THE OS HUMERI. 181 



the bones were to move on the other perpendicularly up and 

 down, without any rotation. Sometimes a movable ligamentous 

 cartilage is found in this joint ; and sometimes such a cartilage 

 is only interposed at the anterior half of it ; and in some old 

 subjects a sesamoid bone has been found here. 



The scapula is connected to the head, os hyoides, vertebrae, 

 ribSj^and arm bone, by muscles that have one end fastened to 

 these parts, and the other to the scapula, which can move it 

 upwards, downwards, backwards, or forwards : by the quick 

 succession of these motions, its whole body is carried in a 

 circle. But being also often moved, as upon an axis perpen- 

 dicular to its plane, its circumference turns in a circle whose 

 centre this axis is. Whichever of these motions it performs, 

 it always carries the outer end of the clavicle and the arm 

 along with it. The glenoid cavity of this bone receives the os 

 humeri, which plays in it, as will be more fully explained here- 

 afterT 



The use of the scapula, is, to serve as a fulcrum to the arm ; 

 and by altering its position on different occasions, to allow always 

 to the head of the os humeri a socket to move in properly 

 situated ; and thereby to assist and to enlarge greatly the motions 

 of the superior extremity, and to afford the muscles which rise 

 from it more advantageous actions, by altering their directions 

 with respect to the bone which they are to move. This bone 

 also serves to defend the back part of the thorax, and is often 

 employed to sustain weights, or to resist forces too great for the 

 arm to bear. 



Os Humeri, or Arm Bone. 



The arm has only one bone, best known by the Latin name 

 of os humeri ; which is long, round, and nearly straight. 



The upper end of this bone consists of ' a large round 

 smooth head, which forms the segment of a sphere, whose 

 axis is not in a straight line with the axis of the bone, but 

 stands obliquely backwards from it. The extent of the head 

 is distinguished by a circular fossa surrounding its base, where 

 16 



