nail; B, the Malpigian; C, horny layer; d, vertical cells, being the youngest and deepest; e, upper and more flattened cell of the Malpigian layer.

In reference to the formation of the nail, QUAIN says: "The nail, like the cuticle, is made up of epithelial cells. The oldest and most superficial of these are the broadest and hardest, but, at the same time, very thin, and so intimately connected together that their respective limits are scarcely discernible. They form the exterior horny part of the nail, and cohere together in irregular layers, so as to give this part a lamellar structure. On the other hand, the youngest cells, which are those situated at the root and under surface, are softer and of a rounded or polygonal shape. The deepest layer differs somewhat from the others, in having its cells elongated and arranged perpendicularly, as in the case of the epidermis. Thus the under part of the nail (Fig. 3, B) corresponds in nature with the Malpigian or mucous layer of the epidermis, and the upper part (C) with the horny layer. As in the case of the epidermis, the hardened scales of the nails may be made to reassume their cellular character by treatment with caustic alkali, and afterwards with water, and then it is seen that they still retain their nuclei.

"The growth of the nail is effected by a constant generation of cells at the root and under surface. Each successive series of these cells being followed and pushed from their original place by others, they become flattened into dry, hard and inseparably coherent scales. By the addition of new cells at the posterior edge the nail is made to advance, and by the apposition of similar particles to its under surface it grows in thickness, so that it is thicker at the free border than at the

root.

epidermis, the question at one time raised whether that membrane is continued underneath it, loses its significance. When a nail is thrown off by suppuration, or pulled away by violence, a new one is produced in its place, provided any of the cells of the deeper layers of the epithelium are left."

The hoof of the horse is developed in a similar manner to that of the nail of the human being. The outer portion, the wall or crust, is developed from the epidermic or epiblastic layer, and is analogous to the enamel of the teeth; while the inner portion is developed from the mesoblastic layer, and is analogous to the dentine of the teeth. The outer layer of the true dermoid structure-the mesoblastic-is formed into laminæ or ridges, and is very vascular. These laminæ fit between other laminæ in the horny portion, and at the point of union the ungualblasts, derived from the Malpigian layer, build the crust. It is formed from within outwardly, and from above downwardly, the bottom of the crust being the thickest. So eminent an authority as FLOWER says on this subject:

“The hoof of the horse corresponds to the nail or claw of other mammals, but is so constructed as to form a complete and very solid case to the expanded portion of the toe, giving a firm basis of support formed of a non-sensitive substance, which is continually renewed by the addition of material from within, as its surface wears away by friction against the ground."

Certain diseases will cause the shedding of the horny portion from the inner portion, and at death this horny portion can also readily be removed. The wearing away of the hoof causes an increased action of the cells formed from the Malpigian layer, a large portion of which, being in the formative state, can be separated at this point with comparative ease. If it were not for the laminæ of the epiblastic layer of the hoof wedging themselves between those of the mesoblastic layer, it would be easily torn off during life.

Another illustration of this governing principle is to be found in the horns of our domestic cattle. The outer layer (the true horn) is epiblastic, while the inner layer is mesoblastic, being formed from within. outwardly. If a section of fresh horn be cut, the different layers may be seen without the aid of a microscope. The outermost layer is invariably the hardest, and the layers gradually get softer and softer as we proceed inwardly, until the pure germinal or formative matter is reached. This is found to be of a gluey or sticky consistency. If the external horn be stricken from its internal structure, as I have frequently seen done, a sticky substance will be found to remain on that portion directly beneath; and if the outer or epiblastic horn layer be at once replaced and bound in position, union will take place, and it will become firm as before. If, however, the epiblastic portion stricken off be not replaced, and any of the formative cells of the deeper layer

remain, they will reproduce a new outer horny layer, though it will be inferior to the original one.

Fig. 4. Hair rudiment from an embryo of six weeks, magnified 350 diameters, after Kolliker in Quain's Anatomy. a, horny, and mucous b or malpigian layer of cuticle; i, limiting or basement membrane; m, cells (some of which are assuming an oblong figure), which chiefly form the future hair.

Fig. 5. Rudiment of a hair of the eyebrow, magnified 50 diameters (Kolliker). a, horny layer or cuticle; b, mucous layer; c, external layer of root-sheath; i, limiting or basement membrane; h, papillæ.

Fig. 6. Hair follicle from the eyebrow, with the hair just protruded. The inner layer of the root-sheath rises to the mouth of the hair follicle. a, horny and b, mucous or malpigian layer; e, hair knob; f, stem and g, point of hair; d, internal layer of root-sheath; h, papilla; i, limiting or basement membrane.

The diagrams 4, 5, 6, taken from Quain's Anatomy, aptly illustrate the growth of hair.

The similarity between the dipping down of the Malpigian layer of the skin into the papillary layer to form the hair follicle, in Fig. 4, and the dipping down of the same layer to form the enamel organ is very striking.

It will be seen that the papilla grows from the corium, or true derm, from below the basement membrane, which is analogous to the dentine germ of the teeth.

The following is an extract on the development of the hair, from the ninth edition of Quain's Anatomy, Vol. II, page 252:

"On the surface of the papilla, at the bottom of the follicle" (at the surface of the dentine, at the bottom of the enamel) "there is a continual multiplication of cells. These, for the most part, lengthen out and unite into the flattened fibres which compose the fibrous part of the hair, and certain of them getting filled with pigment give rise to the colored streaks and patches in that tissue; their nuclei, at first, also lengthen in the same manner, at last they partly become indistinct. The cells next the circumference expand into the scales which form the imbricated cuticular layer. The medulla, where it exists, is formed by the cells nearest the centre.

"The substance of the hair, of epidermic nature, is, like the epidermis itself, quite extra-vascular, but like that structure also, it is organized and subject to internal organic changes."

This is precisely the case with the enamel of the teeth. Admitting that the enamel contains only two and one-half per cent. of organic matter, and that it is extravascular, it certainly must have nourishment to sustain this percentage, small though it is, and it receives it by imbibition, in a manner similar to other extravascular structures. Every dentist knows that the enamel of the teeth is affected by disease through the system just as the hair is. Iron and medicinal acids taken by the mouth do not affect the enamel of the teeth by actual contact with it; they act through the system; and such being the fact, the theory that enamel is dead matter, needing and receiving no nourishment, falls to the ground.

Figs. 7 and 8 are fac similes, as near as could be cut, of a slide containing a section of the jaw of a rat, which was prepared by Mr. Cole, of London, one of the most celebrated and expert microscopical slide makers in Europe. The line surmounting the dentine, however, in Fig. 7, is purely diagramatical. These figures show a temporary and a permanent tooth. Between the enamel and the dentine is a space, which space is also shown in the cut by Dr. Williams presented below; in fact all good sections show the same formation.

When the enamel is not fully developed, or the younger the organ is, the greater the space will be found to be. It becomes less and less

as the tooth matures. This space is occupied by exceedingly young germinal matter, and, as the tooth develops the space becomes filled up, and there is union between the dentine and the enamel. If a tooth be extracted before it is fully matured, and then dried, the enamel and dentine can readily be separated. At this point, also, the formed enamel rods can be seen, and on the under surface the soft germinal matter is plainly visible.

Fig. 7. Vertical section of the jaw of a rat, showing a temporary and a permanent tooth germ.

It appears in the section under the microscope, also in Fig. 8, as though there was a line of demarcation, or an edge to the formed enamel. In reality no such line exists, it being an "optical delusion " in the microscopical examination.