OCR Text |
Show 710 DR. E. LONNBERG ON THE [June 19, have on their anterior surface a layer of enamel which, measured on a longitudinal section about \ m m . from the tip, has a thickness of 400-435 fx. O n the posterior surface the enamel, measured at the same distance from the tip, has also a considerable thickness, namely about two-thirds of that of the anterior layer (fig. 9). This offers a striking contrast to the condition prevailing in the common Calf and the Lamb. In an unborn lamb the anterior layer of enamel, measured near the tip of the incisor, is 330 p in thickness, but the posterior layer is at the middle of the crown only 66 p or one-fifth of the anterior one, and is less than half that thickness Fig. 9. Section through the tip of a milk-incisor of a Musk-or not even one-tenth of the anterior layer. In a new-born common calf the incisors are provided with a layer of enamel on their anterior side which in thickness near the'tip equals that of the Musk-calf (about 400 /< or more) (cf. fig. 10, p. 711). But on the interior surface the layer of enamel is very thin and becomes towards the tip only 20-10 p. Such a thin layer is, of course, very soon worn off. The interior layer of enamel of the milk-incisors in Sheep, and still more in Cattle, can thus with full right be termed rudimentary. This condition is of course secondary ; and Ovibos with its fully developed layer of enamel, represents a more |