150 MUSCULAR SYSTEM. [sect. 82. 



D. Fibro-cartilages and Sesamoid Bones. —The tendons of 

 some muscles (tibialis posticus, peronceus longus) contain, where 

 they run in tendinous sheaths, dense, semi-cartilaginous masses 

 imbedded in their substance, which are known under the name of 

 sesamoid cartilages, and of sesamoid bones when they become ossi- 

 fied, as occasionally happens. The latter occur normally imbed- 

 ded in the flexor tendons of certain fingers and toes, with one 

 surface directed towards an articulating cavity. 



Respecting the intimate structure of the last-mentioned parts, 

 it may be remarked, that the sesamoid bones consist of ordinary 

 fine cancellated osseous tissue, enclosed on one side in tendinous 

 or ligamentous substance, and on the other, projecting into an 

 articular cavity, where the surface is covered by a thin layer of 

 cartilage. The ligaments of the tendons have the same structure 

 as the tendons themselves, with the exception of the more deli- 

 cately constructed rectinacula tendinum. 



The synovial bursa?, which are invariably thin walled, consist, 

 in as far as they possess a special membrane, of variously de- 

 cussating, and often anastomosing, loosely connected bundles 

 of connective tissue, together with fine elastic fibres; whilst 

 the synovial sheaths — in conformity with their double office of 

 synovial bursas at one place, and at another of ligaments of 

 tendons, connected with tendinous sheaths — in their thinner 

 places have the structure of the bursce mucosae, in their thicker, 

 are composed of dense connective tissue. Both kinds of sacs, 

 together with the parts lying in, or otherwise bounding them, 

 are only partially covered by epithelium, which" generally con- 

 sists of a simple layer of nucleated polygonal cells of 0-004'" to 

 0*007 . 



The places destitute of an epithelium, which may be distinguished 

 by their dull lustre and yellowish appearance, and are especially 

 to be found in localities where the tendons and the parts sur- 

 rounding them are exposed to a greater pressure, invariably 

 exhibit, almost in their whole extent, the nature of fibro-car- 

 tilages ; the dense connective tissue of which they are made up, 

 and which is mixed with but few elastic fibres, containing car- 

 tilage-cells, often in very considerable number, whose size and 

 other characters present almost the same variations as those of 

 the true cartilages. Where the cartilage-cells are deposited in 

 very large proportion, the tendons are thickened, or even, as it 

 were, filled with distinct fibrocartilaginous masses (cartilagines 

 sesamoidece). Upon the cuboid bone, at the part where the tendon 



