The Arm.. 59 



while the lower fragment, and, with it the forearm, would 

 be drawn upwards by the biceps and triceps. Should the 

 direction of this variety of fracture be downwards and for- 

 wards, then, the lower fragment will be drawn up behind 

 the upper fragment, whereas, if the break be directed 

 downwards and backwards, the lower fragment will over- 

 ride i.e., lie in front of the lower end of upper fragment. 

 When the break is situated close to, and, below, the inser- 

 tion of the deltoid, it may be the result of muscular action, 

 in which case the line of fracture is very often directed 

 transversely. In fact the humerus is more often broken 

 by muscular action than is any other bone, and, when frac- 

 ture thus occurs, it is the result of a sudden transverse strain 

 being put upon the shaft, as in the act of throwing a ball. 

 The mechanism of the production of fracture of the hum- 

 erus by muscular action is as follows : The upper part of 

 the bone is steadied and firmly held to the scapula and 

 thorax by the deltoid and the pectoral muscles, hence, in 

 the act of throwing, the long lever formed by the lower 

 half of the shaft and the forearm together, exerts a power- 

 ful strain on the bone below these muscular stays, especial- 

 ly when the momentum is suddenly arrested, with the 

 result that a fracture may be produced. There is generally 

 more or less shortening in fracture of the shaft, and this 

 shortening may be ascertained by comparing the relative 

 length of the two limbs, as measured from the "salient 

 angle " of the acromion to the tip of the external condyle. 

 This "salient angle" of the acromion is the point of 

 junction of the acromion process with its spine and is 

 opposite to the acromio-clavicular articulation. It is well 

 to remember in measuring the humerus that the right is 

 on an average about one third of an inch longer than the left, 

 and that the humerus of the male is about one inch longer 

 than that of the female. In fracture of the shaft of 



