VARIOUS MEMBRANES AND TISSUES. 119 



cavities, such as the peritoneal, pericardial, plem-al, &c. ; and, 2nd, those 

 called synovial, which line joints and sheaths of tendons, bursae mucoste, 

 &c. 



In the first named the fluid secreted is identical in general and chemical 

 characters with the serum of the blood or with very dilute liquor san- 

 guinis. It is probable that it is in great measure separated from the 

 blood by simple transudation through the coats of the vessels. 



In the second or synovial membrane the fluid appears to be a process 

 of more elaborate secretion by means of the endothelial cells on its sur- 

 face. It is dense and viscid, and contains abundance of albumen. It is 

 commonly known as joint oil. 



231. Cartilage. 



Cartilage is a semi-transparent homogeneous substance, possessing very 

 considerable strength and consistency combined with some elasticity. 

 In colour it is pearly white, but turns yellow when dried. It is insensi- 

 tive and non-vascular. At least no vessels containing red blood can be 

 discovered in it, even by the aid of the microscope. It is, however, 

 covered with a thin film of synovial epithelium, from the blood-vessels 

 of which it is freely supplied with nutriment. 



Simple cartilage, such as that which covers the articular surface of 

 bones, consists of numerous granular nuclei embedded in a structureless 

 matrix. This description of cartilage is termed articular, and serves to 

 facilitate the gilding motion of bones, and also acts as a cushion between 

 them, preserving their articular surfaces from attrition and the bones 

 themselves from concussion. 



When the articular cartilages of a joint have been eroded or destroyed 

 by inflammation or other disease in the organ, a sort of repair is set up 

 by nature, which consists in the deposit of fibro-cellular tissue in the 

 place of the cartilage. This new material is, however, no sufficient sub- 

 stitute. It generally binds together the ends of the bones, and so pre- 

 vents further irritation and inflammation, but the joint is stiffened. In 

 other cases this fibro-cellular deposit does not take place, but the exposed 

 surfaces of the bones become soldered together, and the joint is firmly 

 and permanently anchylosed. In other instances condensation appears 

 to take place at the ends of bones, which under the influence of friction 

 subsequently become smooth and polished. 



Cartilage is found in various parts of the body, and seems to be 

 employed by nature wherever some degree of elasticity combined with 

 strength is required. 



Cartilage may be either temporary or permanent. Temporary carti- 

 lage is that which is intended afterwards to be converted into bone. In 

 the early stage of foetal life cartilage exists as the sole foundation of the 

 skeleton, bone being in due time deposited in its meshes and substituted 

 for it. Permanent cartilage in particular parts is sometimes absorbed 

 when the horse grows old, and bone is deposited in its place, as, for 

 instance, between the metacarpal bones. Cartilage once absorbed is 

 never reproduced. 



