PARTS OF ANIMALS, IV. x. 



which provide another example of Nature's habit of 

 using an organ for secondary purposes, for she 

 employs the tail not only as a guard and covering for 

 the fundament but also in other serviceable \vays. 



There are differences too in the feet of quadrupeds. Hoofs, etc 

 Some have a solid hoof, some a cloven hoof ; others 

 have a foot that is divided into several parts. Sohd 

 hoofs are present in those animals which are large 

 and contain much earthy substance," which instead of 

 making horns and teeth forms an abscession'' so as 

 to produce nail, and o\\-ing to the abundance of it, it 

 produces not several separate nails but a single one, 

 in other words, a hoof. Because of this, these 

 animals in general have no hucklebone ; and also 

 because the presence of a hucklebone makes it 

 rather difficult to bend the hind leg freely, since a 

 limb that has one angle can be bent to and fro more 

 quickly than one that has several. It is a sort of 

 connecting-rod, and therefore practically interpolates 

 another bit of a limb betAveen the tΛvo, thereby in- 

 creasing the weight ; but it makes the animal's footing 

 more reliable. This explains why, when hucklebones 

 are present, they are present in the hind limbs 

 only, never in the front : the front limbs have to be 

 light and flexible because they go first, while the 

 hind limbs must be reliable and able to stretch. 

 Further, a hucklebone puts more force into a Ηολυ — a 

 useful point in self-defence — and animals >vhich have 

 one use their hind limbs in this way : if anything 

 hurts them they kick out at it. 



Cloven-hoofed animals have a hucklebone, as their 

 hind limbs are on the light side ; and that is the very 

 reason why they are cloven-hoofed : the bony sub- 

 stance stays in the joint and therefore is deficient in 



389 



