The ideas of the French chemists, and more particu larly of Foureroy, although much more accurate than those of Haller and'his contemporaries, do not, however, appear to be perfectly correct. Finding that a large quantity of jelly could be extracted by boiling from many membranous bodies, he was disposed to regard membrane as essentially composed of jelly, or at least as differing from it rather in its physical than its chemical properties. We are indebted to Mr. Hatchett for much valuable in formation on this subject ; from his experiments we learn, that what may be regarded as the basis of mem branous matter, is a substance which, in its chemical pro perties, resembles coagulated albumen, and which seems to differ from the pure albumen of the blood, or the white of an egg, solely in its mechanical structure. Albumen appears, therefore, to be the proper basis of membranous matter, that which gives it its general form, and deter mines its peculiar texture, yet it probably always contains a portion of jelly, and, in many cases, even in greater pro portion than the albumen itself.
Jelly is very soluble in water, especially when heated ; it is thus separated from the albumen, and by the evapo ration of the water, may be obtained in a state of purity. The most characteristic property is that which has obtain ed the specific name of gelatinization, or jellying, where a solution of the substance in hot water concretes as it cools, and is again dissolved by increasing the tempera ture, without undergoing any farther change. Another peculiarity of jelly is the change which it experiences by putrefaction ; instead of acquiring the highly fetid odour of most animal substances, and generating ammonia, it becomes acid. The nature of the acid thus produced has not been accurately ascertained, hut it is supposed to be the acetic.
Mucus, or mucilage, appears likewise to enter into the composition of membrane, or at least to be always at tached to it. This substance, like jelly, is soluble in wa ter; yet it does not possess the property of gelatinizing, and differs from jelly in many of its chemical relations. It is, however, a substance of rather an indefinite nature, at least the term mucus has been hitherto applied in an indefinite manner, and has been used rather as a popular than as a scientific or a technical appellation.
considerable proportion of membrane, as well as of all the other soft parts, consists of water, and it has been supposed by many eminent physiologists, that upon the relative quantity of the water and the solid matter de pend many of the morbid changes of the body, as well as the varieties of the constitution and temperament of dif ferent individuals. These speculations formed a promi nent part of the theories of Boerhaave, and his successor Gaubius; and, to a certain extent, were adopted by Cul len. It is obvious, that when membranous matter no longer forms a part of the living body, its properties are much affected by the quantity of water with which it is combined ; and it is probable that this may be the case during life, with some of the external parts, and those that enjoy only a small portion of vitality, but it is very doubtful how far these principles will apply to the great bulk of the animal body, and affect its powers and func tions, as it was conceived to do by the Boerhaavians.
With respect to the ultimate elements of membrane, we know indeed that it consists of carbon, hydrogen, oxy gen, and azote, but we are not accurately acquainted with the mode of their combination, or with the proportion in which they exist. From its being less disposed to under go the putrefactive fermentation than most of the other soft parts of the body, it has been supposed by the French chemists to contain less azote, but this opinion seems to be founded rather upon hypothesis, than deduced from any exact facts, and is not supported by the recent expe riments of Berzelius, (Med. Chir. Trans. vol. i.)who was unable to detect any material difference between the che mical constitution of fibrin and albumen.
The greater fixedness of membrane must be immedi ately attributed to a stronger attraction between its parti cles, and this probably, in some measure, arises from its being more free from the admixture of heterogeneous bo dies, especially from its containing but a small proportion of either blood or fat, substances which seem always to have a strong tendency to decomposition. The circum stance of membranous matter generally containing less water than most of the solids of the body, may also be one cause why it is less disposed to become putrid. There are, indeed, some reasons for supposing that jelly con tains less azote than other animal compounds ; and, so far as jelly forms a constituent of membranous matter, the same remarks will apply to this substance. It is from this supposed constitution of jelly, that it is frequently said to be less completely animalized than most other of the con stituents of the body, as the chemical composition of animal matter differs from that of vegetables principally in the latter containing little or no azote. In connexion with this substance, it is deserving of our attention, that if we examine the corresponding organs of animals of different ages, those of the young animals will be found to contain a greater proportion of jelly, and those of the older of alhumen. It is on this account that the parts of young animals, such as the feet of the calf, are.princi pally employed in the preparation of jelly as an article of food; and upon the same principle it is that soups pre pared from veal differ so much from soups prepared from beef, in the great quantity of jelly contained in the for mer. We perceive, therefore, that the young animal, not only in its physical and mental powers, but even in its chemical constitution, is less completely possessed of its specific characteristics than when it has arrived at a more mature age, an observation which we shall find to be supported by many other facts, besides the one which has been stated above, respecting the membranous matter.