THE MUSCULAR SYSTEM. 255 



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 existence of con- 

 tinuous 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 membrane. 



D. Fibro- cartilages and sesamoid bones. — The tendons of 

 some muscles (tibialis posticus, peronams longus), in those 

 portions which run in the tendinous sheaths, contain, imbedded 

 in their substance, dense, semi-cartilaginous bodies, which are 

 known under the name of sesamoid cartilages (fibro-cartilagines 

 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, presenting 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 laver 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 fascia and of the tendons themselves, and 

 exhibit occasionally fine elastic 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, together with 



