NERVOUS SYSTEM OF THE HORSE. 



49 



heavier than the spinal cord in certain animals, and it agrees fairly 

 well with the conclusions drawn from practical experience : — 



Teeth. — As horses' teeth have been described and illustrated in 

 Veterinary Notes for Horse-Owners, allusion will be made to them, 

 here, only with the object of explaining certain references to this 

 subject in the chapter on " The Evolution of the Horse." 



A horse's teeth are usually divided into front-teeth {incisors), 

 tushes {canine teeth), and back -teeth. At first there is a milk dentition, 

 generally, of 3 incisors and 3 back-teeth on each side of each jaw ; 

 making a total of 24 milk-teeth. Between the ages of 10 months and 

 5 years, the milk incisors and milk back -teeth become replaced by 

 permanent teeth, tushes spring up in male horses, and 3 more back- 

 teeth make their appearance behind the first 3 back-teeth on each 

 side of each jaw ; the total being 40. The first 3 back-teeth are 

 called premolars, and the second 3, which are not preceded by milk 

 teeth, are termed molars. Occasionally in the upper jaw, but very 

 seldom in the lower jaw, there are 4 premolars, in which case the ist 

 premolar is very small, and is usually called a "wolf's tooth." It is 

 always present in the young foetus. As the early ancestors of the 

 horse had 4 premolars, this premolar is evidently a vestigial tooth. 

 In the mare, the tushes are absent, or only in a rudimentary form. 

 The teeth are numbered from the front backwards. 



The body of a tooth is composed of an ivory-like substance {dentine), 

 which is covered by a layer of very hard material {enamel), by means of 

 which the animal cuts and masticates his food. On the cutting surface 

 of the incisors, the enamel forms a depression (becomes •' cupped "), and 

 thus gives rise to the "mark." The cupped condition of the enamel 

 greatly increases the cutting power of the teeth. There are two 

 depressions in each of the back teeth. The enamel is covered by 

 a layer of cement, which is nearly similar in structure to bone. The 

 cutting surface of a tooth is called the table ; and the portion of the 

 tooth outside the gum, the crown. 



Taking one side of both jaws and allowing for the appearance of a 

 1st premolar, the horse's dental formula is : — 



Incisors 



Canines \ 



Premolars 'tl , 



Molars 



(44) 40. 



