314 nature's teachings. 



On the left hand are seen the upper part of the human thigh- 

 bone and that part of the hip-bone into which it fits. 



The reader will see that at its upper end the bone takes 

 rather a sharp turn, and is then modified into a ball. This 

 ball fits into a corresponding socket, technically named the 

 " acetabulum/ ' and is thereby endowed with freedom of motion 

 in almost every direction. Generally we do not practise our 

 limbs sufficiently to develop that full freedom, but those who 

 have seen any good professional acrobats must have been 

 struck with the wonderful mobility of which the human body 

 is capable. 



The socket is not a deep one, but dislocation of the hip is 

 exceedingly rare, the bone being held in its place by three 

 powers. The first is due to a short ligament, which, however, 

 does not always exist, but, when it is present, is useful in 

 retaining the bone in its place. Then there is the contractile 

 power of the thigh muscles, which are always forcing the ball 

 into the socket. Lastly, there is the pressure of the atmosphere, 

 a force which is seldom taken into consideration, but which has 

 great influence on many parts of the human frame. This part 

 of the subject will be resumed when we come to treat of 

 Atmospheric Pressure. 



The arms are jointed to the shoulder-blades in a very similar 

 manner, the upper arm-bone, or " humerus/' being furnished 

 with a rounded end, and fitting into a cup-like cavity in the 

 shoulder-blade, or "scapula." This formation can easily be 

 seen by separating the different bones of a shoulder of 

 mutton. 



At the bottom of the illustration are given two vertebrae of 

 a snake, separated in order to show their structure. It will be 

 seen that each joint has a ball in front and a socket behind, 

 thus giving the creature that wonderful flexibility which is 

 quite proverbial, and without which it could not seize its 

 prey. 



The following eloquent passage is taken from Professor 

 Owen's work entitled " The Skeleton and the Teeth : " — 



" Serpents have been regarded as animals degraded from a 

 higher type, but their whole organization, and especially their 

 bony structure, demonstrate that their parts are as exquisitely 



