CHAP. V.] JOINTS. 51 



dyloid and the saddle joints admit of motion in every direction 

 except that of axial rotation. 



The different kinds of movement of which bones thus con- 

 nected are capable, are flexion and extension ; abduction and 

 adduction ; rotation and circumduction. 



A limb is flexed, when it is bent; extended, when it is 

 straightened out. It is abducted, when it is drawn away from 

 the middle line of the body; adducted, when it is brought 

 to the middle line. It is rotated, when it is made to turn on 

 its own axis; circumducted, when it is made to describe a 

 conical space, by rotation around an imaginary axis. No part 

 of the body is capable of perfect rotation like a wheel, for the 

 simple reason that such motion would necessarily tear asunder 

 all the vessels, nerves, muscles, etc., which unite it with other 

 parts. 



As the synovial membranes are intimately connected with 

 the joints, it may be well to give a brief description of them 

 here. 



The synovial membranes are composed entirely of connective 

 tissue, with the usual cells and fibres of that tissue. They are 

 distinguished by the nature of their secretion, which is a viscid, 

 glairy fluid, resembling the white of an egg and named synovia. 

 From its nature, it is well adapted for diminishing friction, and 

 thereby facilitating motion. 



These membranes are found surrounding and lubricating the 

 cavities of the movable joints in which the opposed surfaces 

 glide on each other; in these situations they are called articu- 

 lar synovial membranes. They are found forming sheaths for 

 the tendons of some of the muscles, and thus facilitating their 

 motion as they glide in the fibrous sheaths which bind them 

 down against the bones ; they are here called vaginal synovial 

 membranes, or synovial sheaths. Lastly, they are found in the 

 form of simple sacs, interposed, so as to prevent friction, be- 

 tween two surfaces which move upon each other, and in these 

 situations they take the name of bur sal synovial membranes, or 

 synovial bursse. These bursse may be either deep seated or 

 subcutaneous. The former are, for the most part, placed be- 

 tween a muscle and a bone, or between a tendon and a bone. 

 The subcutaneous bursse lie immediately under the skin, and 

 occur in various parts of the body, interposed between the skin 



