SUPPORT AND MOVEMENT 



381 



the clavicle but also the nerves and muscles 

 of the arms which make them such useful 

 appendages today. While the clavicle is 

 firmly attached to the sternum on the front, 

 the scapula has no secure attachment and 

 is loosely slung over the thoracic basket by 

 means of muscles and ligaments. This ar- 

 rangement permits a great deal of move- 

 ment in which the shoulders can be freely 

 rolled over the ribs. The anterior append- 

 ages have much more freedom of move- 

 ment than the posterior appendages, whose 

 primary function is locomotion. 



The upper arm, the humerus, fits into a 

 crude socket made by the union of the 

 scapula and clavicle called the glenoid 

 fossa. The humerus is held in place by liga- 

 ments at its upper end, but since the attach- 

 ment is none too secure, under certain 

 stresses it may be forced out of the socket, 

 resulting in a dislocation. Such stretched 

 ligaments allow dislocation more readily 

 under similar subsequent stresses. The ad- 

 vantage of this junction lies in its loose 

 arrangement which allows more freedom of 

 movement for the arm. For example, the 

 arm may be turned in a complete circle as 

 well as rotated in the socket. A dog, on the 

 other hand, could not possibly perform such 

 a feat, for the arrangement of the bones in 

 its pectoral girdle is much more rigid. 



The two forearm bones, the radius and 

 ulna, form a combination whereby hinge 

 action as well as partial rotation can take 

 place. This means that the forearm can be 

 flexed (bent on itself) in a straight pull or 

 it can twist through 180 degrees. The num- 

 ber of times one performs these movements 

 each day is almost unlimited. At the wrist 

 another hinge is produced by the end of the 

 radius and the carpals, the small wrist 

 bones. Actually this is as much a universal 

 joint as it is a hinge, with the result that the 

 hand can move in all directions with equal 

 facility. The hand with its large, opposable 

 thumb is a primitive but most useful instru- 

 ment and it is hard to imagine life as it is 

 lived today without it. 



The pelvic girdle is the most specialized 

 part of the entire skeleton. A quadruped, 

 running on all fours, does not require as 

 secure an attachment to the column as does 

 a biped, whose pelvis has become corre- 

 spondingly modified. However, in the case 

 of man the pelvis has become not only an 

 excellent support for the entire body but it 

 has also broadened and flared out so that 

 it functions as a support for the organs of 

 the abdominal cavity. This again is a satis- 

 factory method of handling the pendent 

 viscera of tlie upright animal. 



The pelvis is composed of three pairs of 

 fused bones: the large, flat and cupped ilia 

 (singular — ilium), the ischia (singular — 

 ischium, the bones used in sitting), and the 

 pubic bones which complete the girdle in 

 front. The fused vertebrae of the sacrum 

 form a complete circle at the back, leaving 

 a large opening through which all mammal 

 offspring must pass in tlie process of birth. 

 The urinary and digestive tracts pass 

 through here also. The dimensions of this 

 opening are one of the clues used in deter- 

 mining the sex of a skeleton. Not only is the 

 opening larger in females but, in addition, 

 the attachment of the pubic bones is not so 

 broad. Both features are essential to allow 

 such a large object as a fetus to pass 

 through. The ilia also flare outward more 

 abruptly in the female than in the male; this 

 changes the position of the legs somewhat 

 so that the method of walking and running 

 differs in the two sexes. The familiar female 

 , "waddle" is a result of skeletal arrange- 

 ment, not any intention on her part. For 

 the same reason it is highly unlikely that a 

 woman wfll ever run the 100-yard dash in 

 10 seconds. 



The femur is the longest bone in the 

 skeleton. Its proximal end (end nearer the 

 body) is a pronounced ball which lies at 

 an angle to the rest of the bone and which 

 fits into a deep socket in the pelvis called 

 the acetabulum. This is a much more secure 

 arrangement than the one in the shoulder 

 region, although it does not have equiva- 



