INTEODUCTOEY 29 



which it is designed, and resembles the arrangement of beams and 

 trusses used in architectural construction (Fig. 9). A bone is thus 

 enabled to bear all kinds of strains with an economical disposition of 

 its tissue ; it supports weight, and resists the traction of muscles. 

 That the structure of bone is largely the result of mechanical forces 

 is shown by the fact that when bone tissue is destroyed it is replaced 

 by new bone whereof the fibres are arranged along the lines of greatest 

 pressure and greatest tension. 



Bone Marrow. The marrow of the bone is of two kinds : the 

 yellow marrow, which fills the medullary cavities, and the red marrow, 

 which occupies the spongy substance. 



The yellow marrow is almost pure fat ; the red marrow, which is 

 red largely because of the presence of blood-vessels, contains more 

 protoplasmic elements and in growing bones is more abundant. 



Associated with the marrow and Ivina: close to the bone tissue are 



t/ O 



peculiar cellular elements which during growth destroy useless bone 

 tissue, and are hence known as osteoclasts. 1 



PARTS OF BONES. FEATUEES ON BONES. 



Inasmuch as all bones are irregular, to study them thoroughly it is 

 necessary to resolve them into their various parts, to define their sur- 

 faces and their borders, and to describe the elevations and depressions 

 and other features which produce their characteristic irregularity. By 

 parts of a bone are meant those larger divisions which, as a rule, at 

 one time were once separate pieces ; for example, the head of a bone. 

 The classification may be arbitrary, however, and adopted merely for 

 convenience of description. 



In the same manner as a bone itself may have several names, a 

 part of a bone, or a feature on a bone, may have two names : an 

 English technical name and a Latin or a Greek name, as head and 

 caput, border and margo. Most of the English names are common 

 English words used in their ordinary sense, as border, spine, angle ; 

 others are transliterations of Greek terms, as condyle, from the Greek 

 xoi'bv/tos ; apophysis, from the Greek anotyvGic, ; and a few, adopted 

 from the Latin into the language without change, are now used only in 

 anatomy, such as foramen, sinus. 



1 From (dr.) osteon, bone, and klastos, broken. 



