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,— 
Boch 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. at 
| 44. Nearly allied to the Serous mem- af ji 
" branes are the Synovial, which form closed “7% 
k sacs in the interior of joints, covering the H 4 
i ends of the cartilages, and gree Ap 
F 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 ates together, 66 the layers of car- Fig. 12. 
_ tilage with which theyare severally covered, Discnam oraz stave- 
; and the dotted line ¢ ¢ the ial st Basra Spe 
’ ities th 
brane, which is seen to form the sac or * f,cxTemities oC te 
bag ¢ ¢’, whilst at the points cece it is cartilage; >, layer of 
‘Tefiected upon the utilis of the joints. vers ete apni! 
‘In point of fact, however, the Synovial Wr iner of spo. 
‘membrane is not ordinarily traceable as a vial membrane form- 
distinct layer over the surface of these ‘evs! cpsle 
eartilages, but seems to have become incorporated with them ; 
though in the embryo its presence may be distinctly proved. 
