336 GENERAL ANATOMY. 



B. OF THE SHEATHS OF THE TENDONS. 



521. The sheaths of the tendons are ligamentous canals 

 which embrace and fix the tendons in their place. 



Some of them are sufficiently long to form true canals; 

 others, which are much shorter, are called annular ligaments. 

 Some of these annular ligaments are entirely circular; others, 

 as well as the sheaths, are completed by the neighbouring 

 bones, whence result osteo-ligamentous sheaths. They are, 

 together with the tendons which they contain, invested by va- 

 giniform synovial membranes. These sheaths are very solid 

 and strong; they contain each one or more tendons; they are 

 especially more numerous at the free extremities of the limbs, 

 more in the direction of flexure, and also stronger in this di- 

 rection than in that of extension. They keep the tendons in 

 their proper place, prevent their displacement during the 

 action of the muscles and the motions of the joints; they also 

 serve, in some places, as pulleys which change the direction 

 of the tendons and modify that of the motions. 



C. OF THE PERIOSTEUM. 



522. The envelope of the bones, or the periosteum, em- 

 braces the bones in their whole extent, excepting the articular 

 surfaces. The teeth alone, which however are not bones, are 

 destitute of it. 



This envelope is interrupted at the amphiorthrodial and 

 diarthrodial articulations, but is not so at the immoveable 

 ones. 



Its external surface is flucculent, and nearly covered with 

 filaments which confound themselves with the surrounding 

 cellular tissue, and which, in other places, are continuous with 

 the ligaments and tendons. 



The internal surface is fixed to the bone by innumerable 

 prolongations which accompany the vessels into its interior 

 and into its substance. This surface is very firmly attached 

 to the bones wherever they are thick and spongy, but less so 

 in the other parts. Its adhesion is also less firm in children 

 than in adults. 



