518 



SEROUS AND SYNOVIAL MEMBRANES. 



The conjecture just mentioned derives con- 

 siderable support from a comparison of their 

 structure in the adult with their younger and 

 foetal conditions in the same animal. In the 

 latter especially, the quantity of cell- growth 

 on the surface is so great, as, in respect of 

 mere continuousness, almost to merit the 

 appellation of an epithelium. But though its 

 constituent cells lack the angularity, and 

 somewhat the size, of those which belong to 

 the inferior strata ; yet, like the same cells 

 in the adult, they are quite distinguishable 

 from the epithelial covering of the bursa, 

 not only by their appearances with and with- 

 out acetic acid, but by the distance to which 

 the shifting focus follows them. At successive 

 depths, they are seen to become somewhat 

 larger, more angular, and wider apart ; and 

 the same process of fissiparous multiplication 

 may be detected in them as in the adult cells. 

 We shall see that these differences at the 

 different stages of the animal's life experience 

 a close parallel in articular cartilage. 



In a few instances, I have witnessed 

 another form of cell-multiplication in this 

 tissue. It occurred in one or two cats of a 

 few months' age, but I cannot say whether it 

 is limited to any particular period of their 

 life. It is represented in fig. 398, and con- 



Fig. 398. 



Compound Cells of Bursal Fibro-cartilage, From the 

 young Cat. (Magnified 400 diameters.} 



sists of an oval or elongated vesicle or cell 

 of limitary membrane, which is filled with, 

 and usually more or less bulged by, a number 

 of cytoblasts. These compound vesicles 

 were sparingly scattered through the carti- 

 lage-like tendon and periosteum ; similar 

 masses of cytoblasts, of a spherical form, may 

 occasionally be seen in young articular carti- 

 lage ; and, indeed, instances of this form of 

 cell-multiplication might be adduced from 

 many structures, temporary, permanent, and 

 morbid, but their introduction would be 

 foreign to the province of this article. 



The constitution of the synovial sheaths of 

 tendons resembles that of these bursae in 

 many respects ; and, on the whole, offers a 

 still closer approximation to the structure of 

 a joint. In many places, the sheath consists 

 only of a delicate transparent membrane, the 

 tenuity of which approaches that of the 

 serous membranes, and which, like them, rests 

 on a stratum of loose areolar tissue, and is 

 reflected from the parietes of the cavity to 

 the tendon where it enters it. Here they 

 possess an oval or slightly angular epithelium, 

 which constitutes only one layer. But almost 



every such tendon, in some part or other of 

 its course, offers an alteration of direction 

 implying considerable friction, and effected 

 either by a projection of bone, or by a pulley 

 of thick and strong fascia. Such are the 

 grooves and posterior carpal ligament for the 

 extensors at the back of the wrist. And 

 here is again discovered the condition which 

 was previously stated of the obturator tendon, 

 but with some slight modifications; firstly, 

 that the approximation to the structure of 

 cartilage, here visible to the naked eye, affects 

 equally the whole periphery of the tendon, 

 instead of being limited, as heretofore, to one 

 of its surfaces ; and, secondly, that the cell- 

 growth is more plenteous, sufficiently so as to 

 offer scarcely a point of the surface unoc- 

 cupied by cells ; of which the shape, size, and 

 disposition almost exactly resemble those of 

 the surface of articular cartilage in the young 

 mammal or the adult reptile. 



In the cartilaginous-looking portions of the 

 sheath, a similar, but less extensive depth of 

 cell-growth obtains ; and I believe I have 

 recognised the same condition in the surface 

 of the crucial ligaments of the knee-joint. 



The vessels of these synovial sheaths are 

 very numerous, and their capillaries exhibit a 

 tortuous arrangement which is identical with 

 that witnessed in the articular synovial mem- 

 branes hereafter to be described. But this 

 copious supply of vessels is limited to the 

 delicate membranous portions of the sheath, 

 and to those mesentery-like reflections which 

 here and there pass from the parietes to the 

 contained tendon. Wherever the tendons 

 are subject to much friction, and evince the 

 partially cartilaginous structure already de- 

 scribed, there the vessels are absent from those 

 superficial and cell-containing stra.ta, and, so 

 far as I know, are limited to the deeper and 

 non-cellular parts of the tendon. 



SYNOVIAL MEMBRANES. The synovial 

 membranes are structures exceedingly analo- 

 gous to the preceding, and consist of a layer 

 of cell-growth, which covers the inner surface 

 of the ligaments that connect the different 

 segments of the skeleton in the diarthrodial 

 joints, and which thus partially lines the 

 " cavity " or interior of these articulations. 



They have been usually described as re- 

 sembling the burses mucoscE both in the nature 

 and consistence of their secretion, and in 

 their constant adherence to the morphological 

 character of a shut sac ; while the absence of 

 epithelium predicated of these bursa?, has 

 been laid down as the chief anatomical dis- 

 tinction between the two structures. And, 

 on the other hand, they have been likened to 

 the serous membranes by the common pos- 

 session of a tesselated epithelium, and by 

 their continuity over the whole surface of the 

 cavity and its contents ; while they have 

 been severed from them by the difference in 

 the composition and consistence of their 

 secretion, a viscid alkaline fluid, instead of 

 a more limpid and neutral one. Most of 

 these statements can only be received with 

 some modification. 



