Chap. VIII] SPECIAL MEMBRANES AND GLANDS 129 



(2) by secreting serum which acts as a lubricating fluid and tends 

 to lessen friction. 



The inner surface of a serous membrane is free, smooth, and 

 polished ; and in the case of serous membranes proper, the inner 

 surface of one part is applied to the corresponding inner surface of 

 some other part, only a very small quantity of fluid being interposed 

 between the surfaces. The organs situated in a cavity lined by a 

 serous membrane, being themselves also covered by it, can thus 

 glide easily against its walls or upon each other, their motions being 

 rendered smoother by the lubricating fluid. 



SYNOVIAL MEMBRANES 



Synovial membranes are frequently classed as serous membranes, 

 because their function is the same and they have no communica- 

 tion with the surface of the body. They differ, however, (1) in 

 the nature of their secretion, (2) in their structure, and (3) they are 

 associated with the bones and muscles, and not with the viscera. 

 Synovial membrane is composed of fibrous tissue which has on its 

 free surface an imperfect covering of cells that are irregularly 

 shaped, and secrete a viscid glairy fluid that resembles the white of 

 egg, and is named synovia. 



They are divided into the following classes : — 



1. Articular. 



2. Vaginal. 



3. Bursal. 



1 . Articular. — Articular synovial membranes are found sur- 

 rounding and lubricating the cavities of the movable joints in 

 which the opposed surfaces glide on each other. 



2. Vaginal. — Vaginal synovial membranes are found forming 

 sheaths for the tendons of some of the joints, and thus facilitating 

 their motion as they glide in the fibrous sheaths which bind them 

 down against the bones. 



3. Bursal. — Bursal synovial membranes, or synovial bursae, 

 are found in the form of simple sacs, interposed, so as to prevent 

 friction, between two surfaces w^hich move upon each other. 

 These bursse may be either deep-seated, or subcutaneous. The 

 deep-seated are for the most part placed between a muscle and a 

 bone, or between a tendon and a bone. The subcutaneous 



