by John Carey
Yes, it took thirty million years and twenty million reptiles to get one that would stick long enough to develop into something else and let the scheme proceed to the next step.
Then the Pterodactyl burst upon the world in all his impressive solemnity and grandeur, and all Nature recognized that the Cainozoic threshold was crossed and a new Period open for business, a new stage begun in the preparation of the globe for man. It may be that the Pterodactyl thought the thirty million years had been intended as a preparation for himself, for there was nothing too foolish for a Pterodactyl to imagine, but he was in error, the preparation was for man. Without doubt the Pterodactyl attracted great attention, for even the least observant could see that there was the making of a bird in him. And so it turned out. Also the makings of a mammal, in time. One thing we have to say to his credit, that in the matter of picturesqueness he was the triumph of his Period; he wore wings and had teeth, and was a starchy and wonderful mixture altogether, a kind of long-distance premonitory symptom of Kipling’s marine:
’E isn’t one o’ the reg’lar Line, nor ’e isn’t one of the crew,
’E’s a kind of a giddy harumfrodite – soldier an’ sailor too!
From this time onward for nearly another thirty million years the preparation moved briskly. From the Pterodactyl was developed the bird; from the bird the kangaroo, from the kangaroo the other marsupials; from these the mastodon, the megatherium, the giant sloth, the Irish elk, and all that crowd that you make useful and instructive fossils out of – then came the first great Ice Sheet, and they all retreated before it and crossed over the bridge at Behring’s strait and wandered around over Europe and Asia and died. All except a few, to carry on the preparation with. Six Glacial Periods with two million years between Periods chased these poor orphans up and down and about the earth, from weather to weather – from tropic swelter at the poles to Arctic frost at the equator and back again and to and fro, they never knowing what kind of weather was going to turn up next; and if ever they settled down anywhere the whole continent suddenly sank under them without the least notice and they had to trade places with the fishes and scramble off to where the seas had been, and scarcely a dry rag on them; and when there was nothing else doing a volcano would let go and fire them out from wherever they had located. They led this unsettled and irritating life for twenty-five million years, half the time afloat, half the time aground, and always wondering what it was all for, they never suspecting, of course, that it was a preparation for man and had to be done just so or it wouldn’t be any proper and harmonious place for him when he arrived.
And at last came the monkey, and anybody could see that man wasn’t far off, now. And in truth that was so. The monkey went on developing for close upon 5,000,000 years, and then turned into a man – to all appearances.
Such is the history of it. Man has been here 32,000 years. That it took a hundred million years to prepare the world for him is proof that that is what it was done for. I suppose it is. I dunno. If the Eiffel tower were now representing the world’s age, the skin of paint on the pinnacle-knob at its summit would represent man’s share of that age; and anybody would perceive that that skin was what the tower was built for. I reckon they would, I dunno.
Source: The Works of Mark Twain. What Is Man? and Other Philosophical Writings, edited with an introduction by Paul Baender, published for the Iowa Center for Textual Studies by the University of California Press, Berkeley, Los Angeles, London, 1973.
Drawing the Nerves
The greatest Spanish scientist, and the virtual founder of neuroscience, Santiago Ramón y Cajal (1852–1934) was born in one of the poorest regions of Aragon, the son of a country surgeon. A failure at school, where he resisted attempts to teach him Latin and Greek, he was apprenticed to a shoemaker, but developed an interest in human anatomy, encouraged by his father. They used to rob churchyards together, carrying bones home for inspection. After medical school at Zaragoza, Cajal taught anatomy at Valencia, where he learnt about the technique, developed by the Italian neurologist Camillo Golgi (1844–1926), of staining nervous tissue with silver nitrate, in an attempt to reveal its fine structures. As he explains in this extract from his autobiography, Cajal decided to apply the technique, in the first instance, to the brain and sensory centres of embryos and young animals, in order to simplify the problem. Using similar nerve-specific stains, he was able to determine the fine structure of the retina of the eye, and to trace the connections of nerve cells in grey matter and the spinal cord. With Golgi, he received the 1906 Nobel Prize for Medicine for establishing the neuron or nerve cell as the basic unit of nervous structure.
In my systematic explorations through the realms of microscopic anatomy, there came the turn of the nervous system, that masterpiece of life. It is important to remember that the technical resources of those times were quite inadequate for attacking the great and alluring problem effectively. Colouring agents capable of staining selectively the processes of the nerve cells so that they could be followed with some certainty across the formidable tangle of the gray matter were as yet unknown.
Nevertheless, in spite of the weakness of our methods of analysis, the problem attracted us irresistibly. We saw that an exact knowledge of the structure of the brain was of supreme interest for the building up of a rational psychology. To know the brain, we said, is equivalent to ascertaining the material course of thought and will, to discovering the intimate history of life in its perpetual duel with external forces; a history summarized, and in a way engraved in the defensive neuronal coordinations of the reflex, of instinct, and of the association of ideas.
Unfortunately, we lacked a weapon sufficiently powerful to pierce the impenetrable thicket of the gray matter, that constellation of unknowns.
In spite of all this, my pessimism was exaggerated, as we are about to see. Obviously, the desideratum referred to was and is even today an unattainable ideal, but some progress towards it could be made by taking advantage of the technique of the time. As a matter of fact, the instrument of revelation already existed; only I, isolated in my corner, was not acquainted with it, nor had it yet become known to any extent among scientists, in spite of having been made public in the years 1880 and 1885. It was discovered by C. Golgi, the famous histologist of Pavia, through the favour of chance, the muse who inspires great discoveries. In his staining experiments, this savant noticed that the protoplasm of the nerve cells, which is so refractory to artificial staining, possesses the valuable attribute of attracting strongly a precipitate of silver chromate when this precipitate is produced right within the thickness of the pieces of tissue. The modus operandi, which is of the simplest, consists essentially of impregnating fragments of gray matter for several days in solutions of potassium bichromate (or of Müller’s fluid), or better still, in a mixture of bichromate and 1 per cent osmic acid solution, and treating them afterwards with dilute solutions (0.75 per cent) of crystalline silver nitrate. In this way there is formed a deposit of silver bichromate which, by a happy peculiarity that has not yet been explained, picks out certain nerve cells to the absolute exclusion of the others. When one examines the preparation, the granules of the gray matter appear coloured brownish black even to their finest branchlets, which stand out with unsurpassable clarity upon a transparent yellow background, formed by the elements which are not impregnated. Thanks to such a valuable reaction, Golgi succeeded during several years of labour in clarifying not a few points of importance in the morphology of the nerve cells and processes. As I have already mentioned, however, the admirable method of Golgi was then (1887–8) unknown to the immense majority of neurologists or was undervalued by those who had the requisite information about it … I decided to employ the method of Golgi on a large scale and to study it with all the patience of which I was capable. Innumerable tests by Bartual and myself in many parts of the central nervous system and many species of animals convinced us that the new method of analysis had before it a brilliant future, especially if there could b
e found some way of overcoming its highly capricious and uncertain character.* The procuring of a good preparation constituted a delightful surprise and gave rise to jubilant hopes.
Up to that time, our preparations of the cerebrum, the cerebellum, the spinal cord, etc., confirmed fully the discoveries of the celebrated histologist of Pavia, but nothing new of any importance arose out of them. I did not, however, lose faith in the method on that account. I was fully convinced that, in order to make a significant advance in the knowledge of the structure of the nerve centres, it was absolutely necessary to make use of procedures capable of showing the most delicate rootlets of the nerve fibres vigorously and selectively coloured upon a clear background. It is well known that the gray matter is formed by something like a very dense felt of excessively fine threads; and for following these filaments thin sections or completely stained preparations are worthless. What is required for this purpose is very intense reactions which, nevertheless, permit the use of very thick almost macroscopic sections (the processes from nerve cells are sometimes many millimeters or even centimeters long), the transparency of which, in spite of their unusual thickness, is made possible by the exclusive colouration of some few cells or fibres which stand out in the midst of extensive masses of cells that are uncoloured. Only thus does the undertaking to follow a nervous conductor from its origin to its termination become possible.
In any case, we were now in possession of the required instrument. It remained only to determine carefully the conditions of the chrome-silver reaction, and to regulate it so as to adapt it to each particular case. And if the brain and other adult central organs of man and other vertebrates are too complex to permit of scrutinizing their structural plan by the method referred to, why not apply the method systematically to lower vertebrates or to the early stages of ontogenetic development, in which the nervous system should present a simple and, so to speak, diagrammatic organization?
Such was the programme of work which I laid out for myself. It was commenced in Valencia, but only after I had removed to Barcelona was it completed, with a perseverence, an enthusiasm, and a success which surpassed my expectations.
The year 1888 arrived, my greatest year, my year of fortune. For during this year, which rises in my memory with the rosy hues of dawn, there emerged at last those interesting discoveries so eagerly hoped and longed for. Had it not been for them, I should have vegetated sadly in a provincial university without passing in the scientific order beyond the category of more or less estimable delvers after details. As a result of them I attained the enjoyment of the sour flattery of celebrity; my humble surname, pronounced in the German manner (Cayal), crossed the frontiers; and my ideas, made known among scientific men, were discussed hotly. From that time on, the trench of science had one more recognized digger.
How did it happen? The reader will, I hope, forgive me if I devote a few remarks and explanations here to an occurrence so decisive for my career. I declare, in the first place, that the new truth, laboriously sought and so elusive during two years of vain efforts, rose up suddenly in my mind like a revelation. The laws governing the morphology and connections of the nerve cells in the gray matter, which became patent first in my studies of the cerebellum, were confirmed in all the organs which I successively explored. I may be permitted to formulate them at once:
1. The collateral and terminal ramifications of every axis cylinder† end in the gray matter, not in a diffuse network as maintained by Gerlach and Golgi, and most other neurologists, but by free arborizations arranged in a variety of ways (pericellular, baskets or nests, climbing branches, etc.).
2. These ramifications are applied very closely to the bodies and dendrites [tree-like outgrowths from the cell body] of the nerve cells, a contact or articulation being established between the receptive protoplasm and the ultimate axonic branchlets.
From the anatomical laws stated spring two physiological corollaries:
3. Since the final rootlets of the axis cylinders are applied closely to the bodies and dendrites of the neurons,‡ it must be admitted that the cell bodies and their protoplasmic processes enter into the chain of conduction, that is to say, that they receive and propagate the nervous impulse, contrary to the opinion of Golgi, according to whom these parts of the cell perform a merely nutritive rôle.
4. The continuity of substance between cell and cell being excluded, the view that the nerve impulse is transmitted by contact, as in the junctions of electric conductors, or by an induction effect, as in induction coils, becomes inescapable.
The laws mentioned, a purely inductive outcome of the structural analysis of the cerebellum, were afterwards confirmed in all the nervous structures examined (retina, olfactory bulb, sensory and sympathetic ganglia, cerebrum, spinal cord, medulla oblongata, etc.). Later studies by myself and by others revealed that these structural and physiological standards apply equally, without modification, to the nervous system of vertebrates and to that of invertebrates. As happens with all legitimate conceptions, mine become more thoroughly established and gained progressively in dignity as the circle of confirmatory studies was extended.
However, in my eagerness to condense the essentials of the results obtained in brief propositions, I have not replied as yet to the question formulated in preceding paragraphs.
How were these laws discovered? Why did my work suddenly acquire surprising originality and broad importance?
I wish to be frank with the reader. To my successes of those days there contributed, without doubt, some improvements of the chrome silver method, particularly the modification designated the procedure of double impregnation; but the principal thing, the really efficacious cause, was – who would have thought it? – the application to the solution of the problem of the gray matter of the dictates of the most ordinary common sense. Instead of taking the bull by the horns, as the saying is, I permitted myself some strategic subterfuges. This demands explanation.
I have already pointed out that the great enigma in the organization of the brain was the way in which the nervous ramifications ended and in which the neurons were mutually connected. Repeating a simile already used, it was a case of finding out how the roots and branches of these trees in the gray matter terminate, in that forest so dense that, by a refinement of complexity, there are no spaces in it, so that the trunks, branches, and leaves touch everywhere.
Two methods come to mind for investigating adequately the true form of the elements in this inextricable thicket. The most natural and simple apparently, but really the most difficult, consists of exploring the full-grown forest intrepidly, clearing the ground of shrubs and parasitic plants, and eventually isolating each species of tree, as well from its parasites as from its relatives. Such was the approach employed in neurology by most authors from the time of Stilling, Deiters, and Schültze (mechanical and chemical dissociations) to that of Weigert and Golgi, in which the isolation of each form of cell or of each fibre is procured optically, that is by the disappearance or absence of colour or the majority of the interlacing elements in the gray matter. Such tactics, however, to which Golgi and Weigert owed important discoveries, are inappropriate for the elucidation of the problem proposed, by reason of the enormous length and extraordinary luxuriance of the nervous ramifications, which inevitably appear mutilated and almost indecipherable in each section.
The second path open to reason is what, in biological terms, is designated the ontogenetic or embryological method. Since the full grown forest turns out to be impenetrable and indefinable, why not revert to the study of the young wood, in the nursery stage, as we might say? Such was the very simple idea which inspired my repeated trials of the silver method upon embryos of birds and mammals. If the stage of development is well chosen, or, more specifically, if the method is applied before the appearance of the myelin sheaths upon the axons (these forming an almost insuperable obstacle to the reaction), the nerve cells, which are still relatively small, stand out complete in each section; the terminal ramification
s of the axis cylinder are depicted with the utmost clearness and perfectly free; the pericellular nests, that is the interneuronal articulations, appear simple, gradually acquiring intricacy and extension; in sum, the fundamental plan of the histological composition of the gray matter rises before our eyes with admirable clarity and precision. As a crowning piece of good fortune, the chrome silver reaction, which is so incomplete and uncertain in the adult, gives in embryos splendid colourations, singularly extensive and constant.
How is it, one may ask, that scientists did not hit upon so obvious a step? Certainly the idea must have occurred to many. In after years I learned that Golgi himself had already applied his method to embryos and young animals and obtained some excellent results; but he did not persist in his efforts, perhaps not thinking that he could progress by such a path in the elucidation of the problem of the structure of the centres. So little importance did he evidently attach to such experiments that in his greatest work the observations described have reference exclusively to the adult nervous system of man and mammals. In any case, my easy success proves once more that ideas do not show themselves productive with those who suggest them or apply them for the first time, but with those persevering workers who feel them strongly and put all their faith and love in their efficacy. From this point of view, it may be affirmed that scientific accomplishments are creations of the will and rewards of ardour.
Realizing that I had discovered a rich field, I proceeded to take advantage of it, dedicating myself to work, no longer merely with earnestness, but with fury. In proportion as new facts appeared in my preparations, ideas boiled up and jostled each other in my mind. A fever for publication devoured me. In order to make known my thoughts, I made use chiefly of a certain professional medical review, the Gaceta Médica Catalana. The tide of ideas and impatience for publication rising rapidly, however, this outlet became too narrow for me. I was much annoyed by the slowness of the press and the lateness of the dates of appearance. To extricate myself once and for all from such fetters, I decided to publish upon my own account a new review, the Revista trimestral de Histología normal y patológica. The first number saw the light in May, 1888, and the second appeared in the month of August of the same year. Naturally, all the articles, six in number, sprang from my own pen. From my hands emerged also the six lithographic plates which were included. Financial considerations obliged me not to print more than sixty copies altogether at the time and these were distributed almost entirely among foreign scientists.