GENERAL PRINCIPLES APPLICABLE TO TEE STUDY OF ALL THE BONES. 13 



and uniform rules, but the new designations proposed have not been sanctioned 

 by custom. 



Situation. 

 The situation of a bone should be viewed in two ways : 1st, Relative to the 

 median plane of the body ; 2nd, Relative to the other portions of the skeleton. 



A. Situation relative to the median plane of the body. — The designation of 

 median plane, improperly median line, is given to an imaginary vertical plane 

 passing through the middle of the skeleton, which it divides from before to 

 behind, into two equal portions. The bones may be situat|^ on the median 

 plane, in which case there is only one of each kind, and they are called single ; 

 they are also named symmetrical bones, because the median plane divides them 

 into equal lateral halves exactly alike.^ The bones disposed in a double and 

 regular manner on the sides of the median plane bear, for this reason, the name 

 of pair bones ; they are also called asymmetrical bones, because their form does 

 not admit of their being divided, in any sense, into two similar portions. On the 

 contrary, a bone of this kind always offers the most perfect symmetry with its 

 fellow on the opposite side.^ 



B. Relative situation to the other parts of the skeleton. — To indicate the 

 situation of a bone, considered from this point of view, is to make known the 

 place it occupies in the region to which it belongs, and the connections it may 

 have with adjoining regions. Thus, the radius is situated in front of the ulna, 

 between the arm-bone and the carpus. 



Direction. 



This is absolute or relative. 



A. The absolute direction is related to the axes of the bones themselves. Thus 

 it is that a bone may be rectilinear, curvilinear, or twisted. 



B. The relative direction is determined by the relation to the fictitious planes 

 established around or in the interior of the skeleton, or with regard to the 

 neighbouring bones. For example, a bone is vertical, horizontal, or oblique. In 

 the latter case it may be downwards and backwards, or in the reverse direction. 

 Example : the scapula is placed obliquely downwards and forwards. 



Shape of the Bones. 



Form. — This is also absolute or relative. 



A. Absolute Form. — The absolute form of a bone is that which it owes to 

 the relations existing between its three dimensions — length, width, and thickness. 

 a. A bone in which one of its dimensions much exceeds those of the other two is 

 a long bojie (example : the femur), provided it be hollowed out internally by an 

 elongated space— the medullary canal. Long bones belong exclusively to the 

 limbs. In the animal economy there are found bones which resemble them in 

 their dimensions, but they have no medullary canal (example : the ribs). 

 These differ essentially from the true long bones, and are also distinguished from 

 them by the appellation of elongated bones, b. A bone that offers two dimensions 

 much more developed than the third, is a flat or tvide bone (example : the 

 parietal bone). The bones of this category, destitute of a medullary cavity, are 



• Instances have been recorded of asymmetry in single bones. Lesbre has seen the sixth 

 cervical vertebra of the Horse tricuspid on one side and bicuspid on the other, and a last dorsal 

 vertebra with one of its transverse processes having the characters of the lumbar vertebraj. 



* But there might be slight differences in weight, torsion, etc. In Man there is nearly 

 always a difference between the right and the left side. 



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