Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/255

The germ of the first permanent molar appears about the sixteenth week of embryo life ; that of the second permanent molar not until about the seventh month after birth ; whilst that of the wisdom tooth is not formed until about the sixth year The crown of the first molar is the first of the permanent teeth to erupt into the mouth, which it usually does in the sixth year. The incisors appear when the child is seven or eight ; the bicuspids when it is nine or ten ; the canines about twelve ; the second molars about thirteen ; and the wisdom teeth from seventeen to twenty five.

FIG. 24. A, the lower jaw of a child between four and five years old. 5, the last mi k molar, with the successional bicuspid tooth in the cavity of reserve imme diately below it; 6 and 7, the first and second peimanent molars in their sacs; 6, the cavity in connection with which the wisdom tooth is formed. B, the lower jaw "of a child about six years old; 6 and 7, the first and second per manent molars; 8, the papilla of the wisdom tooth developed in connection with its cavity 6. From Goodsir

In his dentition man is diphyodont as regards his incisor, canine, and premolar teeth, but monophyodont in the molar series.

From the description of the development of the teeth, it will have been seen that a tooth is made up of three hard tissues enamel, dentine, and. cement and of the soft vascular and nervous pulp. These tissues are not developed from one layer only of the blasto derm. The enamel is of epiblast origin, whilst the dentine, cement, arid pulp are derived from the mesoblast. A tooth in its funda mental development, as was long ago pointed out by Goodsir, must be referred to the same class of organs as the hairs and feathers. The enamel of the tooth, like the hair, is produced by a differentia tion of the involuted epithelium of the epiblast, whilst the dentine and pulp resemble the papilla of the hair, in proceeding from the mesoblast. The tooth-sac, like the hair-follicle, is also of meso blast origin. Whether the cement, as Eobiii and Magi tot have de scribed, be developed by means of a special cement organ, in the in terior of the tooth-sac, or be formed, as has been stated in this de scription, by the alveolo-dental periosteum, it is on either view de rived from the mesoblast. As to the origin of Nasmyth s membrane, there is a difference of opinion ; some regard it as a special cornifica- tion of the external cells of the enamel organ, in which case it would be from the epiblast ; whilst others consider it to be continuous with though structurally different from, the cement homologous, there fore, with the layer of cement, which in the horse, ruminants, and s .me other mammals covers the surface of the crowns of the teeth.

The tissues of a tooth have not all the same importance in the structure of a tooth. The dentine is apparently always present, but the enamel, or the enamel and cement, may be absent in the teeth of some animals. For example, the tusks of the elephant and narwhal, and the teeth of the Edentata, are without enamel, and in the Eodentia enamel is present on only the anterior sur face of the incisors. But though the enamel is not developed, or forms only an imperfect covering for the crowns of these teeth, yet an enamel organ is formed in the embryo jaws. In 1872 "W. Turner described a structure homologous with the enamel organ in relation with each of the dental papillae in the lower jaw of a fcetal narwhal ; but this organ did not exhibit a differentiation into the three epithelial layers, such as occurs in those teeth in which enamel is developed. Since then C. S. Tomes has seen an enamel organ in the embryo armadillo, and has also pointed out that, in teeth generally, enamel organs exist, quite irrespective of whether enamel subsequently does or does not form.

But further, the involution of the oral epithelium, and the coin cident formation of a primitive groove, take place not only where the teeth subsequently arise, but along the whole curvature of the future jaws ; whilst the production of dental papillae is restricted to the spots where the teeth are formed. Hence it would seem that the inflection of the oral epithelium is not so essential to the development of a tooth as the formation of a papilla. The inflected epithelium marks only a preliminary stage, and it may or may not be transformed into tooth structure. But that which is essential to the formation of a tooth is the production of the papilla which appears at the bottom of the primitive groove.

(w. t.)

DIGITALIS, or Foxglove, a genus of biennial and perennial plants of the natural order Scrophulariacece. The common or purple foxglove, D. purpurea, is common in dry hilly pastures and rocky places and by road sides in various parts of Europe ; it ranges in Great Britain from Cornwall and Kent to Orkney, but it does not occur in Shetland or in some of the eastern counties of England. It flourishes best in siliceous soils, and is not found in the Jura and Swiss Aips. The characters of the plant are as follows : stem erect, roundish, downy, leafy below, and from 18 inches to 6 feet or more in height ; leaves ilternate, crenate, rugose, ovate or elliptic-oblong, and of a dull green, with the under surface downy and paler than the upper ; radical leaves together with their petioles often i foot in length ; root of numerous, slender, whitish fibres ; flowers lf-2/ inches long, pendulous, on one side of the stem, purplish crimson, and hairy and marked with eye-like spots within ; segments of calyx ovate, acute, cleft to the base ; corolla obtuse, with the upper lobe entire or obscurely divided ; stamens four and didynamous (see vol. iv. p. 138, fig. 226); anthers yellow and bilobed ; capsule bivalved, ovate, and pointed ; and seeds numerous, small, oblong, pitted, and of a pale brown. As Parkinson remarks of the plant, " It flowreth seldome before July, and the seed is ripe in August ; " but it may occasionally be found in blossom as late as September. In one variety, common in gardens, the flowers are white ; in another their purple is of a coppery or metallic hue ; and not unfrequently in cultivated plants several of the uppermost blossoms may be united together so as to form a cup-shaped compound flower, through the centre of which the upper part of the stem passes. A figure of D. ptirpurea will be found in vol. iv. plate xi. Many species of foxglove with variously-coloured flowers have been introduced into Britain from the Continent. The plants may be propagated by off-sets from the roots, but are best raised from seed.

The foxglove (Ang.-Sax., foxes-dife, foxes-glofa) is known by a great variety of popular names in Britain. In the south of Scotland it is called bloody fingers ; further north, dead-men s-bells ; and on the eastern borders, ladies thimbles, wild mercury, and Scotch mercury. Among its "Welsh synonyms are menyg-dlyllon (elves gloves), menyg y llwynog (fox s gloves), bysedd cochion (red lingers), and bysedd y civn (dog s fingers). In France its designations are gants de notre dame, and doigts de la Vierge. The German name fingerhut (thimble) suggested to Fnchs, in 1542, the employment of the Latin adjective digitalis as a designation for the plant.

The leaves, gathered from wild plants when about two- thirds of their flowers are expanded, deprived usually of the petiole and the thicker part of the midrib, and dried, constitute the drug digitalis or digitalis folia of the pharmacopoeia. The prepared leaves have a faint odour and bitter taste ; to preserve their properties they must be kept excluded from light in stoppered bottles. They are occasionally adulterated with the leaves of Inula Conyza, Ploughman s Spikenard, which may be distinguished by their greater roughness, their less divided margins, and their odour when rubbed ; also with the leaves of Symphytum oflicinale, Comfrey, and of Verbascum Thapsus, Great Mullein, which unlike those of the foxglove have woolly upper and under surfaces. The powder, infusion, and tinc ture of digitalis are employed both externally and inter nally ; and its active principle, digitalin, may further be used for subcutaneous injection. Digitalin, according to Nativelle, is a crystallizable, neutral, inodorous, bitter substance, of the formula C 05 H 40 15 , insoluble in water and ether, but soluble in alcohol and chloroform. The earliest known descriptions of the foxglove are those given by Fuchs and Tragus about the middle of the 16th century, but its virtues were doubtless known to herbalists at a much remoter period. Gerarde, in his Herbal (1597), advocates the use of foxglove for a variety of complaints ; and John Parkinson, in the Theatrum Botanicum, or Theater of Plants (1640), tells us that