316 SECRETION. 



an irregularly shaped, closed sac, one layer of which is attached to the ten- 

 don, 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. 

 Bursaa are also found beneath the skin, especially in parts where the integu- 

 ment moves over bony prominences, as the olecranon, the patella and the 

 tuberosities of the ischium. These sacs, sometimes called bursas mucosas, are 

 much more common in man than in the inferior animals, and they have essen- 

 tially the same uses as the deep-seated bursas. The form of both the super- 

 ficial and deep-seated bursaa is very irregular, and their interior is frequently 

 traversed by small bands of fibrous tissue. The synovial sheaths, or vaginal 

 processes, line the canals in which the long tendons play, particularly the 

 tendons of the flexors and extensors of the fingers and toes. They have 

 essentially the same structure as the bursas, and present two layers, one of 

 which lines the canal, while the other is reflected over the tendon. The vas- 

 cular folds, described in connection with the articular synovial membranes, 

 are found in many of the bursas and the synovial sheaths. 



The quantity of synovia in the joints is sufficient to lubricate freely the 

 articulating surfaces. In a horse of medium size and in good condition, 

 examined immediately after death, Colin found 1-6 fluidrachm (6 c. c.) in 

 the shoulder-joint; 1-9 drachm (7 c. c.) in the elbow-joint; 1-6 drachm (6 

 c. c.) in the coxo-femoral articulation ; 2 '2 drachms (8 c. c.) in the femoro- 

 tibial articulation; and 1-9 drachm (7 c. c.) in the tibio-tarsal articulation. 



When perfectly normal, the synovial fluid is either colorless or of a pale, 

 yellowish tinge. It is so viscid that it is with difficulty poured from one ves- 

 sel into another. This peculiar character is due to the presence of an organic 

 substance called synovine. When this organic matter has been extracted 

 and mixed with water, it gives to the fluid the peculiar viscidity of the syno- 

 vial secretion. The reaction of the fluid is faintly alkaline, on account of the 

 presence of a small proportion of sodium carbonate. The fluid, especially 

 when the joints have been much used, usually contains in suspension pale 

 endothelial cells and a few leucocytes. According to Robin, the synovia of 

 the human subject contains about sixty-four parts per thousand of organic 

 matter, with sodium chloride, sodium carbonate, calcium phosphate and am- 

 monio-magnesian phosphate. 



The synovial secretion is produced by the general surface of the mem- 

 brane and not by any special organs. The folds and fringes which have been 

 described were at one time supposed to be most active in secreting the organic 

 matter, but there is no evidence that they have any such special office. 



Mucous Membranes and Mucus. A distinct anatomical division of the 

 mucous membranes may be made into two classes ; first, those provided with 

 squamous epithelium, and second, those provided with columnar or conoidal 

 epithelium. All of the mucous membranes line cavities or tubes communica- 

 ting with the exterior by the different openings in the body. 



The following are the principal situations in which the first variety 

 of mucous membranes, covered with squamous epithelium, is found: the 

 mouth, the lower part of the pharynx, the oesophagus, the conjunctiva, the 



