VERTEBRATED ANIMALS. 261 



and consequently pass towards those which radiate from the central cav- 

 ity in the direction of the surface of the dentine, where this possesses a 

 similar vascularity, as was remarkably the case in the teeth of the great 

 extinct Megatherium. In the Human tooth, however, the cementum has 

 no such vascularity; but forms a thin layer (Fig. 446, b), which envelops 

 the root of the tooth, commencing near the termination of the cap of 

 enamel. In the teeth of many herbivorous Mammals, it dips down with 

 the enamel to form the vertical plates of the interior of the tooth; and 

 in the teeth of the Edentata, as well as of many Reptiles and Fishes, it 

 forms a thick continuous envelope over the whole surface, until worn- 

 away at the crown. 



657. Dermal skeleton. The skin of Fishes, of most Reptiles, and of 

 a few Mammals, is strengthened by plates of a horny, cartilaginous, bony, 

 or even enamel-like texture; which are sometimes fitted-together at their 

 edges, so as to form a continuous box-like envelope; whilst more com- 

 monly they are so arranged as partially to overlie one another, like the 

 tiles on a roof; and it is in this latter case that they are usually known 

 as scales. Although we are accustomed to associate in our minds the 

 ' scales' of Fishes with those of Reptiles, yet they are essentially-different 

 structures; the former being developed in the substance of the true skin 

 (with a layer of which, in addition to the epidermis, they are always cov- 

 ered), and bearing a resemblance to cartilage and bone in their texture 

 and composition; whilst the latter are formed upon the surface of the 

 true skin, and are to be considered as analogous to nails, hoofs, etc., and 

 other ' epidermic appendages.' In nearly all the existing Fishes the scales 

 are flexible, being but little consolidated by calcareous deposit; and in 

 some species they are so thin and transparent, that, as they do not pro- 

 ject obliquely from the surface of the skin, they can only be detected by 

 raising the superficial layer of the skin, and searching beneath it, or 

 by tearing oft' the entire thickness of the skin, and looking for them near 

 its under surface. This is the case, for example, with the common Eel, 

 and with the viviparous Blenny; of either of which fish the skin is a 

 very interesting object when dried and mounted in Canada balsam, the 

 scales being seen imbedded in its substance, whilst its outer surface is 

 studded with pigment-cells. Generally speaking, however, the posterior 

 extremity of each scale projects obliquely from the general surface, carry- 

 ing Before it the thin membrane that incloses it, which is studded with 

 pigment-cells; and a portion of the skin of almost any Fish, but especially 

 of such as have scales of the ctenoid kind (that is, furnished at their pos- 

 terior extremities with comb-like' teeth, Fig. 448), when dried with its 

 scales in situ, is a very beautiful opaque object for the low powers of the 

 Microscope (Fig. 447), especially with the Binocular arrangement. Care 

 must be taken, however, that the light is made to glance upon it in the 

 most advantageous manner; since the brilliance with which it is reflected 

 from the comb-like projections entirely depends upon the angle at which 

 it falls upon them. The only appearance of structure exhibited by the 

 thin flat scale of the Eel, when examined microscopically, is the presence 

 of a layer of isolated spheroidal transparent bodies, imbedded in a plate 

 of like transparence; these, from the researches of Prof. W. C. William- 

 son 1 upon other scales, appear not to be cells (as they might readily be 



1 See his elaborate Memoirs ' On the Microscopic Structure of the Scales and 

 Dermal Teeth of some Ganoid andPlacoid Fish,' in "Philos. Transact.," 1849; and 

 ' Investigations into the Structure and Development of the Scales and Bones of 

 Fishes,' in " Philos. Transact.," 1851. 



