CLASSES OF BONES. 13 



cylindrical, and consists of a shell of compact tissue of varying 

 thickness, which encloses the cancellated tissue and medullary 

 canal, and is pierced by the medullary or nutrient foramen; 

 it is smallest in the centre, expanding towai'ds 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 exposed surfoces, 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 getting gradually less dense towards the centre of the 

 shaft, which is occupied by the medullary canal. While 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 

 arrangement 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 arti- 

 cular 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 articu- 

 lating 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. Fiat bones are com- 

 posed 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 



