PROFESSOR HUXLEY'S LECTURES. 289 



This (showing) is the heel of the horse, and here is the great median 

 toe, answering to the third toe in our own foot ; and here we have 

 upon each side two little splint-bones, just as in the fore-limb, which 

 represent the rudiments of the second and the fourth toes rudiments, 

 that is to say, of the metatarsal bones, the remaining bones having 

 altogether vanished. Let me beg your attention to these peculiar- 

 ities, because I shall have to refer to them by-and-by. The result of 

 this modification is, that the fore and hind limbs are converted into 

 long, solid, springy, elastic levers, which are the great instruments 

 of locomotion of the horse. 



As might be expected, and as I have already said, the apparatus 

 for providing this machine with the fuel which it requires is also of a 

 very highly differentiated character. A horse has, or rather may 

 have, forty-four teeth, but it rarely happens that in our existing horses 

 you find more than forty for a reason which I will communicate di- 

 rectly and in a mare it commonly happens that you find no more than 

 thirty-six, because the " tushes," or canine teeth, of the mare are rarely 

 developed. Then there are some curious peculiarities about these teeth. 

 As every one who has had to do with horses knows, the cutting 

 teeth the incisors are six above and six below, and those incisors 

 present what is called a "mark;" at least, that mark is usually pres- 

 ent in horses up to a certain age. It is a sort of dark patch across 

 the middle of the tooth. The presence of that dark patch arises from 

 a great peculiarity in the structure of the horse's incisor tooth. It is 

 in fact considerably curved, witli a deep pit in the middle of the 

 crown, and then a long fang. In the young foal this pit is very deep. 

 As the animal feeds, this pit becomes filled up w T itk its fodder, that 

 fodder becomes more or less carbonized, and then you have the dark 

 mark, and the reason the dark mark serves as an indication of age, 

 for, as the horse feeds, this is more and more worn down, until at last, 

 in an aged horse, the tooth is worn beyond the bottom of the pit, and 

 the mark disappears. Then, as I said, the male horse generally has 

 canine teeth. We need not notice their structure particularly. Fol- 

 lowing that, you may occasionally notice a very small and rudimen- 

 tary tooth, but it is very often absent. It really represents the first 

 tooth of the grinding series. Then there follow six great teeth, with 

 exceedingly long crowns. The crowns, in fact, are so long that the 

 teeth take a very long time to wear down, whence arises the possi- 

 bility of the great age to which horses sometimes attain. This is 

 shown in the side diagram. Then the pattern and structure of a 

 horse's tooth are very curious. The crown of the horse's tooth pre- 

 sents a very complicated pattern ; that is to say, supposing this to be 

 one of the grinders of the left side (illustrating) above, there is a kind 

 of wall like a double crescent. Then there are two other crescents, 

 which fall in that direction, and these are complicated by folds, and 

 all the spaces between these crescentic ridges are filled up by a kind 

 vor,. x. 19 



