History of Civilization in England, Vol. 3 of 3

According to Cullen, all the solids in the human body are either simple or vital. The simple solids retain, after death, the properties which they possessed during life. But the vital solids, which form the fundamental part of the nervous system, are marked by properties, which disappear directly death occurs.830 Hence, the simple solids, having fewer functions than the vital, have also fewer diseases; and the maladies to which they are liable admit of easy classification.831 The real difficulty lies in the vital solids, because on their peculiarities the whole nervous system depends, and nearly all disorders are immediately due to changes in them. Cullen, therefore, made the nervous system the basis of his pathology; and, in speculating on its functions, he assigned the chief place to an occult principle, which he termed the Animal Power, or Energy, of the brain.832 This principle acted on the vital solids. When the principle worked well, the body was healthy; when it worked ill, the body was unhealthy. Since, then, the state of the vital solids was the main cause of disorder, and since the Energy of the brain was the main cause of the state of the vital solids, it became important to know what the influences were which acted on the Energy, because in them we should find the beginning of the series. Those influences were divided by Cullen into physical and mental. The physical were, heat, cold, and effluvia, the three most potent of the material disturbers of the human frame.833 The mental influences, which excited the brain to act on the solids, were comprised under six different heads, namely, the will, the emotions, the appetites, the propensities, and, finally, the two great principles of habit and of imitation, on which he, with good reason, laid considerable stress.834 In arguing from these mental causes, and in generalizing the relations between them and the sensations of the body, he, faithful to his favourite method, proceeded deductively from the metaphysical principles then in vogue, without inquiring inductively into their validity, such an induction being, he thought, no part of his duty.835 He was too anxious to get on with his dialectic, to be interrupted by so trifling a matter as the truth or falsehood of the premisses on which the reason rested. What he did in the metaphysical part of his pathology, he also did in its physical part. Although the blood and the nerves are the two leading features of the human economy, he did not search into them by a separate induction; he subjected them neither to chemical experiments in order to learn their composition, nor to microscopic observations in order to learn their structure.836 This is the more observable, because though we must admit that animal chemistry was then generally neglected, and that its real meaning was scarcely understood until the wonderful labours of Berzelius revealed its importance, still the microscope was ready to Cullen's hands; it having been invented a hundred and fifty years before he completed his pathology, and having been in common scientific use for about a hundred years. But his love of synthesis overcame him. His system is constructed by reasoning from general principles; and of that process, he certainly was a consummate master. Between the premisses and the conclusion, he hardly ever lets error creep in. And, in reference to the results of his speculations, he had one immense merit, which will always secure to him a conspicuous place in the history of pathology. By insisting on the importance of the solids, he, one-sided though he was, corrected the equal one-sidedness of his predecessors; for, with extremely few exceptions, all the best pathologists, from Galen downwards, had erred in ascribing too much to the fluids, and had upheld a purely humoral pathology. Cullen turned the minds of men in the other direction; and though, in teaching them that the nervous system is the sole primary seat of disease, he committed a great mistake, it was a mistake of the most salutary kind. By leaning on that side, he restored the balance. Hence, I have no doubt, he indirectly encouraged those minute researches into the nerves, which he would not himself stop to make, but which, in the next generation, gave rise to the capital discoveries of Bell, Shaw, Mayo, and Marshall Hall. At the same time, the old humoral pathology, which had prevailed for many centuries, was practically pernicious, because, assuming that all diseases are in the blood, it produced that constant and indiscriminate venesection, which destroyed innumerable lives, besides the irreparable injury it often inflicted both on body and mind; weakening those whom it was unable to slay. Against this merciless onslaught, which made medicine the curse of mankind, the Solid Pathology was the first effective barrier.837 Practically, therefore, as well as speculatively, we must hail Cullen as a great benefactor of his species; and we must regard his appearance as an epoch in the history of human comfort, as well as in the history of human thought.

It may, perhaps, facilitate the conceptions of unprofessional readers, if I give, in as few words as possible, a specimen of the way in which Cullen employed his method, in investigating the theory of some one class of diseases. For this purpose, I will select his doctrine of fever, which, though now generally abandoned, once exercised more influence than any other part of his pathology. Here, as elsewhere, he reasons from the solids.838 Disregarding the state of the blood, he says, that the cause of all fever is a diminished energy of the brain.839 Such diminution may be produced by various sedatives, the most common of which are effluvia, whether marsh or human, intemperance, fear, and cold.840 Directly the energy of the brain is impaired, the disease begins. Rapidly passing through the nervous system, its first palpable effect is a chill, or cold fit, which is accompanied by a spasm on the extremities of the arteries, particularly where they touch the surface of the body.841 This spasm on the extreme vessels, irritates the heart and arteries, and the irritation continues till the spasm is relaxed.842 At the same time, the increased action of the heart restores the energy of the brain; the system rallies; the extreme vessels are relieved; while, as a consequence of the whole movement, sweat is excreted, and the fever abates.843 Shutting out, therefore, all consideration of the fluids of the body, the successive stages of languor, cold fit, and hot fit, might, in Cullen's opinion, be generalized by reasoning merely from the solids, which, furthermore, produced his well-known distinction between fevers, the continuance of which is owing to an excess of spasm, and those, the continuance of which is owing to an excess of debility.844

A similar process of thought gave birth to his Nosology, or general classification of diseases, which some have regarded as the most valuable part of his labours;845 though, for reasons already mentioned, we must, I think, reject all such attempts as premature, and as likely to work more harm than good, unless they are simply used as a contrivance to aid the memory. At all events, the Nosology of Cullen, though it exhibits clear traces of his powerful and organizing mind, is fast falling into disrepute, and we may be sure, that, for a long time yet, a similar fate will await its successors. Our pathological knowledge is still too young for so great an enterprise.846 We have every reason to expect, that, with the aid of chemistry, and of the microscope, it will continue to grow more rapidly than it has hitherto done. Without venturing to predict the rate of its increase, we may form some idea of it, by considering what has been effected with resources very inferior to those we now possess. In a work of great authority, published in the year 1848, it is stated, that since the appearance of Cullen's Nosology, our mere enumeration of diseases has almost doubled, while our knowledge of the facts relating to disease has more than doubled.847

I have now only one more name to add to this splendid catalogue of the great Scotchmen of the eighteenth century.848 But it is the name of a man, who, for comprehensive and original genius, comes immediately after Adam Smith, and must be placed far above any other philosopher whom Scotland has produced. I mean, of course, John Hunter, whose only fault was, an occasional obscurity, not merely of language, but also of thought. In this respect, and, perhaps, in this alone, Adam Smith had the advantage; for his mind was so flexible, and moved so freely, that even the vastest designs were unable to oppress it. With Hunter, on the contrary, it sometimes seemed as if the understanding was troubled by the grandeur of his own conceptions, and doubted what path it ought to take. He hesitated; the utterance of his intellect was indistinct.849 Still, his powers were so extraordinary, that, among the great masters of organic science, he belongs, I apprehend, to the same rank as Aristotle, Harvey, and Bichat, and is somewhat superior either to Haller or Cuvier. As to this classification, men will differ, according to their different ideas of the nature of science, and, above all, according to the extent to which they appreciate the importance of philosophic method. It is from this latter point of view that I have, at present, to consider the character of John Hunter; and, in tracing the movements of his most remarkable mind, we shall find, that, in it, deduction and induction were more intimately united than in any other Scotch intellect, either of the seventeenth or eighteenth century. The causes of this unusual combination, I will now endeavour to ascertain. When they are understood, they will not only explain many peculiarities in his works, but will afford materials for speculation, to those who love to examine the development of ideas, and who are able to discern the way in which different schemes of national thought have given different shapes to national character, and have thereby modified the whole course of human affairs, to an extent of which the ordinary compilers of history have not the slightest suspicion.

Hunter remained in Scotland till the age of twenty, when he settled in London; and, though he was abroad for about three years, he abandoned his own country, and became, socially and intellectually, a native of England.850 Hence, the early associations of his mind were formed in the midst of a deductive nation; the later associations, in the midst of an inductive one. For twenty years he lived among a people, who are, perhaps, the acutest reasoners in Europe, if you concede to them the principles from which they reason; but who, on the other hand, owing to their proneness to this method, are so greedy after general principles, that they will accept them on almost any evidence, and are, therefore, at once very credulous and very logical. In that school, and surrounded by those habits, the intellect of John Hunter was nurtured during the most impressible period of his life. Then the scene suddenly shifted. Coming to England, he passed forty years in the heart of the most empirical nation in Europe; a nation utterly abhorring all general principles, priding itself on its common sense, boasting, and with good reason too, of its practical sagacity, proclaiming aloud the superiority of facts over ideas, and despising every theory, unless some direct and immediate benefit could be expected to accrue from it. The young and ardent Scotchman found himself transplanted into a country totally different from that which he had just quitted; and such a difference could not fail to influence his mind. He saw, on every side, marks of prosperity, and of long and uninterrupted success, not only in practical, but also in speculative, life; and he was told that these things were effected by a system which made facts the first consideration. He was ambitious of fame, but he perceived that the road to fame was not the same in England as in Scotland. In Scotland, a great logician would be deemed a great man; in England, little account would be made of the beauty of his logic, unless he was careful that the premisses from which he argued, were trustworthy, and verified by experience. A new machine, a new experiment, the discovery of a salt, or of a bone, would, in England, receive a wider homage, than the most profound speculation from which no obvious results were apprehended. That this way of contemplating affairs has produced great good, is certain. But it is also certain, that it is a one-sided way, and satisfies only part of the human mind. Many of the noblest intellects crave for something which it cannot supply. In England, however, during the greater part of the eighteenth century, it was even more supreme than it is now, and was, indeed, so universal, that, from the year 1727 until nearly the close of the century, our country did not possess, in any branch of science, a speculator who had sufficient force to raise himself above those narrow views which were then deemed the perfection of wisdom.851 Much was added to our knowledge, but its distant boundaries were not enlarged. Though there was an increase of curious and valuable details, and though several of the small and proximate laws of nature were generalized, it must be admitted, that those lofty generalizations, which we owe to the seventeenth century, remained stationary, and that no attempt was made to push beyond them. When John Hunter arrived in London, in 1748, Newton had been dead more than twenty years, and the English people, absorbed in practical pursuits, and now beginning, for the first time, to enter into political life, had become more averse than ever to inquiries which aimed at truth without regard to utility, and had accustomed themselves to value science chiefly for the sake of the direct and tangible benefit which they might hope to derive from it.

That Hunter must have been influenced by these circumstances, will be obvious to whoever considers how impossible it is for any single mind to escape from the pressure of contemporary opinion. But, inasmuch as all his early associations had inclined him in another direction, we perceive that, during his long residence in England, he was acted on by two conflicting forces. The country of his birth made him deductive; the country of his adoption made him inductive. As a Scotchman, he preferred reasoning from general principles to particular facts; as an inhabitant of England, he became inured to the opposite plan of reasoning from particular facts to general principles. In every country, men naturally give the first place to what is most valued. The English respect facts more than principles, and therefore begin with the facts. The Scotch consider principles as most important, and therefore begin with the principles. And, I make no doubt that one of the reasons why Hunter, in investigating a subject, is often obscure, is that, on such occasions, his mind was divided between these two hostile methods, and that, leaning sometimes to one and sometimes to the other, he was unable to determine which he should choose. The conflict darkened his understanding. Adam Smith, on the other hand, in common with all the great Scotchmen who remained in Scotland, was remarkably clear. He, like Hume, Black, and Cullen, never wavered in his method. These eminent men were not acted on by English influence. Of all the most illustrious Scotchmen of the eighteenth century, Hunter alone underwent that influence, and he alone displayed a certain hesitation and perplexity of thought, which seems unnatural to so great a mind, and which, as it appears to me, is best explained by the peculiar circumstances in which he was placed.

One of the ablest of his commentators has justly observed, that his natural inclination was, to conjecture what the laws of nature were, and then reason from them, instead of reasoning to them by slow and gradual induction.852 This process of deduction was, as I have shown, the favourite method of all Scotchmen, and, therefore, was precisely the course which we should have expected him to adopt. But, inasmuch as he was surrounded by the followers of Bacon,853 this natural bias was warped, and a large part of his marvellous activity was employed in observations and experiments, such as no Scotch thinker, living in Scotland, would ever have engaged in. He himself declared, that thinking was his delight;854 and there can be no doubt that, had he been differently situated, thinking would have been his principal pursuit. As it was, the industry with which he collected facts, is one of the most conspicuous features in his career. His researches covered the whole range of the animal kingdom, and were conducted with such untiring zeal, that he dissected upwards of five hundred different species, exclusive of dissections of different individuals, and exclusive, too, of dissections of a large number of plants.855 The results were carefully arranged and stored up in that noble collection which he formed, and of the magnitude of which we may gain some idea from the statement, that, at his death, it contained upwards of ten thousand preparations illustrative of the phenomena of nature.856 By this means, he became so intimately acquainted with the animal kingdom, that he made a vast number of discoveries, which, considered singly, are curious, but which, when put together, constitute an invaluable body of new truths. Of these, the most important are, the true nature of the circulation in crustacea and insects;857 the organ of hearing in cephalopods;858 the power possessed by mollusks of absorbing their shells;859 the fact that bees do not collect wax, but secrete it;860 the semicircular canals of the cetacea;861 the lymphatics of birds;862 and the air-cells in the bones of birds.863 We are also assured, that he anticipated the recent discoveries respecting the embryo of the kangaroo;864 and his published works prove, that, in the human subject, he discovered the muscularity of the arteries,865 the muscularity of the iris,866 and the digestion of the stomach after death by its own juice.867 Although, in his time, animal chemistry was not yet raised to a system, and was consequently little heeded by physiologists, Hunter endeavoured, by its aid, to search out the qualities of the blood, so as to ascertain the properties of its constituents.868 He also examined it in different stages of embryonic life, and by minutely tracking it through its periods of development, he made the capital discovery, that the red globules of the blood are formed later than its other components. His contemporaries, however, were so little alive to the importance of this great physiological truth, that it fell dead upon them, and, being forgotten, it was, about fifty years afterwards, rediscovered, and was announced, in 1832, as a law of nature which had just been brought to light.869 This is one of many instances in the history of our knowledge, which proves how useless it is for a man to advance too far beyond the age in which he lives.870 But Hunter, besides making the discovery, also saw its meaning. From it, he inferred that the function of the red globules is to minister to the strength of the system, rather than to its repair.871 This is now universally admitted; but it was not admitted till long after his death. Its recognition is chiefly owing to the rapid advance of animal chemistry, and to improvements in the microscope. For, by the employment of these resources, it has become manifest, that the red globules, the respiratory process, the production of animal heat, and the energy of the locomotive organs, are but different parts of a single scheme.872 Their connexion with each other is established, not only by a comparison of different species, but also by a comparison of different members of the same species. In human beings, for example, the locomotive and other animal functions are more active in persons of a sanguine temperament than in those of a lymphatic temperament; while, in sanguine temperaments, the globules are more numerous than in lymphatic ones. The knowledge of this fact we owe to Lecanu;873 and to him we are also indebted for an analogous fact, corroborating the same view. He has shown, that the blood of women contains more water and fewer red globules than the blood of men;874 so that here again we discern the relation between these globules and the energy of animal life. Inasmuch, however, as these researches were not made until many years after the death of Hunter, the coincidence between them and his speculative conclusions is a striking instance of his power of generalization, and of that unrivalled knowledge of comparative anatomy, which supplied him with materials from which, in spite of the backwardness of animal chemistry, he was able to draw an inference, which later and minuter researches have decisively verified.875

Having thus, by a comprehensive survey of the animal world, associated its remarkable faculty of movement with the state of its blood, Hunter turned his attention to another aspect of the question, and took into consideration the movements of the vegetable world, in the hope that, by comparing these two divisions of nature, he might detect some law, which, being common to both, should unite into one study all the principles of organic motion. Though he failed in this great undertaking, some of his generalizations are very suggestive, and well illustrate the power and grasp of his mind. Looking at the organic kingdom as a whole, he supposed that its capacity of action, both in animals and in vegetables, was of three kinds. The first kind, was the action of the individual upon the materials it already possessed; and this gave rise to growth, secretion, and other functions, in which the juice of the plant was equivalent to the blood of the animal.876 The second kind of action had for its object to increase these materials; it was always excited by want, and its result was, to nourish and preserve the individual.877 The third kind was entirely due to external causes, including the whole material world, all the phenomena of which were a stimulus to some kind of action.878 By combining, in different ways, these different sources of motion, and by studying every incitement to action, first, in reference to one of the three great divisions just indicated, and, secondly, in reference to the power of action, as distinguished from the quantity of action,879 Hunter believed that some fundamental truths might be obtained, if not by himself, at all events by his successors. For, he thought that, though animals can do many things which plants cannot, still, the immediate cause of action is in both cases the same.880 In animals, there is more variety of motion, but in plants there is more real power. A horse is certainly far stronger than a man. Yet a small vine cannot only support, but can raise, a column of fluid five times higher than a horse can. Indeed, the power which a plant exercises of holding a leaf erect during an entire day, without pause and without fatigue, is an effort of astonishing vigour, and is one of many proofs, that a principle of compensation is at work, so that the same energy which, in the animal world, is weakened by being directed to many objects, is, in the vegetable world, strengthened by being concentrated on a few.881

In pursuing these speculations, which, amid much that is uncertain, contain, I firmly believe, a large amount of important, though neglected, truth, Hunter was led to consider how motion is produced by various forces, such as magnetism, electricity, gravitation, and chemical attraction.882 This carried him into inorganic science, where, as he clearly saw, the foundation of all organic science must be laid. Just as, on the one hand, the human frame could never be successfully studied, except by the aid of principles which had been collected from an investigation of animals below man,883 so, on the other hand, the laws of those very animals must, he said, be approached through the laws of common or inorganic matter.884 He, therefore, aimed at nothing less than to unite all the branches of physical science, taking them in the order of their relative complexity, and proceeding from the simplest to the most intricate. With this view, he examined the structure of the mineral kingdom, and, by an extensive comparison of crystals, he sought to generalize the principles of form, in the same way as, by a comparison of animals, he sought to generalize the principles of function. And, in doing this, he took into account, not only regular crystals, but also irregular ones.885 For, he knew that, in nature, nothing is really irregular or disorderly; though our imperfect apprehension, or rather the backwardness of our knowledge, prevents us from discerning the symmetry of the universal scheme. The beauty of the plan, and the necessity of the sequence, are not always perceptible. Hence, we are too apt to fancy that the chain is broken, because we cannot see every link in it. From this serious error, Hunter was saved by his genius, even more than by his knowledge. Being satisfied that every thing which happens in the material world, is so connected and bound up with its antecedents, as to be the inevitable result of what had previously occurred, he looked with a true philosophical eye at the strangest and most capricious shapes, because to him they had a meaning and a necessary purpose. To him, they were neither strange nor capricious. They were deviations from the natural course; but it was a fundamental tenet of his philosophy, that nature, even in the midst of her deviations, still retains her regularity.886 Or, as he elsewhere expresses it, deviation is, under certain circumstances, part of the law of nature.887