CLASSES OF BONES. 13 



foramen; it is smallest in the centre, expanding towards the 

 extremities, and is circular, oval, or prismoid in form. When 

 a long bone is placed nearly vertically under the body, the 

 internal wall of its shaft is usually the thickest. When 

 obliquely placed, the thick portions of the shaft correspond 

 with the lines which would indicate the direction of pressure 

 produced by the animal's weight. Long bones are never 

 straight ; they may be twisted, as is the humerus, and, if bent, 

 are generally convex on their exbosed surfaces, the shell being 

 thickest on the concave side. 



The extremities of long bones always exceed the shaft in cir- 

 cumference, and are remarkable for the irregularity of their outline; 

 they are expanded and roughened, to afford surface for the 

 attachment of tendons and ligaments, ^their protuberances also 

 materially increasing the mechanical power of muscles by serving 

 as pulleys over which the tendons play. The' extremities are 

 composed of cancellated, with a thin layer of compact, tissue, the 

 cancellated gelling gradually less dense towards the centre of the 

 shaft, which is occupied by the medullary canal. Wliile the 

 extremities exceed the shaft in circumference, their weight is 

 not relatively greater — their increase being due to a diffusion and 

 expansion of material, not to an addition of substance. This 

 arremgement lightens and strengthens the bone, besides filling its 

 cavities with an elastic material to resist concussion. The 

 hardest part of a bone is usually the thin portion lying next to 

 the articular surface; it is only found when the bone is fully 

 developed, and it rests upon a series of arches formed by the 

 cancelli : this thin layer is covered with cartilage. Excepting 

 on their articulating surfaces, the extremities of long bones 

 are copiously pierced by foramina, which chiefly transmit 

 blood-vessels to and from the interior. 



Flat or tabular hones afford extensive surface for muscular 

 attachment, and help to enclose cavities containing important 

 organs. Thus, the cranium protects the brain, the scapula and 

 ribs protect the respiratory organs and heart. Flat bones are 

 composed of two thin expanded plates of compact tissue, 

 rarely quite parallel to each other, and enclosing a cancellated 

 structure between them. The internal is considerably harder 

 than the external plate, but not so thick or tough, the outer 

 being more elastic and less liable to fracture ; the connecting 

 cancellated tissue is plentifully supplied with blood-vessels. 

 In the bones of the cranium, the compact plates are called the 



