JOINTS 



97 



In a diarthrosis the connective tissue between the bones remains com- 

 paratively loose in texture; and a cleft forms within it, containing tissue 

 fluid. This is the joint cavity (Fig. 84). It is bounded in part by flat- 

 tened connective tissue cells, which spread out and form an imperfect 

 epithelium (Fig. 85). This is not a continuous layer of cells, since in 

 many places the fibrous tissue comes to the surface. The connective 

 tissue layer blends with the perichondrium, which in turn passes into 

 cartilage, and a portion of the cartilage, uncovered by perichondrium, 

 helps to bound the joint cavity. 



FIG. 84. PHALANGEAL JOINT 

 FROM A HUMAN EMBRYO OF 

 THB FOURTH MONTH. 



car., Cartilage; j. c., joint cavity; 

 s. f., stratum fibrosum; s. s., 



tsratum synoviale. 



FIG. 85. AN ENLARGED DRAWING OF THE LEFT PART OF THE JOINT 



SHOWN IN FIG. 84. 



b. v., Blood vessel; car., cartilage; j. c., joint cavity; mes. epi. 

 mesenchymal epithelium. 



The articular cartilages are sometimes fibrous (as noted on p. 81) but 

 usually they are hyaline. They vary in thickness from 0.2 mm. to 5 mm., 

 being thinner at the periphery. The cells near the free surface are flat- 

 tened. In the middle strata they are rounded and are often arranged in 

 groups; in the deepest layers they tend to be in rows perpendicular to 

 the surface. The matrix becomes calcified as the cartilage connects 

 with the bone, and a line of demarcation separates the calcified from the 

 uncalcified portion (Fig. 86). In the uncalcified cartilage, cells with 

 processes extending into the adjacent matrix have been described, and 

 the deeper layers of flattened cells may exhibit lobed nuclei. 



The joint capsule consists of an outer layer of dense connective tissue, 

 the stratum fibrosum; and an inner loose layer of which the mesenchymal 

 epithelium is a part, the stratum synoviale (Fig. 84). The fibrous layer 

 is specially thickened in various places to form the ligaments of the joint. 



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