650 THE EXTERIOR OF THE HORSE. 



as a whole, which it is necessary to classify methodically, in order to 

 render its study more simple. We should mention once for all that 

 this knowledge does not include that of the canines or the molars : 

 the former, because they do not exist in all subjects, and because their 

 wear is too irregular ; the latter, because they are practically not very 

 accessible in the examination, and because they have already been 

 discussed. 



A division which naturally presents itself consists in distinguishing, 

 on the one part, the phenomena that pertain to the milk-teeth, and, on 

 the other, those which relate to the permanent teeth ; but the following 

 subdivisions must also be made : 



1st. The period of eruption of the incisors of the first dentition. 



2d. The period of their levelling, their progressive wear, and their 

 expulsion from the alveolus. 



3d. The period of shedding of the deciduous incisors and their 

 replacement by those of the second dentition. 



4th. The period of levelling of the latter. 



5th. The period of the successive forms which their table of friction 

 presents, and the details that are observed there. 



Whence it follows that there are five principal periods from birtli 

 to extreme old age. 



First Period. Eruption of the Incisors of the First Den- 

 tition. At the moment of birth none of the incisors have penetrated 

 through the gums ; the anterior border of the pincers alone is outlined 

 underneath the mucous membrane, which is red and congested in both 

 jaws. (Plate I.) 



About One Week. The pincers generally appear in from six to 

 eight days, and the eruption of the superior precedes that of the 

 inferior. (Plate II.) 



It is of very little importance to examine the teeth at this epoch 

 of life, for the general aspect of the foal, -its gait, which is not yet 

 defined, the state of the hairs and of the mane, all, in a word, indi- 

 cate at a glance that the animal is still young. 



About One Month. The intermediates emerge through the 

 gums in from thirty to forty days, and the anterior border of the 

 pincers shows the effects of wear. (Plate III.) 



About Three Months. The jaws now present four teeth above 

 and below, which have emerged through the gums ; the pincers com- 

 mence to wear on their posterior border. (Plate IV.) 



About Four Months. The incisive arcade is wider trans- 

 versely ; the inferior intermediates have emerged still more through 



