« AnteriorContinuar »
• With all the knowledge we possess of the forms of the body, considered as an instrument, and of the mixture and mutual bearings of the rudiments to one another, yet the cause of most of the phenomena within the animal body lies so deeply hidden from our view, that it certainly never will be found. We call this hidden cause vital power; and like many others who before us have in vain directed their deluded attention to this point, we make use of a word to which we can affix no idea. This
power to live belongs not to the constituent parts of our bodies, nor does it belong to them as an instrument, neither is it a simple power ; but the result of the mutual operation of the instruments on one another—a result which varies as the operations vary, and which often, from small changes and obstructions ceases altogether. When our elementary books inform us, that the vital power in one place produces from the blood the fibres of the muscle; in another a bone; in a third the medulla of the brain ; and in another again, certain humours which are destined to be carried off; we know after this explanation as little as we knew before. This unknown cause of the phenomena of life is principally lodged in a certain part of the animal body, viz, in the nervous system, the very operation of which it constitutes. The brain and the nerves determine altogether the chemical processes within the body; and although it cannot be denied that the exercise of their functions tends to produce chemical effects ; yet we are constrained to confess, that the chemical operations therein are so far beyond our reach, that they entirely escape all our observations. Our deepest chemical researches and the finest discoveries of later times, give us no information on this subject. Nothing of what chemistry has taught us hitherto, has the smallest analogy to the operations of the nervous system, or affords us the least hint toward a knowledge of its occult nature. And the chain of our experience must always end in something inconceivable; unfortunately, this inconceivable something acts the principal part in animal chemistry, and enters so into every process, even the most minute, that the highest knowledge which we can attain, is the knowledge of the nature of the productions, whilst we are for ever excluded from the possibility of explaining how they are produced.'
In estimating the relative importance of the functions of the animal economy, the first place is unquestionably due to that performed by the brain and nerves. The Professor commences, therefore, by inquiring into the structure of these organs. Of their chemical composition our knowledge, which is far from complete, is derived chiefly from the experiments of Thouret, Fourcroy, Jordan, and Bichat; the former having made us acquainted with the composition of the brain itself, as far as the state of science at that time would permit, and Bichat having examined with considerable attention the neurilema, 'or membrane of the nerves.---By subjecting the nerves to the action of caustic alkali, the medullary part of the nerve is dissolved; while the tube formed by the neurilema is left entire.
Next in importance to the nervous stands the sanguiferous system. Many chemists have directed their attention to determine the nature and composition of the blood; but the most explicit analysis made of it was by Deyeux and Parmentier, to which Fourcroy and Vauquelin afterwards added an examination of the colouring matter. Berzelius himself has also contributed considerably to our knowledge of this remarkable fluid; not only by correcting some mistakes into which his predecessors had fallen, but by determining the character of its constituent parts with greater precision. He has ascertained that the fibrin of the colouring matter and the albumen, combined with the mineral acids in excess, form insoluble compounds which become soluble in water, on washing off the excess of acid. He has found that these substances dissolve readily in the acetic and phosphoric acids, both of which prevent the coagulation of the blood by heat ; that fibrin by being boiled in water dissolves in small quantity, while the remaining portion shrinks and becomes insoluble in acetic acid ; and that alcohol and ether change all these substances into peculiar kinds of fat. He has also discovered in blood an alkaline lactate of potash, and some peculiar animal substances, which accompany that salt all the humours of the body, and which he thinks owe their existence in the blood to the absorption of those decayed parts of the body, which are intended to be removed by means of the different secretions. He does not admit either gelatine or sulphur to be constituent parts of the blood, but regards the substance which has been mistaken for the former, as albumen in a state of imperfect coagulation ; while the latter, he thinks, is a part of the albumen from which it has been disengaged by the combined effect of boiling and caustic alkali.
The colouring matter of blood has been long regarded as owing its properties to the presence of iron : but, while Deyeux and Parmentier suppose it is a solution of iron in the free alkali of the blood, Fourcroy and Vauquelin conceive it is more probably a solution of the red subphosphat of iron in albumen. Professor B. states, that though he repeated their experiments with much care, the results were invariably different. He regards their conclusions, therefore, as erroneous, and is of opinion that we know as little now of the manner in which iron is combined with the colouring matter of the blood as when it was first discovered in it. The colouring matter cannot, he thinks, be albumen, much as it resembles it; and the microscopical observations of Leeuwenhoek and others have sufficiently ascertained that it is not dissolved in the blood. Nor did the Professor find it possible to dissolve it in serum, by triturating the coagulated cruor in it; for though the serum becomes coloured by this means, yet the colouring matter afterwards subsided leaving the serum perfectly clear as before the experiment. Serum dissolves metallic oxides, especially those of iron, to a certain extent; but none of them impart to it the colour of blood; nor does serum impregnated with iron possess the peculiar characters of the colouring matter. As, therefore, the most delicate tests of iron do not detect its presence in the colouring matter, and as the strongest acids do not separate from the blood, or from its charcoal, either the iron, or calcareous phosphate which the ashes of blood abundantly contain, Professor B. concludes that neither of these substances exist in the blood in a saline form. It becomes probable, therefore, that blood contains merely the elements of these salts, combined in some way with which we are unacquainted. And as the subphosphate of lime which enters so largely into the composition of bone, cannot be separated from dried blood by any dilute acid, he concludes that it is only produced from the decomposition of the immediate constituents of the blood ; which is effected at the precise spot where it is wanted.
On the very obscure subject of the coagulation of fibrin, Professor B. does not throw any new light. All inquiries into its cause have been hitherto fruitless ; nor does it appear, as far as we yet know, to be connected with any chemical change whatever. The chemical examination of the fibrin, the albumen, and the colouring matter, proves them to be so nearly allied in composition, that it is probable they are convertible into each other by processes going on within the body, and that they are all equally applicable to the purposes of the animal economy either in the offices of secretion, or in the reproduction of parts. On instituting an accurate comparison between the blood of man and that of the ox, Professor B. found that they possessed a remarkable resemblance, as well in the constituent parts themselves, as in the proportions ; but there was a striking difference in the chemical character of some of the constituents. This difference the Professor regards as an indication of a larger proportion of nitrogene in the blood of the ox; a circumstance very remarkable, when the respective nature of their food is considered. The Professor's supposition receives strength from the fact that the charcoal of the blood of the ox, when burnt slowly, constantly gives out carbonat of ammonia, though it may have been heated in an open vessel, and freely exposed to the air. This fact of itself appears almost conclusive of the compound nature of nitrogene.
From the examination of the blood Professor B. proceeds to examine that of the arteries, and more especially to determine the nature of that portion of the arterial structure which has been generally supposed to consist of small annular fibres possessing the character and office of the muscular fibre This was the opinion of Haller, and his theory of the pulsation of the arterial system is founded upon it; but it was rejected by John Hunter, and has been proved to be erroneous by Bichat, who discovered that no perceptible motion was produced in the arteries of a living animal by those chemical or mechanical stimuli, which excite contraction in the muscular fibre ; and who also, having endeavoured to investigate the character of this and the other membranes of the artery by maceration, came to the conclusion, that the pulsation of the artery depended solely on the action of the heart, and that the artery had merely a limited degree of loco--motion. In this state of the inquiry Professor B. entered upon a chemical investigation of the nature of the artery, from which he has obtained the most satisfactory and decisive results; and has thus afforded a very happy proof of the assistance which chemical science may bestow in perfecting our anatomical knowledge of the ultimate structure of parts. He has placed it beyond doubt that the fibrous membrane of the artery is not muscular; since instead of being soft and flaccid like muscle, and containing a large proportion of water, it is dry and elastic. Neither does it possess the same chemical properties as the muscular fibre, which is soluble in acetic acid, and forms scarcely soluble compounds with the sulphuric, nitric, and muriatic acids. The arterial fibre, on the contrary, is not soluble in acetic acid, though it is readily so in the mineral acids diluted with water, from which it is not precipitated by alkali, gr alkaline prussiats, which may be regarded as the tests of fibrih. It must be concluded, therefore, that the elasticity of this coat supplies the want of muscular structure, and that the phenomena of the pulse are altogether dependant upon it, since it must be dilated during the systole of the heart, and resume its original state during the diastole. Whether this structure extends to the minute capillary vessels, can only be conjectured in the present state of our knowledge. Analogy would lead us to suppose that it does.
On the subject of respiration, we are not presented with any new facts ; but the Professor has given a very neat historical detail of what has been donė by others; and offers a conjecture relative to the use of the colouring matter of the blood, which, we think, both novel and ingenious. He observes that the principal effect produced by blood upon the air, is effected by the colouring matter; and as this colouring matter does not penetrate any of the reproducing, and but few of the secreting capillary vessels, he supposes it is principally useful in the production of animal-heat. On this principle, he observes, it is easy to account for the diminished heat of the body after any severe loss of blood. Whether the diminution of temperature in such circumstances is real or apparent only, may admit of question, but of the fact itself, there is no doubt. We have seen it in a very remarkable degree in a boy who had been bled to a degree beyond what the powers of the system could sustain: he was in a state of perpetualshivering, until the time of his death.
Subsequent to the period at which this memoir was probably drawn up, some attempts have been made in this country by Mr. Brodie to overturn the hypothesis, which attributes the production of animal heat, to the function of respiration. For our own part, we do not consider them as by any means decisive. In attributing the production of aniinal heat to the influence of the brain, there appears to be this strong objection to the theory, that the relative magnitude of this organ compared to that of the animal, is much less in other animals, than in man, while their temperature is precisely the same; a fact which seems to be hardly compatible with the hypothesis. The labours of other experimentalists on this particular subject have merely determined the consequences resulting from the division of the eighth pair of nerves, which it is probable influences the blood, only secondarily, since its primary effect is, by degrees, to render the respiration uneasy and laborious.
From the extreme minuteness of the absorbent vessels, we have but little knowledge of their anatomical structure, ap! still less of their chemical composition. The lymph which they contain however bas been examined by Emmert and Reuss, who found it to be a complete chemical solution, which, after some time, becomes a coagulum, altogether resembling the fibrin of the blood. And as this fluid is absorbed after having served particular purposes in the animal economy, Professor B. infers that the colourless portion of the blood which penetrates the reproducing capillary vessels must also contain fibrin in a state of complete solution. The examination of this fluid as it eirculates in these vessels, and after it has been taken up by the absorbents, he observes, would be of great consequence, as also to determine the nature of the decayed and useless parts, which are removed by their means. From an examination of the hu. mours in the muscles, and of the urine, Professor B. thinks it likely that most animal matters are changed into lactic acid, phosphoric acid, and other animal matters, which are soluble in water, alcohol, which are found with the lactates in the various humours, and constitute the viscid extracts generally obtained in their analysis. Should this be the case, a much larger proportion of this viscid extract must be contained in the lymph of the absorbents than in the serum.
The secretions which are formed from that portion of the circulating fluid, which is not intended to be conveyed back again into the circulation, vary considerably in their character, though they still retain some of the characters of the fibrin and albumen,