CHAPTER II. 



AETHROLOGY. 



The several bones which form the skeleton are united by means 

 of certain soft structures, forming a series of articulations or 

 joints, the study of which is termed Arthrology or Syndesmology. 



Before considering the different forms of joints, it will be advis- 

 able to describe briefly the various tissues, other than bone, which 

 enter into and contribute to'^ards their formation. These are 

 chiefly cartilage, connective and elastic tissues, and fat. 



In health, one bone never comes directly in contact with another, 

 cartilage or fibrous tissue being always interposed ; an exception to 

 this exists in the adult skull, most of the bones of which become 

 firmly united by ossification of the interposed soft material. 



CARTILAGE. 



Cartilage, known also by the familiar name of gristle, is a firm, 

 bluish-white, elastic animal substance, somewhat translucent, 

 resilient, and flexible, possessing great cohesive power. That 

 which forms the original basis of the bony framework is termed 

 temporary, and that which persists in the adult, permanent 

 cartilage; the former disappears as it is replaced by bone, but 

 the latter, of which alone we have to treat here, never under 

 normal circumstances becomes ossified. Cartilage consists of 

 corpuscles or cells, usually imbedded in an intercellular substance 

 or matrix. The cells, which are contained in lacunae in the matrix, 

 are oval, round or fusiform, and nucleated, the nuclei, which 

 appear under the microscope as small spots, containing still 

 smaller objects called nucleoli. Cartilage when boiled yields 

 chondrine, a substance varying slightly from gelatine. 



There are three varieties of the permanent kind — hyaline, 

 fihro, and cellular cartilage. In the first, the matrix is homogen- 

 eous, or void of definite structure, appearing slightly granular under 

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