56 SYNOVIAl MEMBRANES. CILIATED EPITHELIUM: 



by the 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 mlli 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- 

 ivards 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 

 extremely variable ; the largest that 



their cina are seen at o, tneir nave been observed being about 

 nuclei at c -, at a is shown'or.e of l-500th of an inch in length, and 



these cells unusually elongated. ^ gmaUest M^OOOth. When in 



motion, each filament appears to bend from its root to its 

 point, returning again to its original state, like the stalks of 



B 



Fig. 13. CILIATED EPITHELIUM 



CELLS; as seen sideways at A, 



