INTRODUCTORY. 15 



cells. The compound membranes are together made up 

 therefore of internally a vasculo-areolar layer, centrally 

 basement membrane, externally epithelial cells. They 

 are of four kinds — serous, synovial, mucous, and cuticular. 



Serous membranes are the most simple. They line 

 closed sacs, do not secrete fluid from the blood, but merely 

 are moistened by some halitus or vapour which serves to 

 allow them to move freely upon each other without friction, 

 for two layers of serous membrane generally come in 

 contact whereby motion of the contents of a cavity without 

 motion of its walls is admissible. The epithelium of this 

 form of membrane consists of a single layer of cells spread 

 over the surface of the basement membrane, closely fitted 

 together, whereby each assumes the hexagonal form charac- 

 teristic of tessellated epithelimn. The pleura, pericardium, 

 and arachnoid are good examples of serous membranes. It 

 is supposed that they are merely dilated extremities of 

 lymphatics. Synovial membranes differ from serous in their 

 greater complexity and in the nature of their secretion, for 

 they separate from the blood a fluid which, in consequence 

 of the fatty matter it contains, serves to lubricate freely mov- 

 able joints. They therefore line the cavities of such joints 

 covering the inner surface of its ligaments, and in young 

 animals even extending over the cartilage of the articular 

 surfaces of the bones. In some parts, either to fill up space 

 or to afford increased secreting surface, the vascular layer 

 of the membrane is thickened, rendering it prominent and 

 heightened in colour, forming the synovial fring'es. Besides 

 lining the cavities of joints synovial membranes form the 

 thecae or sheaths of tendons, lining the channels through 

 which they pass, and being reflected over the tendons,the two 

 portions being connected by double folds of membrane 

 termed /roe?^a. Whenever a tendon plays over a prominent 

 surface whereby the direction of its action is altered it has 

 placed between it and the prominence a small synovial 

 closed sac termed a bursa, the third form of synovial cavity ; 

 a marked bursa is situated between extensor pedis tendon 

 and the antero-inferior part of metacarpi magnum, the 

 synovial apparatus between flexor brachii and the humerus 

 is intermediate in character between a theca and a bursa. 

 Sometimes two or even three of the different forms of 

 synovial cavity communicate. Synovial membranes always 

 present simple tessellated epithelium. It is on the Mucous 



