SEROUS AND SYNOVIAL SECRETIONS. 767 



cold; in other situations, it is much more viscid, as the intestinal or 

 vesical mucus, and the nasal mucus ; that from the air-passages in 

 cases of cold is very viscid, and contains albumen. The use of mucus 

 is chiefly mechanical, assisting in the acts of mastication, deglutition, 

 or speech, or, as in certain animals, in the capture of prey. It aids 

 in taste and smell, preserving the moisture of the parts, and acting 

 also as a solvent. It is likewise protective, both mechanically and 

 chemically, by offering resistance to the action of the digestive fluids, 

 which do not easily dissolve it. Sometimes it behaves as a ferment, 

 possibly assisting the salivin, pepsin, and pancreatin, though not pos- 

 sessing very active powers. It, or something mixed with it, undoubt- 

 edly determines the retrograde decomposition of the renal excretion, 

 when this is retained longer than usual in the body. Mucin, not being 

 readily soluble or digestible, cannot, strictly speaking, be nutritive or 

 absorbable. 



Serous and Synovial Secretions. 



These fluids, which cover and moisten the surface of the serous and 

 synovia! membranes, differ so little in composition from the plasma of 

 the blood, that their formation has by some been regarded, not as a 

 process of secretion, l)ut as one of transudation through those mem- 

 branes. A certain modification of the plasma of the blood, as it is 

 exuded from the capillaries, is here accomplished, however, by the 

 action of the epithelial cells, which, in a single layer, cover these 

 membranes. These fluids may sometimes contain, in certain morbid 

 conditions, excretory materials, such as urea, lactate of soda, sugar, 

 and traces of bile pigment, all of which are manifest transudations. 



The serous fluids, which must not be confounded with the serum of 

 the blood, contains as much as 99 per cent, of water, a few salts, some 

 albumen, and a substance slightly soluble in alcohol. They are thin 

 and scanty in normal conditions, in the cavities of pleura, pericar- 

 dium, peritoneum and arachnoid spaces, their use being merely to 

 prevent friction. The aqueous humor of the eyeball may be regarded 

 as a serous secretion, adapted by its locality to a very special purpose. 

 When accumulated in abnormal quantity, from inflammation, serous 

 fluids cause internal dropsies, and, in that case, generally contain 

 traces of fibrin, which will slowly coagulate after removal from the 

 body. 



The synovial fluid, or synovia, is much more viscid than the serous 

 fluid; it contains nearly 6.5 per cent, of albumen, together with fatty 

 matter, salts resembling those of the blood, epithelial cells, corpuscles 

 like the pale corpuscles of the blood, and, it is said by some, a sub- 

 stance closely resembling mucin. The use of this fluid, found alike in 

 joints, in the so-called bursae mucosae, and in the sheaths of tendons, 

 whether these move in grooves on the bones lined with cartilage, or in 

 soft parts only, is chiefly mechanical, to prevent the eifects of friction ; 

 but in the joints it may act nutritively on the cartilages. 



