« PředchozíPokračovat »
gether to form a single proximate principle, and how disposed the same is to undergo chemical change when certain conditional circumstances are removed; whence result new compounds, most of them being binary in their nature.
Rarely is it the case that proteine exists pure in vegetables : it is united with phosphorus and sulphur, but the manner of combination is ill understood. The compounds there formed are, vegetable albumen, vegetable fibrine, and vegetable caseine, with gluten. The analogies of these in the animal are, animal fibrine, albumen, and caseine; and the difference between the principles are very minute proportions of sulphur and phosphorus.
These principles or compounds of proteine being found in the blood, it might be anticipated that other parts of the frame would be constituted of them, and it has been ascertained that animal fibre is a proteine compound : in the brain, liver, kidneys, and many other organs, the same compound is also met with under the form of albumen ; and we find a similar one to exist in horns, nails, hairs, and cuticle. These principles are, in fact, the bases of the nitrogenized constituents of the animal frame; the neutral quaternary organic principles of Dumas and Boussingault.
In contradistinction to these, in the vegetable are met with what are designated ternary compounds; and under this head are placed woody fibre, starch, or dextrine, and the varieties of sugar, with certain bodies obtained from these; all of which are made up of carbon, hydrogen, and oxygen, varying not very considerably in the quantities of each, yet, their molecules being differently arranged, dissimilar compounds are formed; still they are easily convertible the one into the other.
These are partaken of by animals, and either undergo combustion within the living organism, to keep up the heat of the body, which is essential to life; or their elements are deposited in the form of fat, the supply of oxygen not being sufficient for the first named purpose, or nature not requiring it.
“ The stall fed animal,” says Liebig, “ eats, and reposes merely for digestion. It devours, in the shape of nitrogenized compounds, far more food than is required for reproduction or the supply of waste alone ; and, at the same time, it eats far more of substances devoid of nitrogen than is necessary merely to support respiration, and to keep up animal heat. Want of exercise and
diminished cooling are equivalent to a deficient supply of oxygen ; for when these circumstances occur, the animal absorbs much less oxygen than is required to convert into carbonic acid the carbon of the substances destined for respiration. Only a small part of the excess of carbon thus occasioned is expelled from the body of the horse, and ox in the form of hippuric acid ; and all the remainder is employed in the production of a substance which, in a normal state, only occurs in small quantity, as a constituent of the nerves and brain : this substance is fat.”
Again, he says, “It is evident that the formation of fat in the animal body is the result of a want of due proportion between the food taken into the stomach and the oxygen absorbed by the lungs and skin.”
This has led him to divide all alimentary substances into two classes, under the heads of Elements of Nutrition, and Elements of Respiration.
On the other hand, Dumas asserts, that fat, like all other animal principles, is generated by vegetables ; " that they pass ready formed from them into the bodies of animals, that there they may either be burnt immediately, in order to supply the heat which the animal requires, or that they may be laid up in the tissues more or less modified, to serve as a reserve for respiration. With a view to verify this idea, we instituted many experiments, which all led us to recognise in the food of the herbivorous animals subjected to experiment, quantities of fatty matter superior to those found in the milk of the milch-cow, for example, or stored up in the tissues of the ox put up to fatten. By keeping an account of the fatty matter contained in the dung, and adding it to the quantity fixed, the sum obtained is still inferior to the quantity of fat which analysis discovers in the food of the animal. With these facts before us, it appeared to us natural to admit that animals assimilated directly the fatty substances of vegetables without modifying them at all, or modifying them but little.”
The truth is, the formation of fat in animals has awakened a controversy between the two great chemists of Germany and France,-Liebig and Dumas. Liebig is of opinion, as has been already stated, that graminivorous animals produce fat from the elements of starch or sugar partaken of by them, these nor being burnt up by the oxygen taken in during respiration ; while Dumas,
with whom is associated Boussingault, considers it a fixed rule that animals produce neither fat nor any other substance capable of ministering to nutrition, but that every principle exists ready formed in the vegetables on which the animals feed : and certainly the multiplied and delicate analysis they have made of hay, maize, and other provender given to fatten cattle, fowls, &c., prove the existence of a substance closely resembling fat, as one of their constituents.
They contend that the fatty matters are elaborated in the leaves of plants in the form of a waxy principle, which, passing into the bodies of the hebivora, undergoes a partial oxidation, whence result the stearic and oleic acids met with in tallow. In carnivorous animals these are oxidised anew, and margaric acid is formed, which characterises their fat. Lastly, by a still higher oxidation, the volatile fat acids are formed which appear in the blood and perspiration.
This difference of opinion appears likely to be set at rest by a discovery lately made by Pelouge and Gelis, which is, that butyric acid—an acid hitherto derived from animal fat-may be formed by modifying the fermentation of sugar. This particular result is brought about by the presence of caseine in the matter undergoing the process of fermentation, and “ the change of sugar into butyric acid takes place without any considerable increase of temperature, and without the presence of those energetic substances which could destroy the equilibrium or affect the vitality of the animal tissues ; while this transformation is effected under very simple conditions, and in substances employed by Nature herself.”
Now, if we associate with this the fact, that farinaceous matters undergo a ready conversion into a saccharine principle, we have a clue to explain how it is that these substances become so fattening to animals as it is well known they do.
The lecturer next proceeded to shew how digestion and other functions carried on in the animal economy receive elucidation by an application of chemical laws; also the formation of the various secretions, with their uses, particularly referring to bile and urine. Of the first of these he remarked :
The use of bile in the living organism has been the subject of much inquiry. That it is of considerable importance viewed as a secretion from the blood, by which this fluid becomes depurated, and also from the part it subsequently performs in the animal economy, since it has not been met with in the egesta, is unquestionable.
It has been found, that, when bile has been prevented flowing into the bowels by the formation of a fistula opening outwards, animals have become emaciated, and ultimately died. On the other hand, when the function of the liver is interfered with in a peculiar way, that fat in an undue quantity is deposited. This is seen in the enlarged liver of geese, for making the páté de fois gras, and in the first stages of rot in sheep. It is evident, therefore, that the presence or non-presence of bile has something to do with the production of fat.
Dr. Kemp has lately advanced some ingenious views on this subject. He concurs with Liebig, that animal fat is formed by the assimilation of the non-nitrogenized portions of the food, of which starch may be taken as the type. It is, however, he says, obscure, since the composition of starch and fat is not the same, there being a large excess of oxygen in the first named when compared with the last; this element, therefore, must enter into combination in a manner as yet not understood. If the whole of the assimilated portion of the food pass through the thoracic duct into the system, it is evident that, in the chyle, we must have all that is necessary for the fresh deposit continually going on in the frame, whether of tissues or fat. Now the analysis of this fluid by Marcet and Macaire proves its elements to be so proportioned as to represent proteine in combination with another body capable of forming fats, and legitimately expressed by an empirical formula containing the elements of starch and bile. The presence of this last-named fluid appears to be necessary, otherwise the starch would be split up into binary forms, as carbonic oxide and acid, or carburetted hydrogen, as seen in hoven ; or it would form sugar, as in diabetes.
It hence follows, that if proteine be withdrawn—that which constitutes the basis of the nitrogenized parts of the frame—the other principle must be employed for the formation of fat. Dr. Kemp concludes, “ that in graminivorous animals, the bile, during the digestive operation, is deprived of its nitrogen in the form of ammonia, absorbs oxygen, then enters into combination with the amylaceous parts of the food, and is thus absorbed, together with the proteine compounds. Further, that the object of the amylaceous portion of the food is not immediately to form fat, but, by combination with bile, to produce a compound containing the elements of fat, with an excess of oxygen for cartilage, gelatine, and other allied structures, which may be represented by proteine plus oxygen.”
A clue is thus obtained to the oft repeated assertion, that the liver has to do with the deposition of fat in the body; and we may perhaps trace the formation of the fatty acids in butter and the production of milk to the same cause.
In reference to urine, he observed, that this fluid may be said to contain the soluble constituents of the ashes of the elements, the fæces the insoluble. This secretion in man has an acid reaction, the cause of which is still disputed. In the horse and cow it is alkaline, depending on the presence of the hippurate, benzoate, or carbonate of soda or potash. And we need not be at any loss to account for the alkalinity of this fluid in those animals, if we only bear in mind the character of the food they eat, it abounding with the carbonate of soda or potash.
In the urine of the herbivora the phosphates are seldom met with ; but in the graminivora they may be present, as the soluble phosphates exist in grain ; and these, too, have an alkaline reaction.
Thus we trace the character of the secretions to the nature of the food of animals.
According to Liebig, the organic acids in the urine of man and the carnivora are the uric and hippuric, with another nitrogenous matter (probably the colouring principle of urine), which latter substance, upon the access of air, resolves itself into acetic acid and a substance resembling resin.
The hippuric acid he considers a product of the organism, to the formation of which the elements of the nitrogenous aliments give birth. By decomposition it is resolved into benzoic acid; and the same metamorphosis takes place when the horse or cow is exercised or put to labour, since in both of these animals this acid is found.
Hippuric acid requires about 400 times its weight of cold water to cause it to undergo solution ; it therefore cannot find its way