Irritability is a property commonly attributed to protoplasm, but it is a little doubtful whether there be not again some danger of an illogical use of terms. An amœba, one of the unicellular organisms found in ponds, has the power of moving. If a piece of a water-plant—the stalk of duck-weed is a suitable object—be examined with the microscope, these little animals are usually to be found upon its surface. They feed upon algæ more minute than themselves. When they come in contact with something suitable for food, their body-substance flows around it. The food is coagulated. So much of it as is digestible is digested; the remainder is extruded. Constantly parts of the body-substance are protruded, other parts retracted, in the search for food. Such movement is a response to stimulus. Stimuli received at one part of the body-substance are transmitted to another. The body-substance is irritable. It acknowledges stimuli; it conducts them. But if the amœbæ are watched until, owing to lack of oxygen or other cause, they die, their irritability comes to an end. It is a phenomenon of life. Again the physiologist is in a dilemma. Either protoplasm is not protoplasm when death has supervened, or protoplasm is not irritable as such. It is somewhat paradoxical to ascribe to the physical basis of life a property which depends upon its being alive.
Yet the influence on protoplasm of anæsthetics makes it difficult to understand how it can be either physically or chemically a substance which loses its form or changes its constitution whenever it ceases to display the usual evidences of its existence. Chloroform and similar agents suspend irritability. Yet irritability returns as their influence passes off. They appear to hold it in check without—at any rate visibly—changing the nature of the irritable substance.
All parts of the minute body-substance of an amœba are equally irritable. In higher animals irritability is concentrated in the nervous system. The form of irritability to which consciousness is adjunct is restricted to the cortex of the great brain.
Chloroform and similar agents are termed “anæsthetics” because they abolish the irritability of the cortex of the great brain, before their effects upon other parts of the nervous system are sufficiently pronounced to endanger the working of the animal machine. Pain ceases to be felt before the dose of anæsthetic is sufficient to suspend the irritability of the centres of reflex action. All protoplasm, whether animal or vegetable, is susceptible to the influence of these agents. They cause it to enter into a state which resembles death in all respects save the impossibility of revival. There is a great demand in the Paris flower-market for white lilac in the winter. The plant cannot be forced until after a period of rest. By withholding water and placing the bushes in a cool, shady place, horticulturists endeavour to send them prematurely into their winter sleep. Recently it has been found that from three to four weeks can be gained by placing the bushes for a couple of days in an atmosphere charged with the vapour of ether. Some change of state is evidently produced in protoplasm by anæsthetics. It ceases to be capable of receiving or transmitting stimuli. But we cannot picture the change as being sufficiently pronounced to justify the hypothesis that so long as it is irritable protoplasm is a complex substance which is resolved, as it loses its irritability, into simpler compounds familiar to the chemist. Perhaps it would be more correct to say, we cannot picture these chemical substances as reuniting into protoplasm when the effect of the anæsthetic passes off. Rather are we driven to think of living matter as a mixture of many substances in a state of molecular interchange, and to suppose that the activity of this interchange is diminished by anæsthetics.
Chemical activity is a property of protoplasm. In its network combinations and decompositions are effected more extensive in range than any which a chemist can cause to occur in his laboratory. From ammonia, carbonic acid, and water, a plant makes albumin. A chemist cannot make albumin, no matter how complex may be the nitrogenous substances which he endeavours to cause to combine. Albumin is resolved by animals into water, carbonic acid, and urea. Cells of the gastric glands set a problem which puzzles the chemist by making hydrochloric acid from sodic chloride without the intervention of a “stronger” acid. Many other illustrations of the same kind might be cited. Although the tissues of animals act chiefly as destroying agents, their protoplasm is not without constructive power. There is apparently no limit to the capacity for synthesis of plants. The chemistry of living things may be divided into two provinces, absolutely antagonistic in the series of reactions which they comprise. The one series is constructive, synthetic; the other destructive, analytical. Construction involves the locking up of energy. It is endothermal. Destruction results in the setting free of energy. It is exothermal. To accomplish synthesis energy must be added. Plants obtain it from the sun’s rays. Animals disperse energy, set free by the analysis of substances formed in plants, in maintaining their bodies’ warmth and movement.
The chemistry of the laboratory and the chemistry of protoplasm present certain contrasting features. A chemist reaches the compound which he wishes to form by effecting a series of interchanges. For example, he wishes to form uric acid by uniting a nucleus contained in lactic acid with urea. First he introduces chlorine and ammonia into the molecule of lactic acid. He makes trichlorlactamide. Then he heats (supplies energy to) a mixture of trichlorlactamide and urea. Two of the chlorine atoms carry off hydrogen atoms from the urea. A third leaves the trichlorlactamide with its ammonia. Water also breaks away. Uric acid remains.
| Trichlorlactamide | Urea | Uric Acid | |||
| CCl₃CH.OH.CO.NH₂ + | 2(NH₂)₂CO = | C₅H₄N₄O₃ | + NH₄Cl | + 2HCl | + H₂O. |
In this example the trichlorlactamide may be said to exchange its chlorine and ammonia for urea. When he planned the reaction, the chemist foresaw what would happen. He knew that if he weakened the grip of the lact radicle upon them, chlorine and hydrogen, chlorine and ammonia, oxygen and hydrogen, would take the opportunity of getting away together. The lact radicle and urea would be left with dangling arms, which must “satisfy their affinities” by linking up. It would be rash to assert that any reaction is impossible to Nature’s chemistry; but it may safely be said that the reactions which protoplasm effects are, so far as we know them, of a different type from this laboratory example. Uric acid is the chief excrement of birds. It is made in the liver. If the liver is shut off from the circulation, lactate of ammonia is excreted in the place of uric acid. It is therefore, in all probability, lactate of ammonia which the liver transforms into uric acid. We cannot pretend to say how this is done, although an empirical formula for the change might be drafted easily enough.
Lactate of ammonia has the formula NH₄C₃H₅O₃. Uric acid, C₅H₄N₄O₃, contains a much higher percentage of nitrogen. It could be produced from lactate of ammonia by the condensation of the nitrogen-containing nucleus and the addition of a sufficient amount of oxygen to complete the oxidation of the superfluous carbon and hydrogen into carbonic acid and water. It is of little interest to count the number of atoms concerned in this process. If a bird be fed upon urea, or even upon various salts of ammonia, its liver will change them into uric acid. Lactate of ammonia is the nitrogen-containing compound with which the liver has normally to deal. It can handle almost any other combination of nitrogen with equal ease. In the protoplasm of the liver the atoms in the molecule of lactate of ammonia are rearranged. The molecules are condensed; water is set free; oxidation occurs. It seems almost as if molecules, when in contact with protoplasm, lose their individuality. Their atoms fall into new groups. Chains which the chemist finds so difficult to break—chains from which he can remove a link only by insinuating another and a stronger—are, when in contact with protoplasm, groups of isolated links. The links rearrange themselves. They join into new circlets, larger, smaller, more open, closer. As grains of sand on a metal plate group themselves in harmony with the vibrations caused in the plate by drawing a violin bow across it, so the atoms answer to the forces which set protoplasm vibrating. There is no waste of force. The chemist may need to enclose sawdust and lime in a crucible heated in an electric furnace if he wishes to compel them to combine as carbide. He supplies energy enormously in excess of the amount which the new compound will lock up. Under the influence of protoplasm the reactions which occur are exactly proportional to the amount of energy supplied. Or, if it be a reaction by means of which energy is set free, it occurs spontaneously. No energy is absorbed in setting it going. All the energy liberated is effective. The chemist very frequently needs to heat a substance in order to cause it to decompose, even though it be falling from a less stable to a more stable state.
Vital chemistry and mineral chemistry are so widely different in their methods that one is tempted to think of them as different in kind. We find it very difficult to look at both from the same point of view. Men’s minds are preoccupied with the things that they have to do for themselves. The chemistry of the laboratory is seen as a science circumscribed by the laboratory walls. If it were possible to stand outside, it would be evident that it is only a part of the science of molecular change. Matter changes its state under the influence of force. Many rearrangements are effected by the chemist which do not occur in nature. He has an almost infinite range of action. Yet many of the rearrangements of matter and force which are occurring in the dandelion on his window-sill (if the fumes of sulphuretted hydrogen have not killed it) he is unable to reproduce. It is largely a question of waste. Nature works with greater precision than the chemist; but the chemist could do all that Nature does if he had but the same control of force.