OSTEOLOGY 3 



Internal or mesial : near to the mesial plane ; and external or 

 lateral : farther away from the mesial plane. 



Preaxial and postaxial : these terms were originally used in 

 connection with the embryo. Preaxial, proximal, or cephalic 

 means towards the head (superior), and postaxial, distal, or 

 caudal means towards the tail (inferior). 



Morphology. — This is the science which investigates the form 

 and structure of parts and organs in reference to the series of 

 changes through which they pass during their development. In a 

 morphological sense parts of man's body may be compared with 

 one another, or parts of the body of one animal may be compared 

 with apparently similar parts in the body of another animal, and 

 in this way their actual identity may be established. 



Homology. — Parts or organs which are represented in different 

 animals are said to be homologous (homogenetic) when they are 

 constructed on the same t^-pe and have the same embryonic origin, 

 although they may differ in function. For example, the upper 

 . limb of man and the fore-limb of a quadruped are homologous (homo- 

 genetic). Parts in the body of man are said to hejerially homologous 

 (homodynamic) when they are repeated. For example, the humerus 

 is serially homologous with the femur. 



Analogy. — This is identity in function, but not necessarily in 

 structure. The gills of a fish are analogous with the lungs of 

 man, though differing in structure. 



Segmental Type. — This consists in the repetition longitudinally 

 of a series of segments similar to each other — e.g., vertebrae, and 

 ribs. 



Chemical Composition of Bone. — Osseous tissue belongs to the 

 connective tissues, and it consists of an organic matrix or 

 ground substance, impregnated with mineral matter. The mineral 

 matter is composed chiefly of calcium salts, and so it petrifies the 

 ground substance. The organic matrix is usually spoken of as 

 the animal matter, and it forms about 33 per cent, of the entire 

 bone. The mineral matter is spoken of as the earthy matter, and 

 it forms about 67 per cent, of the bone. 



The animal matter imparts flexibility and elasticity to the bone, 

 and is composed of very delicate fibres, which are collected into 

 bundles, held together by cement substance. The fibres consist of 

 collagen, which is converted into gelatin by boiling. The animal 

 matter can be separated from the earthy matter by steeping a 

 bone for some time in dilute hydrochloric acid, the effect of the 

 acid being to dissolve out the earthy matter. When bone is so 

 treated it is said to be decalcified. There is left a tough, flexible, 

 elastic substance, which can be bent and twisted in various direc- 

 tions, and even tied into a knot, but no amount of force applied 

 to it, or pressure laid upon it, would cause it to break. A bone 

 when so treated retains its original shape, but it loses weight to 

 the extent of about two-thirds, and it also loses its property of 

 hardness, so that it is incapable of beaiing weight. 



