194 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



327. We have seen that the teeth are formed, in the first instance, 

 upon the surface of the Mucous membrane of the mouth ; and conse- 

 quently they really form a part of the external or dermo-skeleton, and 

 not of the internal or osseous skeleton. They correspond, therefore, 

 with the external skeletons of the Invertebrata ; and thus the analogy 

 which has been pointed out, between the enamel of teeth and the pris- 

 matic cellular substance of the shells of Mollusca, and between the den- 

 tine and the shells of the higher Crustacea, holds good also in regard 

 to the situation of these structures. Although the teeth are the only 

 ossified portions of the dermo-skeleton in Man, we find the body par- 

 tially or completely enclosed in an armour of bony scales or plates, in 

 certain Mammalia, Reptiles, and Fishes ; and in some species of the 

 last-named class, which have now ceased to exist, the scales seem to 

 have had the texture of Enamel. 



328. In connexion with the Teeth, the structure and development of 

 the Hair may be described ; this substance being generated very much 

 in the same manner as dentine, by the conversion of a pulp enclosed 

 in a follicle ; though the product of the transformation is different. 

 The Hair-follicle is formed by the inversion of the Skin, as the Tooth- 

 follicle is by an inversion of the Mucous membrane ; and it is lined by 

 a continuation of the epidermis. From the bottom of the follicle, a 

 sort of papilla rises up, formed of cells ; the exterior of this, which is 

 the densest part, is known as the bulb ; whilst the softer interior is 

 termed the pulp. The follicle itself is extremely vascular ; and even 

 the bulb is reddened by a minute injection; though no distinct vessels 

 can be traced into it. It has been imagined until recently, that the 

 Hair, like the other extra-vascular tissues, is a mere product of secre- 

 tion ; its material, which is chiefly horny matter of the same composition 

 with that of the epidermis and its other appendages ( 227), being ela- 

 borated from the surface of the pulp. This, however, proves to be a 

 very erroneous account of it ; as is shown by the results of microscopic 

 inquiries into its structure. Although the Hairs of different animals 

 vary considerably in the appearances they present, we may generally 

 distinguish in them two elementary parts ; a cortical or investing sub- 

 stance, of a fibrous horny texture; and a medullary or pith-like sub- 

 stance occupying the interior. The fullest development of both sub- 

 stances is to be found in the spiny hairs of the Hedge-hog, and in the 

 quills of the Porcupine, which are but hairs on a magnified scale. The 

 cortical substance forms a dense horny tube, to which the firmness of 

 the structure seems chiefly due ; whilst the medullary substance is com- 

 posed of an aggregation of very large cells, which seem not to possess 

 any fluid contents in the part of the hair that is completely formed. 

 The structure of the feather of Birds is precisely analogous : the cor- 

 tical horny tube existing alone in the quill, but being filled with a cel- 

 lular medulla in the stem of the feather itself. The smaller hairs of 

 the Sable (Fig. 56, B) show the cortical and medullary substances in a 

 very characteristic form ; the former being here plainly seen to be made 

 up of flattened imbricated cells resembling those of the epidermis ; whilst 

 the cells of which the latter is composed are nearly globular. In the 

 hair of the Musk-deer (Fig. 56, A), we find the medullary substance to 



