Section I.
FOBM AND DISPOSITION.
We perceive from the preceding enumeration, that the serous membranes surround the organs which are the most essential to the support of life ; und it is re- markable, that these organs arc generally lined on the opposite side by the mucous membranes ; there are, hovfever, exceptions to this arrangement, for in somt places, OS around the brain, the heart, and the testes, the serous membrane ia met with separately, while other parts, as the nostrils, the mouth, and the oisophaguH, are only provided with a mucous lining. Bichat, regarding the serous membranes collectivelyf concludes that they are more extensive than the membrniies belonging to the mucous system, but the correctness of this opinion may be questioned, especially when the ramified dis- position of the latter structure in the interior of the lungs, of the Hver, of the kidneys, &c., is compared willi the simple covering which is ftimishcd on the exterior of these organs by the serous tissue.
In the general description of the serous system, the principal facts connected with the membranes under consideration have been mentioned. Each of them forms a closed sack, which is in some instances ren- dered very complicated by the reflexions it undergoes ; one portion of the membrane gives a covering to the walls of the canty in wiiich it is placed, and another portion affords an envelope to the contained viscera, so' that every serous bag possesses a parietal and a tnscerat layer, which arc continuoiw with each other at tlio point of reflexion. The pleura, the pericardium, and
m3
ABACHNOID AKO PEKimNBTM.
the tunica \iig)nali3 are sin)|:4e in tbor omngenieot, and are consequentlj readily comprehended, but the coanexio&s of tbe tunica arachnoides, and etpeciaDy of the periloncum, arc more intricate, and require a very careful examination before tbL-y can be uodcntood. In proceeding with such an investigation, the principal characters of the serous oiembranea must be horoe io mind : vix. that they are closed sack», that they arc reflected, and that the 'i5cera are placed on th^ ex- ternal surface.
Tlie tunica arachnoides covers the nurface of the l»ajn, anil, pairing into the aperture discoi'ered br Bichat. it lines the ventricles ; it is also reflected on thi- inner part of the dura mater. At the base of the encephalon it 5tx- itisheH a sheath to each of the nerves, which extends as- finr as the p<iint where the uvrve quits the cranium, at which place it is reflected to the dura mater. The anich-' noid also covers the spinal cord, and sends off" processes,.] -which invest the nerves, till tliey quit the theca vertebndia,« j ■when tlie mumbrnne passes on to the spinid portion of theN dura muter which it lines throughout its whole extent.
Tlie connexion which exi&ts between the periloncum.j and itie viscera of the abdomen, is liable to considerable variation, but it may be staled, that in jieneral those or<> guns which are placed near the panetes have a very imper- fect covering, and arc lixe<l in their situation ; whilst those* which are farther remo^'ed from the walls, possess a.j more perfect peritoneal tunic, and iilao enjoy a (freater extent of motion, Tlie peritoneum is united in such a-j manner with the viscera, that it allows of those changes, in the sixe of the organs, which so frequently occur ;', for example, the two layers of membrane ihnl form Ihe' muaentery, the great umeutum, and the broad ligament-
ADMKKKNT AND FKRB SURFACES.
165
of the uterus, ure so loosely joined to each other by the cellular texture, that they readily separate when the oi^n connected with them is enlarged or distended beyond a certain degree. Thus, if the stomach be much increased in size, it glides between the layers of 'the omentum, and n similar change is exhibited, but in a more striking degree, during pregnancy, in which state the enliirgod uterus gruduully separates the folds of the broail Ugamcrits, till e^-cntually they become ob- literated.
.. The splanchnic neroxis membranes have an adherent land a free surface. The external or attached surface is connected either to the parietes of the ca*ities, or to the exterior of the viscera : this union depends priiici- ptdly on the subserous cellular texture, but partly on tlie vCBseb of these membranes. The firmness of the ad- bcKion is not every where the same; in many places it is so inconsiderable, that a sepanition can be readily ef- fected, as between the peritoneum and the muscles of the abdomen, or between the pleura and the walU of the chest, but the visceral layer is so closely attached that some care is required in raising it from the sub- jacent organ. The connexion is still more strict be- tween the serous membranes and the fibrous structures; thus it is extremely ditKcult to depurate the tunica arach- Doidea from the dura mater, the reflected portion of the pericardium from the fibrous structure which strengthens it, or the tunica vagiuolis testis from the tunica albu- ginea.
The free surface, which is polished and moistened by ft thin fluid, gives to the viscera that shining appearance by which they arc characterized, and which is so re- markable, that the limits of the serous membrane may
I4d * OftQANlZATtON.
be distinguished on any oi^an, as the urinary bladder, by the extent of tlm polish. It has been stated in «| previous part of this chapter, that the fifee surfocc io- sulateif the organs which arc placed in juxta positioi with each other, an the stomach and the liver ; and that it facilitates in an especial manner, the movemeat of the parts which are provided with such a covering'^'
Sbction II.
OROANIZATION.
T»E9E membranes are essentially compoacd of con- densed cellular tissue, into which substance they may be reduced by niaccration. Each of them is formed of, one layer, the texture of which is very close towards thef internal surface, whilst externally it becomes loose and flocculent, and in confounded with the odlulsr mem-j brane, by which it is attached to the surrounding parts.
The peritoneum and the pleura appear to contain aa | immense number of vessels, which are so minute that] they do not ordinarily admit the coloured part of tb« ! blood. They are rendered apparent by injection, aod particularly by inflammation. It is necessary, however, to distinguish the vessels of the subserous cellular tissue from those of the membrane itself; the fonner are very numerous, and as they admit the red particles, they are readily perceived through the transparent serous mem* brane. The vascularity of these organs, and &<i])eciaUy of the tunica arachnoideii, has been denied by Rudolpht, llibes, and others, who state that no vestige of vessel .CI& be dlscowred by the microscope in (he substance of
the serous tissue. 1 had an opportunity, about two years since, of examining the arachnoid in an extremely opake and thickened condition. After die arteries of the brain had been tilled with a fine injection, a great number of vessels were perceived, apparently raraifying In the mem- hrane; but, on a careful inspection, it was e'ident tiiat most of them were placed in the opake mutter which was effused between the arachnoid tunic and the pia ma- ter, »() that 1 could not satisfactorily determine if any arteries penetrated into the former membrane. 1 am, however, inclined to admit the existence of vessels, be- cause it is difficult otherwise to account for the jiroduc-' tion of the opacity, and other morbid changes of stnic^ ture, which are so fre((uently observed in the timicd ftiachnoides, the pleura, Stc. The presence of lyrnpiia- tica is indicated by the active absorption which is con- stantly going on in the iiernua membranes. No nerves have been traced into the thickness of these structures, and they possess little or no sensibility dnring health, although they become painful under disease ; but it is probable that the principal part of the severe pain which is experienced in pmtonitis. Sic. shonld be refi'rred W the pressure which thcBe membranes, in conse(inence of tire strictness of their adhesion, exert on the subjacent parts. •
The serous membranes, when Ihey are dried. bL-eOffH? yellowish ; md if they were previously opake, they ttre rendered transparent. Maceration in water restores them to their former appearance; and if it be contmi^, it makes them soft, opake, and pulpy, and e%*entuallT it diseolves them.
PHOPEKTIES AND ANALYSIS