The enamel consists of very small six-sided fibres, set closely, side by side, nearly at right angles to the surface of the dentine, and covering the crown of the tooth as far

A. Enamel fibres viewed in transverse section.
Enamel fibres separated and viewed laterally.

B.

C. A section of a tooth at the junction of the dentine (a) with the cement (e); b, c, irregular cavities in which the tubules of the dentine end; d, fine tubules continued from them ;f, g, lacunæ and canaliculi of the cement. (Magnified about 400 diameters.)

U

as the neck, towards which the enamel thins off and joins the cement (Fig. 98, A, B).

Enamel is the hardest tissue of the body, and contains not more than 2 per cent. of animal matter.

The cement coats the fangs, and has the structure of true bone; but as it exists only in a thin layer, it is devoid of Haversian canals (Fig. 98, C).

13. The development of the teeth commences long before birth. A groove appears in the gum of each side of each jaw; and, at the bottom of this groove of the gum, five vascular and nervous papillæ arise, making twenty in all. The walls of the groove grow together, between and over each of the papillæ, and thus these become enclosed in what are called the dental sacs.

Each papilla gradually assumes the form of the future tooth. Next, a deposit of calcific matter takes place at the summit of the papillæ, and extends thence downwards, towards its base. In the crown the deposit takes on the form of enamel and dentine; in the root, of dentine and cement. As it increases it encroaches upon the substance of the papilla, which remains as the tooth pulp. The fully formed teeth press upon the upper walls of the sacs in which they are enclosed, and, causing a more or less complete absorption of these walls, force their way through. The teeth are then, as it is called, cut.

The cutting of this first set of teeth, called deciduous, or milk teeth, commences at about six months, and ends with the second year. They are altogether twenty in number-eight being cutting teeth, or incisors; four, eye teeth, or canines; and eight, grinders, or molars.

Each dental sac of the milk teeth, as it is formed, gives off a little prolongation, which becomes lodged in the jaw, enlarges, and develops a papilla from which a new tooth is formed. As the latter increases in size, it presses upon the root of the milk tooth which preceded it, and thereby causes the absorption of the root and the final falling out, or shedding, of the milk tooth, whose place it takes. Thus every milk tooth is replaced by a tooth of what is termed the permanent dentition. The permanent incisors and canines are larger than the milk teeth of the same name, but otherwise differ little from them. The permanent molars, which replace the milk molars, are

small, and their crowns have only two points, whence they are called bicuspid. They never have more than two fangs.

14. We have thus accounted for twenty of the teeth of the adult. But there are thirty-two teeth in the complete adult dentition, twelve grinders being added to the twenty teeth which correspond with, and replace, those of the milk set. When the fifth, or hindermost, dental sac of the milk teeth is formed, the part of the groove which lies behind it also becomes covered over, extends into the back part of the jaw, and becomes divided into three dental sacs. In these, papillæ are formed and give rise to the great permanent back grinders, or molars, which have four, or five, points upon their square crowns, and, in the upper jaw, commonly possess three fangs.

The first of these teeth, the anterior molar of each side, is the earliest cut of all the permanent set, and appears at six years of age. The last, or hindermost, molar is the last of all to be cut, usually not appearing till twenty-one or twenty-two years of age. Hence it goes by the name of the "wisdom tooth."

15. Muscle is of two kinds, striated or striped, and smooth, plain, or unstriated. Striated muscle, of which all the ordinary muscles of the trunk and limbs consist, is composed of a number of long parallel cylindrical fibres, called elementary or ultimate muscular fibres, which are bound together by connective tissue into small bundles. These small bundles again are united into larger bundles, and these into one aggregate, by connective tissue, which supports the vessels and nerves of the muscle, and usually forms at one or both ends of the muscle a tendon (see Lesson VII.), and sometimes gives rise to a dense sheath or fascia on its exterior.

Into the ultimate muscular fibre neither vessels, nor connective tissue, enter. Each fibre is, however, enveloped in a sheath formed by a tough, elastic, transparent structureless membrane, the sarcolemma (Fig. 99, D, b).

The sarcolemma is not contractile, but its elasticity allows it to adjust itself, pretty accurately, to the changes of form of the contractile substance which it contains.

This contractile substance, when uninjured, presents a very strongly-marked transverse striation, its substance

appearing to be composed of extremely minute disks of a partially opaque substance, imbedded at regular intervals in a more transparent matter. A more faint striation, separating these disks into longitudinal series, is also observable. When the sarcolemma is torn, the contractile substance of dead muscle may, under some circumstances, be either divided into disks (Fig. 99, C), but it may be

A, a muscular fibre, devoid of sarcolemma, and breaking up at one end into its fibrilla; B, separate fibrilla; C, a muscular fibre breaking up into disks; D, a muscular fibre, the contractile substance of which (a) is torn, while the sarcolemma (6) has not given way. (Magnified about 350 diameters.)

more readily broken up into minute fibrilla (Fig. 99, A, B), each of which, viewed by transmitted light, presents dark and light parts, which alternate at intervals corresponding with the distances of the transverse striæ in the entire fibre. Nuclei are observed here and there in the contractile substance within the sarcolemma.

In the heart, the muscular fibres are striated, and have the same essential structure as that just described, but they possess no sarcolemma.

A. Seen longitudinally. The width of the meshes corresponds to that of an ultimate fibre. a. small artery; b, small vein.

B. Transverse section of striated muscle. a, the cut ends of the ultimate fibres; b, capillaries filled with injection material; c, parts where the capillaries are absent or not filled.

Smooth muscle consists of elongated band-like fibres, devoid of striation, each of which bears a rod-like nucleus. These fibres do not break up into fibrillæ, and have no sarcolemma (Fig. 101).

16. Nervous tissue contains two elements, nerve-fibres and ganglionic corpuscles. Ordinary nerve-fibres, such as constitute the essential constituents of all the cerebrospinal nerves except the olfactory, are during life, or when perfectly fresh, subcylindrical filaments of a clear, somewhat oily, look. But shortly after death, a sort of coagulation sets up within the fibre, and it is then found to be composed of a very delicate, structureless, outer membrane (which is not to be confounded with the neuri