84 ZOOLOGY 



organic fibrils. Vaso-dentine is permeated with blood-vessels, and 

 consequently appears red and moist in the fresh condition. Osteo- 

 dentine approaches bone in its structure and mode of development. 

 The free surface of the tooth is usually capped by a layer of enamel 

 (ZS), a dense substance, either structureless or presenting a delicate 

 fibrillation, containing not more than 3 to 5 per cent, of animal 

 matter, and being, therefore, the hardest tissue in the body. The 

 cement (ZC) coats that portion of the tooth which is embedded in 

 the tissues of the jaw, and sometimes forms a thin layer over the 

 enamel ; it has practically the structure of bone. At the inner end 

 of the tooth there is frequently an aperture (PH f ) leading into a 

 cavity (PH) filled in the fresh condition by the tooth-pulp, a sort 

 of connective-tissue plug abundantly supplied with nerves and blood- 

 vessels. 



In the development of a tooth (Fig. 778, B) the deep layer of the 

 buccal epithelium becomes invaginated and grows inwards into 

 the sub-mucosa in the form of a narrow cord, the enamel-organ 

 (SK). The distal end of this enlarges into a flask-like form, and 

 the bottom of the flask becomes invaginated (Ma) by the growth 

 of a conical process of the sub-mucosa, the dental papilla (ZK). 

 Mesoderm cells accumulate on the free surface of the papilla and 

 form a distinct layer of cells called odontoblasts (0). From these 

 the dentine is formed in successive layers, which gradually accumu- 

 late between the layer of odontoblasts and the inner or invaginated 

 layer of the enamel-organ. The lower, or proximal, part of the 

 papilla remains uncalcified and forms the tooth-pulp. The enamel 

 is formed by the deposition of successive layers of calcific matter 

 from the inner or invaginated layer of the enamel-organ, the cement 

 by the ossification of the tissue immediately surrounding the papilla. 

 Thus the tooth is partly of ectodermal, partly of mesodermal origin. 



In some Fishes the scales or elements of the dermal exo- 

 skeleton pass insensibly into the teeth over the ridges of the jaws, 

 and agree with them in structure, so that there can be no doubt 

 as to the homology of the two. Teeth are, in fact, to be looked 

 upon as portions of the exoskeleton which have migrated from 

 the skin into the buccal cavity, and even into the pharynx, and 

 have there increased in size and assumed special functions. 



The tongue is a muscular elevation of the floor of the mouth, 

 supported by the basi-hyal, and usually more or less protrusible. 

 The roof of the buccal cavity in the embryo sends off a pouch, the 

 pituitary diverticulum (Fig. 769, A, pty. s.), which grows upwards, 

 and, losing its connection with the mouth, becomes attached to 

 the ventral surface of the brain as the pituitary body (pty. &.). It 

 may correspond with the neural gland of Urochorda. 



In terrestrial Craniata buccal glands are present, opening by 

 ducts into the mouth : the most important of these are the racemose 

 salivary glands which secrete a digestive fluid saliva, capable of 



