200 MUSCULAR AND SKELETAL STRUCTURES. 



rounded by synovial sheaths or sacs in which they smoothly 

 glide. 



Bursae are small synovial sacs situated at points exposed to 

 friction ; they facilitate movement by the gliding of their 

 opposite surfaces over each other, lubricated by their glairy 

 contents. They occur at exposed points underneath the skin, 

 as over the patella, and interposed between tendons and bones, 

 as over the bicipital tuberosity of the radius. 



The structure of bones and cartilages, which make up the 

 chief part of the skeletal framework, has been already suf- 

 ficiently considered. 



The ligaments, which bind the parts of the skeleton together, 

 mostly consist of dense white fibrous tissue, and are character- 

 ized by firm, unyielding strength ; a few ligaments, however, 

 the ligamentum nuchse of animals and the ligamenta subflava 

 of the spine, consist of elastic tissue and are serviceable by 

 virtue of their elasticity. 



Fasciae are chiefly composed of interlacing white fibrous 

 tissue. 



The articulations of the bones with one another are of three 

 kinds synarthroses, in which the joint is immovable ; amphi- 

 arthroses, in which there is limited motion ; and diarthroses, 

 in which motion is more or less free. 



The synarthroses, or immovable joints, are exemplified by 

 the bones of the skull and face, immovably united or dove- 

 tailed together by sutures. 



In the amphiarthroses a layer of fibro-cartilage is interposed 

 between the uniting bones (forming a " synchondrosis "), the 

 articulation being reinforced by ligaments. There is no synov- 

 ial joint-cavity and no surfaces moving on each other, the 

 limited amount of motion possible being effected by the flexi- 

 bility of the cartilaginous junction. The inter vertebral junc- 

 tions and the pubic and sacro-iliac symphyses are amphiar- 

 throdial joints. 



The intervertebral discs contain in their centre a pulpy 

 reticular mass, the remains of the chorda dorsalis ; outside 

 of this is a broad zone composed of lamina? of fibro-carti- 

 lage, which merges at the periphery into dense white fibrous 

 tissue. 



The diarthroses, or ordinary joints, are characterized by the 



