SYNOVIAL MEMBRANES AND SYNOVIA 277 



Secretions and Excretions. The products of the various glands 

 may be divided, according to their uses, into secretions proper and excre- 

 tions. Some of the true secretions have certain mechanical uses ; some, 

 like mucus, are thrown off in small quantity without being actually 

 excrementitious ; while others, like the digestive secretipns, are produced 

 at certain intervals and are taken up again by the blood. 



The serous cavities are now regarded as sacs connected with the 

 lymphatic system ; and the liquids of these cavities are not classed with 

 the secretions. 



Synovial Membranes and Synovia. The true synovial membranes 

 are found in the diarthrodial, or movable articulations; but in various 

 parts of the body are found closed sacs, sheaths etc., which resemble 

 synovial membranes both in structure and in their office. Every mov- 

 able joint is enveloped in a capsule, which is closely adherent to the 

 edges of the articular cartilage and even is reflected over its surface for 

 a short distance ; but it is now the common opinion that the cartilage 

 which incrusts the articulating extremities of the bones, though bathed 

 in synovia, is not itself covered with a distinct membrane. 



The fibrous portion of the synovial membranes is dense and resist- 

 ing. It is composed of ordinary fibrous tissue, with a few elastic fibres 

 and bloodvessels. The internal surface is lined with small cells of flat- 

 tened endothelium with rather large rounded nuclei. These cells exist 

 in one, two, three or sometimes four layers. 



In most of the joints, especially those of large size, as the knee and 

 the hip, the synovial membrane is thrown into folds which contain 

 adipose tissue. In nearly all the joints, the membrane presents fringed 

 vascular processes called synovial fringes. These are composed of 

 looped vessels of considerable size ; and when injected they bear a 

 certain resemblance to the choroid plexus. The edges of these fringes 

 present a number of leaf-like membranous appendages, in a great 

 variety of curious forms. They usually are situated near the attach- 

 ment of the membrane to the cartilage. 



The arrangement of the synovial bursae is very simple. Wherever 

 a tendon plays over a bony surface, there is a delicate membrane in the 

 form of an irregularly-shaped closed sac, one layer of which is attached 

 to the tendon, and the other, to the bone. These sacs are lined with an 

 endothelium like that found in the synovial cavities, and they secrete a 

 true synovial fluid. Bursae are also found beneath the skin, especially 

 in parts where the integument moves over bony prominences, as the 

 olecranon, the patella and the tuberosities of the ischium. These sacs, 

 sometimes called bursae mucosae, are much more common in man than 

 in the inferior animals, and they have essentially the same uses as 



