Читать книгу Possible Worlds and Other Essays - J. B. S. Haldane - Страница 10

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KINGS and editors commonly speak in the first person plural. If we all habitually did so, and thought so, we should understand a good deal more about how we work. For each of us is a community of about a hundred million million cells, whose co-operation is our life. This co-operation is brought about in part by the nervous system, and its beauty and delicacy is apt to blind us to the fact that a great many cells—in fact the majority—are not supplied with nerve fibres. Their behaviour is determined by two things, the mechanical and electrical forces exerted on them by their neighbours, and the chemical composition of the fluid that surrounds them or is given to them by their colleagues. How great is the importance of the non-nervous influences is shown by the fact that the other parts of an embryo develop perfectly before any nerves grow out to them from the brain and spinal cord, and will continue to do so nearly normally even if the nervous system does not develop at all. If we may use the well-known comparison of the body and the state we may say that most of our own citizens are not state employees but act from economic and other motives without any direct orders from the central government. What is more, many of the cells in the brain, the seat of government, are alert to the smallest changes in their chemical environment; and react to them by transmitting orders for some such activity as an increase or decrease of the breathing, which will bring their environment back to their normal conditions.

If we observe single cells, such as the protozoa, bacteria, or the diatoms and other microscopic plants in sea-water which are the ultimate source of almost all the nourishment of sea-beasts, we find that they are often remarkably hard to keep alive. The tiniest changes in the fluid around them, especially in its alkalinity, will kill them or greatly alter their behaviour. Indeed they are quite as dependent on the presence of the right amount of potassium and calcium salts around them as on that of oxygen or food. As a matter of fact they spend a great deal of their energy in overcoming the defects of their surroundings. For example, water almost invariably leaks through the skins of fresh-water protozoa, and they require a special organ, the contractile vacuole, to expel it. Placed in salt-water they only empty this quite rarely to get rid of waste products.

Our own cells are much more efficient than protozoa at their particular functions, but they require an extremely constant and artificial environment. It is the business of various organs, such as the lungs, liver, intestine, kidneys, and thyroid gland to keep it constant. In the same way a civilized man is generally far more efficient at his particular vocation than a savage, but only on condition that most of his needs are met by bakers, builders, tailors and so forth. Our internal environment is the blood, or rather its fluid part, the plasma in which the corpuscles are suspended. Some of the activities concerned in its regulation escape our consciousness. For example if the amount of sugar in it becomes too small, the liver makes fresh sugar from a starchlike substance called glycogen which is stored in the liver cells. If the amount of any soluble constituent in it becomes too great, the kidneys eliminate the excess, and so on. Sometimes however our consciousness and will are concerned. A shortage of water leads to thirst, a shortage of sugar which the liver cannot immediately remedy to hunger, a shortage of oxygen to panting, which may be so intense as to occupy our whole attention and will.

The blood plasma of many marine animals is almost the same as sea-water, with the addition of a little sugar and other foodstuffs on the way from the gut to the cells, and waste products on the way from the cells to the excretory organs. A cockle’s heart will continue to beat if placed in sea-water, though quite a small change in its chemical composition, say a precipitation of the calcium (lime) salts, would render the sea-water poisonous to it. We vertebrates have a blood plasma which has much the same composition as sea-water diluted with three times its volume of fresh-water. Such a liquid can safely be injected into the human veins in quite large quantities. The chemical agreement is far too striking to be a coincidence. Whereas all cells contain more potassium than sodium, the plasma contains 15 times as much sodium as potassium, the corresponding figure for sea-water being 27. Similarly the ratio of sodium to calcium is 39 in plasma, 27 in the sea. With regard to magnesium the agreement is not so good. It is suggested that just as the plasma of modern marine invertebrates is very nearly sea-water, so our own represents the sea of a remote period when our marine ancestors first began to develop gills impermeable to sea-water. Modern fish, even those which live in the sea, have a plasma much like our own in its low salt content, so presumably it was their and our common ancestor that first effectively shut itself off from the sea. As the sea is always receiving salt from the rivers, and only occasionally depositing it in drying lagoons, it becomes saltier from age to age, and our plasma tells us of a time when it possessed less than half its present salt content.

It is not only our tissue cells that lead this aquatic existence. Most marine animals, both vertebrate and invertebrate, shed their eggs and spermatozoa into the sea, and rely for fertilization on the numbers and swimming power of the latter. We have cut down our output of eggs to one or two a month, but we still continue, in contrast with many insects and crustaceans, to produce spermatozoa which have to swim great distances to their goal, and are therefore required in fantastically vast numbers. Their marine ancestry is shown by the fact that they can only live in a fluid containing much the same salts as the plasma. And after our development has started from the fusion of an egg and a spermatozoon we pass our first nine months as aquatic animals, suspended in and protected by a salty fluid medium. We begin life as salt-water animals.

There are two of our sense-organs which bear striking testimony to our marine ancestry. Under the skin of a fish are a number of tiny tubes occasionally opening to the exterior. There is a complicated system on the head, and one on each side of the body, often marked by a conspicuous stripe on the skin above it, as in trout.

These tubes contain bunches of microscopic hairs, richly supplied with nerve fibres, and far too delicate to be left on the outer surface of the body. The fishes’ own movements through the surrounding water, and also local currents and vibrations in the water itself, are communicated to the fluid in the tubes, and bend the hairs over. Thus the fish learns of the speed and rhythm of the water movement in the tubes, as a cat might gauge the strength of a wind by the degree of bending of its whiskers.

Two parts of the tube system on each side of the head are deeply buried in the skull and highly specialized. One is adapted to respond to fine and rapid vibrations in the water, in fact to sounds. The other consists of three loops at right angles, the so-called semi-circular canals. These organs are only connected with the sea by a long narrow tube sometimes closed in the adult. But when the fish turns round, the water in one or more of the semi-circular canals is left behind, like the water in a glass which is suddenly rotated, and presses on the hair cells in the canal. Thus while the organs in the external system inform the fish of its movements relative to the water round it, those in the semi-circular canals are stimulated by its turning movements.

We land vertebrates have lost most of the fishes’ canal system, but the two pairs of specialized organs in the head remain as our internal ear, open to the surrounding water in early embryonic life, but closed long before birth or hatching. The ear-drum and an elaborate system of tiny bones transmit aerial vibration to the water in one part of it. The corresponding vibrations of this water act on hair cells at the end of the auditory nerve fibres, and these in turn stimulate those parts of the brain concerned with hearing. When we turn our heads the swirling of the salt-water in the semi-circular canals presses on the hair cells. An elaborate system of nerve fibres in the brain links them up to the muscles which move our eyeballs, and as we turn our heads our eyes turn in the opposite direction, so that the direction of our gaze is unaltered. This is a reflex action uncontrollable by the will; in fact it is impossible to turn one’s head suddenly while keeping the eyes fixed relatively to it.

The semi-circular canals can play us false. In a rotating bowl the water gradually comes to rest with regard to the bowl, i.e. takes up the bowl’s rate of spin. The same happens to the fluid in our internal ears if we rotate uniformly. Hence the stimulus to the eye-muscles ceases and we can gaze steadily at any object rotating with us, for example the face of a partner in the pre-war type of waltz, while surrounding objects at rest cannot be fixed. When, however, the bowl or the man stops rotating the fluid does not, and the eyes execute involuntary movements which lead us to believe that everything is spinning round us. One can also become giddy in a vertical plane by turning round several times with the head bent forward and thus causing the fluid to swirl in a plane which becomes vertical when one lifts one’s head up. The reflex now let loose involves the muscles of the limbs and trunk, and would be appropriate if one were falling over; actually however it often makes us fall in the opposite direction.

In many ways a magnetic or gyrostatic compass would be a better balancing organ, but life has never used either the wheel or the magnet.

The evolution of the human body resembles that of the British constitution. It is full of relics of the past, as curious as the judges’ wigs or the city companies, but for most of these vestiges a new function has been evolved.