SEROUS AND SYNOVIAL MEMBRANES. 55 



A similar arrangement exists in the great cavity of the ab- 

 domen ; but the number and the complex relations of the 

 viscera which this contains, give to the disposition of ita 

 serous membrane, termed the peritoneum, a peculiar complica- 

 tion. The cavity of the skull also is lined by a serous mem- 

 brane, termed the arachnoid, and this is prolonged over the 

 surface of the brain, and enters its lateral ventricles ( 458). 

 The chief purpose of these membranes appears to be to faci- 

 litate the movements of the included organs, by forming 

 smooth surfaces which shall freely glide over each other ; this 

 is evidently of great importance, where such constantly- 

 moving organs as the heart and lungs are concerned. Their 

 surfaces are kept constantly moist with a serous fluid which 

 exudes from the blood ; but in the state of health this fluid 

 does not accumulate in their cavities, being absorbed as fast 

 as it is poured out. Various forms of dropsy, however, 

 such as "water on the brain," "water on the chest," and 

 " ascites," or dropsy of the abdomen are the result of the 

 increased outpouring of fluid into the serous cavities of the 

 arachnoid, the pericardium, the pleura, and 

 the peritoneum respectively. 



44. Nearly allied to the Serous mem- T / 

 branes are the Synovial, which form closed 

 sacs in the interior of joints, covering the 

 ends of the cartilages, and then lining the 

 fibrous capsule which passes from one bone 

 to the other. The mode of their arrange- 

 ment will be understood from the accom- 

 panying diagram ; in which a a represent 

 the extremities of the two bones which 

 are jointed together, b b the layers of car- Fi s- 12 



tilage with which they are severally covered, DIAGRAM OF THE STRUC- 



1,, , , . , ,. * ,, '. , ' TURE OF A JOINT. 



and the dotted line c c the synovial mem- 

 brane, which is seen to form the sac or 

 bag cf cf, whilst at the points cccc it is 

 reflected upon the cartilages of the joints. 

 In point of fact, however, the Synovial 

 membrane is not ordinarily traceable as a 

 distinct layer over the surface of these 

 cartilages, but seems to have become incorporated with them ; 

 for though in the embryo its presence may be distinctly proved 



cartilage ; b b, layer of 

 cartilage closely co- 

 vered with synovial 

 membrane ; c c 1 c, re- 

 flected layer of syno- 

 vial membrane form- 

 ing synovial capsule. 



