1248 THE DIGESTIVE SYSTEM. 



by absorption, particularly in front ; what remains of the temporary tooth is shed ; the 

 permanent tooth passes onwards through the enlarged alveolus, and, making its way to 

 the surface, appears above the gum. After some time, when the tooth has taken its final 

 position, the alveolus is again re-formed, first around its neck, and later on, as the root is 

 built up, around it also, and thus the tooth is permanently fixed. 



What the force is which causes the eruption, is a question that has not been answered 

 satisfactorily. That the growth of the root pushes up the crown was formerly the 

 favourite explanation. For several reasons, unnecessary to detail, this view is now dis- 

 carded, and a theory which attributes the impelling force to the blood pressure is looked 

 upon with more favour, although even this is not altogether satisfactory. (See Tomes' 

 Dental Anatomy, 5th Edition, page 211.) 



MORPHOLOGY OF THE TEETH. 



In most vertebrates below mammals all the teeth are alike in form ; such a dentition is said 

 to be homodont. In the majority of mammals, on the other hand, the teeth are arranged in 

 groups of different size and form ; such a dentition is heterodont. 



Again, mammals have, neglecting exceptional cases, but two functional sets of teeth ; they are 

 consequently said to be diphyodont. Most vertebrates below mammals, on the other hand, have 

 a continuous succession of teeth throughout life, and hence are said to be polyphyodont. 



Seeing that practically all lower vertebrates are provided with simple conical teeth, the evolu- 

 tion of the muiti-tuberculate mammalian molar has given rise to much speculation. The jaws of 

 the earliest fossil mammals found are furnished with tri-tubercular teeth, the three tubercles being 

 placed in an antero -posterior line ; by a rotation of two of the tubercles to the lingual or labial side, 

 as the case may be (a condition found in certain other fossil skulls), we arrive at a tri-tubercular 

 form, from which the transition to an ordinary mammalian molar is not difficult. As to how 

 the tri-tubercular tooth arose from the simple cone, two different views are advanced : one, that 

 it was formed by the union of several conical teeth as a result of the shortening of the jaw and 

 the crowding of the teeth together ; the other, that the single conical tooth developed on its 

 crown two subsidiary tubercles, one in front and the other behind, and that these tubercles 

 growing larger, the tooth assumed the tri-tubercular form. 



The complete or typical mammalian dentition, in its highest development, as in the horse, is 

 represented by the following formula : i. f, c. f, pm. f, m. = 44. In the dentition of man, there- 

 fore, one incisor and two premolars are wanting. Different views are held as to which teeth 

 have been suppressed most probably they are the second incisors, and the first and second or first 

 and last premolars. 



In general it may be said that the dentition of the lower races differs from that of the 

 higher, in that the dental arches are squarer in front, the teeth larger and more regular, the 

 canines stronger, the last molars better developed, and the tubercles on the molars more perfect, in 

 the lower than in the more civilised races. It may be mentioned, however, that the teeth of a 

 savage man, if seen in the mouth of a European, would be looked upon as an " exceedingly 

 perfectly formed set of teeth " (Tomes). 



To express the proportion in size of the crowns of the premolars and molars to that of the 

 skull in different races, Flower compared the distance from the front of the first premolar to the 

 back of the last molar, in situ, with the distance from the front of the foramen magnum to the 

 naso-frontal suture (basi -nasal length), in the form of a " dental index " 



Length of teeth x 100 _ 



Thus : ,. & . ^ 7= = Dental index. 



Basi-nasal length 



and by this means he has divided the various races into microdont (index 42 to 43, Europeans, 

 Egyptians, etc.), mesodont (index 43 to 44, Chinese, American Indians, Negroes, etc.), and macro- 

 dont (index 44 and upwards, Australians, Melanesians, etc.). 



DEVELOPMENT OF PRIMITIVE PHARYNX AND PHARYNGEAL PORTION OF THE 



MOUTH. 



The anterior blind termination of the foregut in the head region constitutes the 

 primitive pharynx. 



Its roof is formed by the tissues covering the under aspect of the mid- and fore-brain, 

 and its floor by the tissues overlying the heart and pericardium. Each side wall is a 

 lamina of tissue extending from the floor to the roof, continuous, in front, with the 

 bucco-pharyngeal membrane, which forms the anterior wall of the pharynx and separates 

 it from the stomodeum. 



In the roof are formed the tissues which form the hinder part of the base of the skull. 



In the side wall and in the floor extensive changes occur, connected with the appear- 

 ance of structures known as the visceral arches and pouches, and with the origin of 

 numerous structures from them and the development of the tongue. 



