732 COMPARATIVE ANATOMY 



may be found wherever there is cartilage or bone to hold them. In the 

 higher forms of animals, in fact, in all the amniotes, with the exception 

 of some squamata, teeth are found only on the margin of the jaws. 

 Turtles and all present forms of birds are toothless, though many extinct 

 birds, of which fossil remains have been found, did have teeth. It is in- 

 teresting to note that even in turtles and birds that have no teeth, the 

 dental ridge in which the teeth, of toothed animals do develop, is never- 

 theless present in the embryonic stages, it being assumed that this is 

 proof of their descent from toothed ancestors. 



It will be observed in Figure 429 that at first ectoderm thickens. 

 The layer of cells immediately beneath the ectoderm pushes downward 

 into the mesenchyme. These latter cells by multiplying rapidly, push 

 portions of this ingrowing plate of cells back up and form a sort of finger- 

 like projection covered by the plate it has pushed before it. The mesen- 

 chymal finger-like projection forms the pulp of the tooth, while the plate 

 of cells which covers it becomes what is known as the enamel organ. 

 The pulp forms several layers of cells, the outer ones becoming odonto- 

 blasts, so called because it is from these that the bone-layer-substance 

 dentine or ivory of the tooth is formed. This latter substance is a secre- 

 tion from the ends of the odontoblasts and it is this which causes it to 

 be somewhat prismatic in form. 



At the base of the enamel organ a denser substance called enamel 

 is secreted. This fits like a cap over the top and sides of the dentine. 

 The dentine continues to grow and forces the tooth up through the epi- 

 thelium so that the tip or crown then comes into position for use. The 

 nerve supply of the tooth comes from branches of the trigeminal or fifth 

 cranial nerve. Both nerves and blood vessels enter through the base of 

 the tooth. Usually, as soon as the teeth are fully formed, the odonto- 

 blasts cease growing. However, there are exceptions to this rule. The 

 tusks of elephants and the incisors of rodents function through life and 

 therefore continue to grow. 



In mammals an additional layer of modified bone, the cement, is 

 formed around the root of the tooth. It may even extend through the 

 crown. The teeth in the mouths of skates and some other elasmobranchs 

 are arranged very much like the scales on the surface of the jaw, that is, 

 in groups of five. In most of the vertebrates there is a succession of 

 teeth. 



Some animals, such as the shark and turtle, continue to renew and 

 shed their teeth. Such teeth are called deciduous. 



In mammals a second set of teeth usually arises directly behind or 

 above and below the first set, so that the ends of the second set, which 

 are to force their way through the jaw, push against the roots of the 

 teeth which are already in use. 



The group of first teeth which is formed in man is called the milk 

 dentition. The second is known as the permanent dentition. In some 



