240 SPECIAL HISTOLOGY. 



membrane can only be detached in parts, and is found to be almost 

 wholly wanting exactly at the points where the mutually gliding parts 

 are in contact. Precisely the same thing obtains in the synovial sheaths, 

 among which the common sheaths of the flexor tendons of the fingers 

 and toes, only in a certain measure, retain the form of a so-termed 

 serous sac, although, even in this case, many parts of the surface of the 

 tendons are without any such membranous lining. Whence it would 

 appear, that in this case, as in many others, the old doctrine of the ex- 

 istence of continuous serous sacs requires thorough emendation. In 

 most of the synovial sheaths, and in many mucous bursse, are found 

 occasionally, particularly in the retinacula, smaller or larger, reddish 

 fimbriated processes, exactly resembling those of the joints, and which, 

 in like manner, are nothing but vascular processes of the synovial mem- 

 brane. 



D. Fibro-cartilages and sesamoid bones. The tendons of some muscles 

 (tibialis posticus, peronceus longus], in those portions which run in the 

 tendinous sheaths, contain, imbedded in their substance, dense semi-carti- 

 laginous bodies, which are known under the name of sesamoid cartilages 

 (fibro-eartilagines sesamoidece\ and when, as occasionally happens, they 

 become ossified, of sesamoid bones (ossa sesamoidea) ; the latter occur 

 normally, imbedded in the flexor tendons of the fingers and toes, pre- 

 senting one surface towards an articulation. 



Respecting the more intimate structure of the last-mentioned parts, 

 the following is to be remarked. The sesamoid bones consist of common, 

 finely cancellated osseous substance, are on one side closely surrounded 

 by tendinous or ligamentous tissue, and on the other, which is invested 

 with a thin layer of cartilaginous substance, project into an articulation. 

 The ligaments of tendons, in correspondence with their function, possess 

 exactly the same firm structure as that of the tendinous portions of the 

 fasciae and of the tendons themselves, and exhibit occasionally fine elas- 

 tic fibres in process of development, or the round formative cells of such 

 fibres disposed in rows. The retinacula tendinum have a more delicate 

 structure ; their function being rather to convey vessels to the tendons, 

 they consequently contain chiefly a more lax connective tissue, with fine 

 elastic fibres, and also fat-cells. The mucous bursse, which are invariably 

 thin-walled, consist, in as far as they possess a distinct membrane, of 

 fasciculi of connective tissue, crossing each other in the most various 

 directions, loosely connected, and in many places anastomosing, toge- 

 ther with some fine elastic fibres. The mucous sheaths, on the other 

 hand, in agreement with their double function, which in one respect is 

 that of a mucous bursa, and in another that of ligaments confining the 

 tendon or of tendinous sheaths, present in their thinner parts the struc- 

 ture of bursce mucosce, and in the thicker, an unmixed, dense connective 

 tissue, frequently with cells disposed in rows, which pass into elastic 



