56 SYNOVIAL MEMBRANES.—CILIATED EPITHELIUM: 
by tbe continuity of its blood-vessels over the entire car¬ 
tilage, yet these are found to retreat gradually as the joint 
is brought into use, until at last they only form a circle round 
the border of the cartilage. Some of the Synovial mem¬ 
branes, as that of the knee-joint, are furnished with little 
fringe-like projections, somewhat resembling the villi of 
mucous membranes (§ 41); these are extremely vascular, 
and are furnished with an epithelium which very readily 
falls off; and there is a strong probability that they are 
concerned in the secretion of the synovial fluid, which is 
much denser than the ordinary serous transudation, having 
from 6 to 8 per cent, of additional albumen, and presenting a 
glairy appearance like that of white of egg. It is interesting 
to see that the same purpose may thus be served by the 
extension of the membrane in either direction, either out¬ 
wards into a villous filament, or inwards into a follicle; the 
function being determined in each case rather by the 
attributes of the cells, and by the 
supply of blood, than by the form 
which the secreting surface may 
happen to present. 
45. The cells of Epithelium, 
whether flattened or cylindrical, 
are observed to be furnished in 
particular situations with a fringe 
of delicate filaments, which are 
termed cilia . These, although of 
extreme minuteness, are organs of 
great importance in the animal 
economy, on account of the extra¬ 
ordinary motor powers with which 
they are endowed. The form of 
the cilia is usually a little flattened, 
and' tapering gradually from the 
base to the point. Their size is 
Big. 13.— Ciliated Epithelium , , \ n 1 , 
cells; as seen sideways at a, extremely variable ; the largest that 
fy d . in transverse section at B; b ave been observed being about 
their cilia are seen at b, their . , . , n 
nuclei at c; at a is shown one of 1-500th of an inch m length, and 
these cells unusually elongated. the smallest 1-13,000th. When in 
motion, each filament appears to bend from its root to its 
point, returning again to its original state, like the stalks of 
