DENTAL FORMULA. 81 



formula of a given animal. The dental formula of a child 



23 -^ 1 o 2 



over two years of age is thus — di. ^ dc. : — - dm. -^ = 20 ; 



which means that the child should have two incisors, one 

 canine, and two molars on each side of each jaw. 



The neck of the sac of each deciduous tooth gives off a 

 diverticulum, in which one of the permanent teeth is de- 

 veloped; as it grows, it causes the absorption of the fang 

 of the corresponding deciduous tooth, which thus becomes 

 shed, and is replaced from below by the permanent tooth. 

 The same letters, but without the prefix d, are used for the 

 permanent incisors and canines; but the permanent teeth, 

 which replace the deciduous molars, are called premolars, 

 and have the symbol pm. Furthermore, three or, it may be, 

 four permanent grinding teeth, on each side of each jaw, ani 

 developed altogether behind the milk molars, and thus comt- 

 into place without replacing any other tooth from below. 

 These are called molars, and have the symbol m. Thus the 

 formula of the permanent dentition in Man is written : 

 . 2.2 1—1 2.2 3.3 -. ^, , . 



*-o~o''-i — 1 P'"^- ^m ™- oo—^^ i there bemg two mcisors, 



one canine, two premolars, and three molars on each side 

 above and below. It is a rule of very general application 

 among the Mamm.alia, that the most anterior molar comes 

 into place and use before the deciduous molars are shed. 

 Hence, when the hindermost premolar, which immediately 

 precedes the first molar, comes into use by the shedding of 

 the last milk molar, the crown of the first molar is already a 

 little ground dovm ; and this excess of wear of the first molai 

 over the adjacent premolar long remains obvious. The fact 

 that, in the permanent dentition, the last premolar is less 

 worn than the first molar which immediately follows it, is 

 often a valuable aid in distinguishing the premolar from the 

 molar series. 



No vertebrate animal has teeth in any part of the alimen- 

 tary canal save the mouth and pharynx — except a snake 

 {Rachiodon), which has a series of what must be termed 

 teeth, formed by the projection of' the inferior spinous pro- 

 cesses of numerous anterior vertebrae into the oesophagus. 

 And, in the highest Vertebrata, teeth are confined to the pr& 

 maxillae, maxillae, and mandible. 



The Circulatory Organs. — The heart of the vertebrate 

 embryo is at first a simple tube, the anterior end of which 



