; I broke the tooth, and removed the pulp, and made the surface dry. The fragments of dentine and cement (the enamel having been in great part broken off) were then placed in a warm room to dry, and in the course of a few hours lost one part in eight by weight, without having suffered any loss in bulk.

From these considerations it is seen that osseous and dental tissue are in the form and arrangement of their cells and tubes very closely allied, but the relations of the ultimate tissues are yet closer; for dentine and cementum, and probably enamel, are built up, like bone, of more or less spherical gra