418 THE TISSUES. 



give origin to them, and thus become aponeuroses ; e. g. the inter- 

 osseous membrane of the forearm and leg. They are also, in cer- 

 tain parts, thickened into ligaments (as on the dorsal aspect of the 

 carpus), to retain the tendons in place. In all these cases, they pre- 

 sent the character and structure of tendons and ligaments. 



In some instances, however, the distinct muscular sheaths are 

 formed of areolar tissue, and are hence extensible, and allow of a 

 considerable amount of motion of the muscle within them ; e. g. 

 sterno-cleido-mastoideus, and other muscles manifesting a consider- 

 able amount of motion under the skin. 



2. The Ligaments of the Tendons. 



These retain the tendons in their place in certain parts ; e. g. the 

 dorsal and palmar ligaments of the carpus, which retain the exten- 

 sors and the flexors of the fingers and the corresponding ligament 

 of the foot. 



Small bundles of white fibrous tissue also strengthen the tendin- 

 ous sheaths next to be described, as minuter ligaments. 



All the ligaments, mentioned under this head, present the struc- 

 ture and character of those in relation with joints (p. 278), and con- 

 tain fewer vessels than the tendons. 



3. The Tendinous Sheaths and Synovial Sacs. 



The sheaths of the tendons (as of the flexors and extensors of 

 the fingers and toes) are usually described as closed serous mem- 

 branes, of which one portion invests the tendon and the other lines 

 the special canal in which it glides to and fro. Kolliker, however, 

 finds that no membrane at all exists on the greater part of the sur- 

 faces hitherto supposed to be covered by it. Fimbriated processes 

 are, however, found projecting here and there into the cavity con- 

 taining the synovial fluid, and from which the latter is doubtless 

 secreted. These processes are very vascular, like those of the syn- 

 ovial capsules of the joints (p. 345). 



The bursce mucosce, or synovial capsules of the muscles, invest the 

 opposed surfaces of muscles and bones, as under the psoas, iliacus, 

 deltoid, gluteus maximus, &c. (p. 195). These also appear in the 

 form of closed sacs, but are usually not everywhere constituted of 

 a serous membrane, any more than the preceding tendinous sheaths. 

 In both, the epithelium is almost never complete ; and both are, 

 therefore, to be classed with the synovial capsules of the joints. 



