178 MANUAL OF ANIMAL BIOLOGY 



Teeth. The teeth of all Vertebrates are regarded as homologous 

 structures which in their evolutionary history are related to the 

 placoid scales of certain Fishes. They are composed of a thin, 

 outer covering of enamel, which forms from the epithelium lining 

 the mouth, and dentine, which develops from the underlying meso- 

 dermal tissue. The latter constitutes the bulk of a tooth. Teeth 

 in the various Vertebrate groups are subject to very great varia- 

 tion in number, shape, and function. In certain of the higher 

 Vertebrates, including Man, the teeth are placed in deep pits, or 

 alveoli, in the jawbones. The portion of the tooth within an 

 alveolus is known as the root, while the exposed enamel-covered 

 portion is termed the crown. We recognize four types of teeth 

 in the higher forms, which, beginning anteriorly, are known as the 



INCISORS, CANINES, PREMOLARS, and MOLARS. (W. f. 104.) 



In Man and most of the Mammals, two sets of teeth are formed. 

 The first set in Man, the milk teeth, are replaced in the early years 

 of childhood by the permanent teeth which develop in the jaws 

 below the first set. The number of milk teeth varies somewhat, 

 with a normal of 20, while in the permanent set there are 32 teeth, 

 divided so that each half of each jaw contains 2 incisors, 1 canine, 

 2 premolars, and 3 molars. In order to state concisely the number 

 of teeth in an animal a dental formula is made use of in which 

 the letters i, c, p, and m indicate the incisors, canines, premolars, 

 and molars respectively, and the numbers of each kind in half of 

 the upper and lower jaws are shown by figures above and below a 

 line. Thus the dental formula of Man is indicated as if, c J, 

 p f, m f = 32. 



B. Vertebral Column. The vertebral column of the Frog is 

 very short. It consists of nine typical vertebrae, together with a 

 narrow, rod-like, unsegmented posterior extension, known as the 

 urostyle. Each vertebra consists of a solid oval portion, the 

 centrum, the center of which occupies the original position of the 

 notochord ventral to the spinal cord. Dorsally and laterally on 

 each centrum a bony structure, known as the neural arch, is 

 developed around the spinal cord. The space between the neural 

 arch and the centrum in each vertebra constitutes the neural 

 canal in which the spinal cord lies. Each neural arch has a dorsal 

 projection, the neural spine, and, except in the first vertebra, a 

 lateral projection on each side, the transverse process, to which 

 the muscles are attached. ( W. fs. 101 ; 103.) 



