272 HUMAN ANATOMY. 



draw the diaphyseal fragment strongly towards the chest-wall, so that its upper end 

 may be found beneath the coracoid process. The shape of the opposing surfaces of 

 the epiphysis and diaphysis lessens both the frequency and the amount of the dis- 

 placement. The two surfaces usually remain in contact at some point: (i) on 

 account of that shape ; (2_) because the humerus on the epiphyseal line is broader 

 than at any other part of its upper end. 



The deformity will be recurred to in connection with that of the conditions 

 which it most closely resembles, — fracture of the surgical neck and dislocation of 

 the humerus, — which (on account of the importance of muscular action in their 

 production and in their treatment) will be considered after the muscles have been 

 described. 



It might be expected that, as the chief growth of the humerus takes place from 

 its upper epiphysis, arrest of growth and development should be a usual sequel. 

 The upper epiphysis from the tenth year to adult life will, according to Vogt, add 

 from seven to ten centimetres to the length of the humerus, the lower epiphysis 

 during the same time adding but one-fifth as much. The activity of the upper 

 epiphysis is shown by the frequency of conical stump after amputation through the 

 upper end of the humerus.^ Despite these facts, in comparatively few cases of 

 disjunction is atrophy or arrest of growth reported as a result. It has been sup- 

 posed, too, that necrosis of the epiphysis should follow this injury on account of 

 deficient blood-supply to the head ; but, through the tuberosities, through the 

 connection of the reflected capsule to the articular cartilage, and through portions 

 of untorn periosteum, the blood-supply is ample. Firm bony union is therefore 

 the usual result in well-treated cases. This is favored by the fact, already alluded 

 to, that the opposing surfaces are nearly always in contact at some point. 



The portion of the shaft just beneath the head and tuberosities is known as the 

 " surgical neck" because it is so often the seat of fracture. 



It contains, as will be seen on examining a longitudinal section of the humerus 

 (Fig. 283), a considerable quantity of cancellous tissue, the absorption of which in 

 old persons leaves the bone weak at that point. The factors already described as 

 favoring epiphyseal separation are operative in this case (page 271). 



The upper curve of the bone, beginning on this level, ends inferiorly at about 

 the lower margin of the deltoid tubercle. Its convexity is forward and outward. 

 The lower curve is concave forward. Both curves may be markedly increased 

 in rickets. The middle of the bone is not only the point of union of these curves, 

 but is also the smallest and hardest and least elastic portion of the shaft ; hence 

 it is most frequently broken, though fractures of the shaft at various levels below 

 and above this point are not uncommon. The deltoid tubercle, when unusually 

 developed, should not be taken for an exostosis. The region is, however, a fre- 

 quent seat of bony outgrowths on account of the insertion and origin, respectively, 

 of the coraco-brachialis and deltoid, and the brachialis anticus and internal head of 

 the triceps. 



The close attachment of the periosteum to the shaft which is thus necessi- 

 tated favors the development of osteo-periostitis, and thus of osteophytes as a 

 consequence of repeated muscular strains. Other favorite seats of exostoses are near 

 the insertion of the pectoralis major, the latissimus dorsi, and the third head of 

 the triceps. 



Tumors of a more serious variety, especially the sarcomata, attack the hu- 

 merus. The central sarcomata are found in the upper extremity chiefly at the 

 upper end of the humerus and at the lower ends of the radius and ulna. It may 

 be interesting to note that those are the extremities towards which the respective 

 nutrient arteries are not directed, and therefore, in accordance with the general 

 rule, the extremities at which bony union of the epiphyses and diaphyses takes 

 place latest. 



The close attachment of the periosteum at the middle of the shaft has been said 

 to account for the fact that non-union after fracture occurs in this region more fre- 

 quently than in the shaft of any other long bone of the skeleton. This has also been 

 attributed to interference with the nutrient artery (which enters the bone near its 



' Owen, Lejars, and others, quoted by Poland. 



