IN THE HUMAN FRAME. 69 



The joint at the shoulder compared with the joint at the 

 hip, though both ball and socket joints discover a differ- 

 ence in their form and proportions, well suited to the dif- 

 ferent offices which the limbs have to execute. The cup 

 or socket at the shoulder is much shallower and flatter 

 than it is at the hip, and is also in part formed of cartilage 

 set round the rim of the cup. The socket, into which the 

 head of the thigh bone is inserted, is deeper, and made 

 of more solid materials.* This agrees with the duties as- 

 signed to each part. The arm is an instrument of motion, 

 principally, if not solely. Accordingly the shallowness of 

 the socket at the shoulder, and the yieldingness of the car- 

 ed to something more resembling that of a quadruped, where the heel 

 never reaches the ground, and where the paw is nothing more than 

 the phalanges of the toes. 



This arch of the foot, from the heel to the toe, has the astragalus, 

 resembling the keystone of an arch ; but, instead of being iixed, as in 

 masonry, it plays freely betwixt two bones, and from these two bones, 

 a strong elastic ligament is extended, on which the bone rests, sinking 

 or rising as the weight of the body bears upon it, or is taken off, and 

 this it is enabled to do by the action of the ligament which runs under 

 it. 



This is the same elastic ligament which runs extensively along the 

 back of the horse's hind leg and foot, and gives the fine spring to it, 

 but which is sometimes ruptured by the exertion of the animal in a 

 leap, producing irrecoverable lameness. 



Having understood that the arch of the foot is perfect from the heel 

 to the toe, we have next to observe, that there is an arch from side to 

 side ; for when a transverse section is made of the bones of the foot, 

 the exposed surface presents a perfect arch of wedges, regularly form- 

 ed like the stones of an arch in masonry. If we look down upon the 

 bones of the foot, we shall see that they form a complete circle hori- 

 zontally, leaving a space in their centre. These bones thus form three 

 different arches — forward ; across ; and horizontally : they are wedged 

 together, and bound by ligaments, and this is what we alluded to Avhen 

 we said that the foundations of the Eddystone were not laid on a better 

 principle ; but our admiration is more excited in observing, that the 

 bones of the foot are not only wedged together, like the courses of 

 stone for resistance, but that solidity is combined with elasticity and 

 lightness. 



Notwithstanding the mobility of the foot in some positions, yet when 

 the weight of the body bears directly over it, it becomes immoveable, 

 and the bones of the leg must be fractured before the foot yields. 



BelVs Treatise on Animal Mechanics. 



* The socket for the head of the thigh-bone is indeed deeper than 

 that at the shoulder, but the " materials" which form the concavities 

 are the same ; both are solid bone covered by cartilage, and both have 

 a rim of a strong fibro-cartilaginous texture, not only for the purpose 

 of rendering the socket deeper, but for preventing fractures of the 

 rim in robust exercises, to which, were it bony, it would be very li- 

 able. Faxton, 



G 2 



