. 272 OSTEOLOGY. 



and coracoid process, the rest of the bone is thin, except along the axillary margin ; but strength is 

 imparted to the body by the manner in which the spine is fused at right angles to its dorsal surface. 

 The Humerus. The body consists of a layer of compact bone surrounding a long medullary 

 cavity. The outer shell, thickest in the distal third of the bone, gradually thins until it reaches 

 the proximal epiphysial line, where it forms a layer no thicker than stout paper. Distally 

 the external shell is thicker and stouter than it is proximally, until it reaches the epicondyles, 

 distal to which the articular surfaces are formed of a layer of compact spongy bone. The proximal 

 end of the medullary cavity is surrounded by loose spongy tissue, the fibres of which arch inwards 

 from the inner surface of the compact outer layer, whilst at the distal end the spongy tissue 

 which springs from the outer shell sweeps distally in a radiating fashion on either side 

 of the olecranon fossa towards the epicondyles. Proximal to the olecranon fossa there is a 

 number of laminae of dense bone which arch across from one side to the other, the con- 

 vexity of the arches being directed distally. The proximal epiphysis, formed of spongy 

 bone, is united to the body by a wavy line, concave laterally and convex medially, leading 

 from the base of the greater tuberosity on the lateral side to the distal articular edge on the medial 

 side. The mass above this includes the head and the two tubercles. The spongy tissue of the 

 head is fine, and is arranged generally in lines radial to its surface ; that of the greater tubercle 

 is more open, and often displays large spaces towards its interior, which in old bones communi- 

 cate freely with the medullary cavity of the body. The general direction of the fibres is 

 parallel to the lateral surface of the tuberosity. The distal articular end is formed of fine spongy 

 tissue, more compact towards the surface, and arranged in lines more or less at right angles to its 

 articular planes. In the adult the principal nutrient canal, viz., that which opens on the 

 surface near the insertion of the coraco-brachialis, traverses the outer compact wall of the body 

 obliquely distally for a distance of two and a quarter inches before it opens into the 

 medullary cavity. 



The Ulna. The weakest parts of the bone are the constricted portion of the semilunar 

 notch, and the body in its distal third, the bone being most liable to fracture at these 

 points. On section the medullary cavity is seen to extend proximally as high as the base of the 

 coronoid process ; distally it reaches the proximal end of the distal fifth of the bone. The walls of 

 the body, which are formed of dense bone, are much thicker on the dorsal surface than on the 

 volar. Proximally they are continuous with the volar surface of the coronoid process and the 

 dorsal surface of the olecranon, where they are composed of layers of looser texture, which, however, 

 gradually become thinner as the points of these processes are reached. Distally they gradually 

 taper until the head and styloid process are reached, round which they form a thin shell, con- 

 siderably thickened, however, in the region of the groove for the extensor carpi ulnaris muscle. 

 The bulk of the proximal extremity is formed of loose spongy bone, arranged in a series of arcades, 

 stretching from the interior to the dorsal wall over the proximal end of the medullary canal. 

 Proximal to the constricted part of the semilunar notch the bone displays a different structure ; 

 here it is formed of spongy bone, of closer texture, arranged generally in lines radiating from the 

 articular surface. At the point of constriction of the semilunar notch the layer immediately 

 subjacent is much denser and more compact. 



The distal fifth of the bone is formed of loose spongy bone, the fibres of which have a general 

 longitudinal arrangement ; towards its extremity the meshes become smaller. 



The Radius. The neck is the narrowest part of the bone ; here fracture may occur, though 

 not commonly. The point at which the bone is usually broken is about one inch proximal to the 

 distal extremity. This is accounted for by the fact that the radius supports the hand at the 

 radio-carpal articulation, and 'the shocks to which the latter is subjected, as in endeavouring to 

 save oneself from falling, are naturally transmitted to the radius. On section, the medullary 

 cavity is seen to extend to the neck ; distally, it reaches to the level of the distal fifth of 

 the bone. Its walls are thick as compared with the diameters of the bone, particularly along the 

 interosseous border, thus imparting rigidity to the curve of the body ; these walls thin out 

 proximally and distally. Proximally, the surface of the tuberosity is formed of a thin shell of 

 bone, which, however, thickens again where it passes on to the neck. The proximal extremity 

 is formed of spongy bone arranged in the form of arcades, reaching distal to the level of the 

 tuberosity medially, but not extending distal to the level of the neck laterally. Beneath the 

 capitular articular surface there is a dense layer, thickest in the centre, and thinning towards 

 the circumference ; this is overlain by a very thin layer of less compact bone. 



The distal fifth of the body and distal extremity are formed of loose spongy bone 

 arranged more or less longitudinally. Immediately subjacent to the carpal articular surface 

 the tissue is more compact, and displays a striation parallel to the articular plane. The nutrient 

 canal of the shaft pierces the volar wall of the proximal part of the medullary cavity obliquely : 

 running proximally for half an inch. 



The Carpus. The bones are formed of fairly compact spongy tissue, surrounded by a thin 

 shell of denser bone. They are very vascular, and their non-articular surfaces are pierced bj 

 many foramina. 



The Metacarpus. Similar in arrangement to that of long bones generally, though it ma) 

 be noted that the compact walls of the body are thicker in proportion to the length of the bon< 

 than in the other long bones of the upper extremity. 



The Phalanges. Each phalanx has a medullary cavity, the walls of the body being formed o 

 dense compact bone, especially thick along the dorsal aspect. The extremities are made up o 

 spongy bone within a 'thin dense shell. 



