132 



NORMAL HISTOLOGY. 



naked cartilage, over whose surfaces of contact not even the imper- 

 fect endothelial covering is continued ; tendons or other structures 

 traversing the joint-cavity receive an investment of the synovial 

 membrane. The marginal zone, embracing the attachment of the 

 membrane to the cartilage, is marked by the gradual alteration of 

 the tissues of the synovial membrane to assume the characters first 

 of fibre-cartilage, and finally of the typical articular cartilage of which 

 the membrane then seems a part. 



The synovial sacs, originating as clefts within the mesoderm 

 surrounding the extremities of the young bones, exhibit a structure 

 corresponding to slightly condensed connective tissue. The mem- 

 brane is composed chiefly of closely-felted bundles of fibrous tissue, 

 mingled with elastic fibres, containing the usual connective-tissue 

 elements ; the free surface of the membrane possesses an imperfect 

 covering of connective-tissue cells, which, when closely placed, 

 as in the younger tissue, present the characters of an endothelium ; 

 when less densely arranged, they retain their processes and appear 

 as branched elements, resembling those of other dense fibrous tissues ; 

 in the vaginal membranes the cells are often elongated to correspond 

 with the axis of the sheath. 



Cleft folds of the synovial membrane project into the serous cavity 

 as the Haversian fringes ; they are free processes of the membrane 

 containing vascular loops and, in the larger ones, fat ; the smaller 

 secondary fringes, or villi, often present as finger-like processes 

 attached to the edges of the larger folds, contain no blood-vessels, 

 but consist principally of small, irregularly-round cells, separated by 

 a scanty intercellular substance. In some cases these villi enclose a 

 denser core, which consists of fibrous bundles ; occasionally the entire 

 villus is formed of fibro-cartilage, the superficial round cells being 

 wanting. 



Blood-vessels are quite numerous within the synovial mem- 

 branes, as well as in the subjacent tissues, nearly all parts of the 

 joints being generously supplied. Many of the Haversian fringes 

 contain vascular tufts, while the termination of the blood-vessels 

 around the margin of the cartilages is marked by vascular loops 

 possessing greatly dilated terminal arches. 



The nerves of the synovial membranes, by no means numerous, 

 form a loose plexus beneath the free surface ; in connection with the 

 joints, peculiar special nerve-endings, the articular end-bulbs of 

 Krause, have been found attached to the nerve-fibres ; Pacinian 

 corpuscles have likewise been observed in relation with the synovial 

 membranes. 



The serous surfaces lining the blood-vessels and the lymphatic 



