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 its 
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¬ 
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¬ 
tilage with which they are severally covered, Diagram of the struc- 
and the dotted line c c the synovial mem- TURE ° F A ° INT * 
i if ? , i a a, extremities of the 
brane, which is seen to form the sac or tones, covered with 
bag c' c', whilst at the points CCCC it is cartilage; ^Mayerof 
reflected upon the cartilages of the joints, vered with synovial 
In point of fact, however, the Synovial Sanyer of’sy™- 
membrane is not ordinarily traceable as a yiai membrane form- 
distinct layer over the surface of these lng s y novia capsue. 
cartilages, but seems to have become incorporated with them.; 
for though in the embryo its presence may be distinctly proved. 
Fig. 12. 
