The physical properties which more especially belong to membrane, are, cohesion, flexibility, extensibility, and elasticity. By its strong cohesive power, it is well fitted to strengthen and support the different organs of the body, and to render their union firm and durable, to serve as a complete covering for them; and, in short, to perform all those offices where much strength is requisite. Its flexible nature peculiarly adapts it for the structure of those parts where much motion is exercised, as about the joints and muscles, in the large blood vessels, and in the cellular substance. Besides affording a capacity for mo tion in general, the flexibility of the membranous matter enables the different parts to yield to external violence, and thus to sustain much less injury than if they had pos sessed a more rigid texture. The advantages which we derive from the flexible nature of membrane, are inti mately connected with its extensibility. This property is essential to the structure of a system, which is princi-• pally composed of soft parts, perpetually in motion, and constantly altering their form and bulk, where some are contracted, while others are necessarily stretched beyond their ordinary size, and which are all surrounded and held together by membranes. This quality is peculiarly un portant in the different organs which are destined for the reception of fluids, whether in the form of pouches or of tubes ; the quantity of fluid which they contain is per petually varying; and, according to their present constitu tion, the size of the recipient is always exactly fitted to the bulk of the contents No less important to the animal system is the elasticity of the membranous matter. As we advance in our knowledge of the subject, we shall be better able to estimate the importance of this property ; at present it will be sufficient to remark, that it serves an important purpose in the action of the organs of circula tion and of respiration, that it frequently co-operates with the muscles in the motions of the joints, and that it is employed to restore the situation of parts which had been previously removed by muscular contraction from their natural position.
Besides the above properties which membrane pos sesses in common with various kinds of matter, it has been thought by some physiologists, especially those of the French school, to exhibit qualities which are more pro perly of a vital nature, or such as belong only to bodies that form part of the living animal system. Bichat con ceives that membrane is contractile, and adduces some facts in support of his opinion ;• but, we apprehend, that when they are duly considered, they will be found to be all referable to elasticity. Blumenbach, to whom the science of physiology is so much indebted, also ascribes to membrane a specific power, which he terms the vis cellulose, which consists in the reaction of a membrane that has been distended, when the stretching force is with drawn vt but all cases of this kind, when carefully ex amined, may, we conceive, be referred, like those ad duced by Bichat, to the effect of elasticity.
The sensibility of membrane is a point respecting which very various opinions have been entertained by phy siologists, and especially by those of the last century, when it became the subject of a warm controversy between Haller and his pupils on one hand, and Whytt, in con junction with his countrymen, on the other. Haller in stituted a variety of experiments on living animus, from which he deduced the conclusion, that mere membrane is altogether without sensation, as he was unable to ex cite any appearance of it, by applying the most powerful stimuli, either chemical or mechanical. \Vhytt, in oppo
sition to the experiments of Haller, brought forward a number of well known facts, connected with the diseased condition of these parts, where, by being inflamed, they produce the most acute pain. Upon the whole, the opi nion of Haller is to be regarded as the one that is literally correct, because it is generally admitted that sensation is confined to the nervous matter ; and it is known that mem brane is very sparingly furnished with nerves, and must therefore have a corresponding degree of insensibility. To what cause we are to ascribe the extreme pain which attends certain morbid conditions of tendons and other similar structures, is a question to which, at present, we are, perhaps, not able to give a satisfactory answer.
It is probable that the erroneous opinions of the mo dern anatomists, at least of those who lived shortly after the revival of letters, were, in a considerable degree, pro duced by the mistaken notions of the ancients respecting the nature of membranes, and the relation of this substance to the nerves. They had generally but a very imperfect acquaintance with the nervous system, and were in the habit of confounding tendons with nerves; and hence it was laid down as a principle, that the tendons are among the most sensible organs of the body. Long after the distinction between these parts was clearly ascertained, the influence of the old doctrine was felt, not only in our physiological speculations, but even in the details of sur gical practice, and gave rise to operations which were extremely painful and dangerous, and which were em ployed for the purpose of avoiding the problematical evil of dividing a membrane.
So very imperfect was the knowledge of animal che mistry, even as late as the time of Haller and Cullen, that they supposed all the soft parts to consist of the same chemical substance, differing only in its mechanical ar rangement. Haller had an opinion, that membrane, being the least complicated part of the • body, consisted prin cipally of simple fibres, which served as a kind of basis to the whole system, and that the fibre itself was composed of earthy particles, cemented by gluten. The discoveries of the pneumatic chemists, and especially of the French, who have assiduously cultivated this branch of the science, proved that Haller's opinion is totally fallacious, and that earth is not ari essential constituent of membrane. The hypothesis of the connecting gluten is equally gratuitous, and quite contrary to the more correct notions of modern chemistry. The particles of membrane, as well as those which compose any other solid, are held together by their affinity for each other, not by any connecting medium. Membrane, indeed, acts mechanically in uniting the dif ferent parts of the body to each other, and in maintaining the proper form of the substances, which are of so deli cate a consistence as not to afford a sufficient degree of adhesion between their particles to keep them in a com pact state. The soft pulp of the nerves, for example, and the adipose matter, seem to be retained in their present form, merely by the membrane in which they are imbed ded ; but this is altogether independent of the consistence of the membrane itself.