278 SECRETION 



the deep-seated bursae. The form of both the superficial and deep- 

 seated bursae is irregular, and their interior frequently is traversed by 

 small bands of fibrous tissue. The synovial sheaths, or vaginal pro- 

 cesses, 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 bursae, and present two 

 layers, one of which lines the canal, while the other is reflected over 

 the tendon. The vascular folds, described in connection with the 

 articular synovial membranes, are found in many of the bursae and the 

 synovial sheaths. 



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

 the articulating surfaces. When perfectly normal it is either colorless 

 or of a pale yellowish tinge. It is so viscid that it is with difficulty 

 poured from one vessel into another. This peculiar character is due 

 to the presence of an organic substance called synovin, a kind of 

 mucin. When this organic matter has been extracted and mixed with 

 water, it gives to the liquid the peculiar viscidity of the synovial secre- 

 tion. The reaction of the synovia is faintly alkaline, on account of the 

 presence of a small quantity of sodium carbonate. The secretion, espe- 

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

 pale endothelial cells and a few leucocytes. The synovia of the human 

 subject contains about sixty-four parts per thousand of organic mat- 

 ter, with sodium chloride, sodium carbonate, calcium phosphate and 

 ammonio-magnesian phosphate. 



The synovial secretion is produced by the general surface of the 

 membrane and not by any special glands. The folds and fringes 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 the mucous membranes line 

 cavities or tubes communicating with the exterior. 



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 female urethra and the vagina. In these situations the membrane 

 is composed of a chorion made up of inelastic and elastic fibrous tissue 

 with capillaries, lymphatics and nerves. The elastic fibres are small 

 and quite abundant. The membrane itself is loosely united to the 

 subjacent parts. The chorion is provided with vascular papillae, more 

 or less marked ; but in all situations except the pharynx, the epithelial 



