SEROUS AND SYNOVIAL MEMBRANES. 



513 



A mixture of these two elements consti- 

 tutes the areolar tissue, which enters so largely 

 into the formation of almost all the organs. 

 The bands of the one and the fibres of the 

 other are closely interwoven, although with- 

 out mutual continuity; each givingoff branches 

 which again unite with the other neighbouring 

 subdivisions of the same kind, so as to form 

 a complicated interlacement of the two net- 

 works. This arrangement results in an innu- 

 merable series of meshes, which everywhere 

 communicate with those in their immediate 

 proximity, and the size and shape of which 

 varies within very wide limits. And these 

 limits are frequently still further extended, 

 since the separation of some of these micro- 

 scopic meshes, and the approximation and 

 condensation of others, gives rise to the for- 

 mation of a secondary net-work, which is 

 visible to the naked eye, and which, though 

 still open in every direction, possesses, espe- 

 cially in inflated and dried preparations, an 

 appearance sufficiently resembling that of 

 cells to remind one of the name formerly 

 applied to this structure, which was called, 

 as if KUT (^DXI'/V, " the cellular tissue." 



The proportion in which these two consti- 

 tuents are mixed varies greatly in the areolar 

 tissue of different parts of the body ; the pre- 

 ponderance of one over the other following 

 that of the conditions which were previously 

 stated to regulate their separate presence. 

 Thus, the likelihood of its frequent and great 

 distention is often a requisition of increased 

 elasticity, and is then accompanied by an 

 increased proportion of the yellow element. 



Similarly, the amount of this compound 

 structure present in different parts appears to 

 depend mainly on its uses. 



Its offices of uniting the different textures, 

 and of convoying the vessels and nerves, render 

 it necessary that more or less of the tissue 

 should always be present on the exterior of 

 an organ ; and the same circumstances would 

 lead us to expect a slight penetration of its 

 surface. 



In the interior of organs, however, its 

 absence is by no means infrequent, and is 

 very significant of its use. Thus, the minute 

 elements of the osseous tissue are physically 

 insusceptible of movement; the permissive and 

 facilitating structure becomes unnecessary 

 and impossible ; and is therefore absent. The 

 highly delicate nervous pulp not only pos- 

 sesses no inherent mobility, but, by the ex- 

 treme delicacy of its structure, offers a 

 physiological obstacle to movement of equal 

 importance with the preceding, and is accom- 

 panied by a similar absence of the tissue. 

 The intimate mutual connection of the mus- 

 cular fibres of the heart, and their association 

 in a common and nearly simultaneous move- 

 ment, is associated with a like deprivation of 

 this interstitial structure. The same absence 

 at once of the necessity and of the tissue is 

 seen in glandular organs, the situation of 

 which shields them from injurious external 

 force, as appears to be the case with the liver. 



But where opposite circumstances obtain, 



VOL. IV. 



where extent and variety of movement imply 

 considerable mobility of the neighbouring 

 muscles of a limb, or situation exposes an 

 organ to external violence, a large quantity 

 envelopes these different textures, penetrating 

 between the different muscles and isolating 

 their several fibres, or breaking up the gland 

 into numerous subdivisions, moveable on each 

 other : of this latter, the mamma is a familiar 

 instance. 



A similar relation might be traced in the 

 wider circumstances of its application. Not 

 only does it form a web of union to the whole 

 body, but it also presents a special layer of 

 considerable thickness, which invests its sur- 

 face, and partitions which isolate its muscles. 

 And something of a corresponding minimum 

 is found in those animals whose locomotive 

 movements are few and simple, or whose 

 situation and habits little expose them to 

 external violence. So that a rough gradation 

 might be traced through fishes, cetaceans, 

 and reptiles, to mammals ; in which last class 

 man stands pre-eminent in the number and 

 complexity of his voluntary motions, and in 

 the remarkable amount of this subservient 

 tissue. 



An increase in the freedom of movement 

 of contiguous parts is associated with an 

 increased laxity of this web, the meshes of 

 which become both longer and wider, so as 

 to be more capable of stretching. They thus 

 allow a greater amount of separation to take 

 place between the parts which are attached 

 to the extremities of their irregular net-work. 



BURS;E. Here and there throughout the 

 bociy, where integument or tendon glides 

 over a bony prominence, a further provision 

 occurs, as the development of distinct cavities, 

 which are lined by a smooth shining surface. 

 By dissecting their parietes from the sur- 

 rounding looser cellular tissue, they may be 

 artificially exhibited as a membrane ; and 

 hence these sacs, closed at all points of their 

 circumference, have received the appellation 

 of burses. In the majority of instances, their 

 interior is almost void of contents ; but, in 

 exceptional cases, they contain a considerable 

 quantity of a glairy, mucus-like fluid, which 

 closely resembles that before alluded to, as 

 naming, from its consistence, the synovial 

 membranes. This similarity of their contents 

 was till lately supposed to be the only analogy 

 borne by these structures to the synovial 

 membranes of the articulations ; and hence 

 they have been included by Henle* and other 

 systematic writers in a class of " Pseudo- 

 serous membranes," and characterised as lack- 

 ing an epithelium on their inner surface. 



Reicherfj", however, detected a layer of 

 nucleated cells lining their interior. He 

 stripped off a fine layer from this surface 

 under water, and, upon expanding and com- 

 pressing it, found that it was covered by nu- 

 merous darkish nuclei, of somewhat elon- 

 gated shape, and upon which acetic acid 



* Allgemeine Anatomie, S. 364. 

 t Mailer's Archiv., 1843. Jahresbericht fur Mi- 

 kroscopisch Anatomic, S. 339. 



I. L 



