In low organisms the contractile protoplasm fulfils the functions of both nerve and muscle, but as we ascend in the scale differentiation becomes more and more complete. From their original common origin, however, we might expect that the poisons which act on the muscles would also act on the motor nerves, and vice versa, and we should hardly expect any poison to act entirely on the one without affecting the other. This is to a considerable extent the case, for very many substances paralyse them both. But, as one would also expect from the differentiation they have undergone, muscle and nerve are not equally affected in the higher animals. Thus we find that although most of the salts of ammonium, and the iodides, chlorides, and sulphates of the compound ammonias into which methyl and ethyl enter, paralyse both muscle and nerve, yet they paralyse the nerve before the muscle. In some cases the nerve is affected so much before the muscle that at first sight it might appear that the nerve alone was paralysed and the muscle left unaffected. More careful observation, however, shows us that most of the compound ammonias, and probably most of the organic alkaloids, affect muscle, motor nerves, and nerve-centres, and, if their action can be continued long enough, will paralyse all three. The symptoms they produce ma}7, however, be entirely different, because these depend upon the order in which the different parts of the nervous system are affected, as has already been pointed out at p. 26. The symptoms produced, for example, by strychnine and methyl-strychnine are utterly different, the former causing tetanic convulsions, and the latter gradually-increasing torpor, weakness, and paralysis. Strychnine stimulates the spinal cord, and methyl-strychnine paralyses the motor nerves; yet if their action continue long enough it is found that both of them will ultimately cause paralysis of both spinal cord and motor nerves. The final result is thus the same in both cases, but the order in which the various parts of the nervous system are affected is different.

In the example just given, the drugs appear to exert a selective influence on the spinal cord and motor nerves respectively, and consequently produce very different symptoms. But we find that a number of drugs appear to act upon muscles motor nerves, and nerve-centres, in a given order, although there may be slight variations in the action of the individual drugs. These substances are generally found to act as protoplasmic poisons, arresting the movements of amoebae and white blood-corpuscles, as well as proving fatal to higher animals.

In the protoplasm of these minute organisms we are unable at present to distinguish any evidences of differentiation. As we ascend in the animal kingdom we find a differentiation between muscle, nerve, and nerve-centre; and the higher up we ascend in the scale the more complex do the nerve-centres become. As Hughlings Jackson has well put it, 'evolution is a passage from the most simple to the most complex, from the lowest to the highest centres.' It is a passage from the most automatic to the most voluntary; but the lowest centres are at the same time the most stable, or, as Jackson calls it, the ' most organised centres'; while the highest centres are the most unstable or least organised. This is represented diagrammatically in Fig. 51, where the centres for the heart and respiratory apparatus and for the sphincters are represented as very simple in their organisation, but very stable, as indicated by the size of the ganglia and thickness of the nerves in the diagram. The spinal cord is represented as more complex, but with thinner lines, in order to show its lesser stability; while the high complexity and small stability of the cerebral cortex is indicated by the great number and thinness of the lines in the figure. According to Jackson, the lowest nervous centre extends from the aqueduct of Sylvius to the lower end of the spinal cord; and in this all parts of the body are directly represented, so that a discharge of nervous energy from any part of it only requires to overcome the resistance in the motor nerves and the muscles themselves. What he regards as the middle motor centres are evolved out of the lowest, and re-represent all parts of the body in more complex and special combinations. The highest centres evolved out of the middle re-re-represent all parts of the body in still more complex and special combinations. A discharge from the highest centres, in order to act on the periphery, has to overcome the resistance of the middle and lowest centres, as well as of the muscles.

Action Of Drugs On Nerves General Action Of Drugs  86

Fig. 51. - Diagram to illustrate Hughlings Jackson's views of the nervous system.

In the action of such poisons as alcohol, the nervous system appears to be paralysed in inverse order of its development: the highest centres going first, next the middle, and then the lowest. After this comes paralysis of the motor nerves, and lastly of the muscles themselves. In the case of alcohol, the dose required to paralyse motor nerves and muscles is so great that, as a rule, we can only observe its effect by directly applying the drug to the nerves and muscles themselves. To such a process of paralysis as this, Jackson applies the term of dissolution.

In the case of drugs which excite nervous centres, we also notice a certain similarity of action. Thus strychnine not only causes convulsions by its stimulating action on the medulla spinalis, but stimulates also the nerve-centres for the respiration and circulation in the medulla oblongata and in the heart itself.