128 LOCOMOTION. [CHAP. vi. 



their osseo-fibrous sheaths in the hand or foot. One layer of the 

 synovial sheath adheres to the wall of the osseo-fibrous canal ; the 

 other, to the contained tendon ; and, the free surface being lubricated 

 by synovia, the tendon plays freely within the canal. In deep- 

 seated whitlow, when the inflammation extends to one of these 

 synovial sheaths, and gives rise to the formation of adhesions within 

 its cavity, the motion of the tendon within is completely destroyed, 

 and a stiff finger is the result. Similar sheaths on a larger scale 

 envelope the tendons which pass beneath the annular ligaments of 

 the wrist and ankle. 



Synovia. The synovial membranes, in health, contain only suffi- 

 cient fluid to keep their free surfaces moist. It is, therefore, diffi- 

 cult to collect the synovia in sufficient quantity for examination. 

 It is a transparent, yellowish-white fluid, viscid like the white of an 

 egg, whence its name (<rvv, cum ; u>ov, ovum). Lassaigne and 

 Boissel, who have published an analysis of human synovia, state 

 that it does not coagulate spontaneously ; that it is an alkaline 

 fluid, containing albumen and salts, such as are found in the serum 

 of the blood : and M. I/Heritier has lately analysed two speci- 

 mens of this fluid, and completely confirmed the statement of those 

 chemists.' 34 ' 



It is plain, from the description above given, that synovial mem- 

 branes contribute to the locomotive function by lubricating the 

 articular surfaces, so that they may glide smoothly on one another 

 with the least possible friction, and also by facilitating the play 

 of some tendons over prominent surfaces, and of others within 

 sheaths. 



Serous Membranes. The movements of the viscera within the 

 great cavities of the trunk are provided for by an arrangement 

 similar to that described in the joints. Between the wall of the 

 cavity and the surface of the contained viscus (the thorax and the 

 lungs, for example,) a closed sac is placed, one layer of which is 

 parietal, and the other visceral. These are respectively attached 

 to the wall of the cavity and to the surface of the viscus by fine areo- 

 lar tissue ; and their continuity is shewn at certain reflections where 

 the one passes into the other. The free surface, as in the synovial 

 membranes, is continually moistened by the proper secretion, which, 

 containing a larger proportion of water than synovia does, resem- 

 bles serum of blood. The serous membranes are, the arachnoid, in 

 the head and spine ; the pleura and pericardium, in the thorax : the 

 peritoneum, in the abdomen ; and the tunica vaginalis testis, in the 

 * Berzelius Chemie Organ, t. vii. 





