NATURAL HISTORY NERVOUS SYSTEM AND SENSE ORGANS CHAPTER LVII GENERAL PRINCIPLES-NERVOUS SYSTEMS OF GENERAL PRINCIPLES Some of the properties of living matter or protoplasm have already been pretty fully considered, in sections which may be regarded as expansions of part of the brief sketch of Human Physiology given in vol. i, pp. 24-59. We have seen that protoplasm is a very complex and eminently unstable substance, which is continually breaking down into simpler compounds, with the result that stored or potential energy is transformed into actual or kinetic energy, without which movement and other life-manifestations would be impossible. The breakingdown process ultimately results in the formation of waste products, which being physiologically useless are cast out of the body. One such product is carbonic acid gas or carbon dioxide, primary object of Breathing or Respiration is to get rid of this. But Breathing also includes the taking in of free oxygen, without which the breaking down of the complex body-substance would not take place at the rate necessary for liberating the energy required. We have also seen that the gradual wasting of the body associated with the breaking-down process requires to be made good; hence the necessity for Food, which is built up into fresh protoplasm. In cases where Growth is taking VOL. IV. 1 95 place the food taken in must obviously be larger in amount than when it is merely a question of compensating for waste. By a process of over-growth with subsequent separation from the parent-body new individuals are developed, capable of leading independent existences, and ultimately giving rise to a further generation in their turn. Another very characteristic property of protoplasm is Contractility, i.e. spontaneous change of shape. Hence all the various kinds of Animal Movement, without which food could not be secured, enemies escaped, or unfavourable surroundings quitted. The present section is an expansion of the last part of the brief sketch of Human Physiology already mentioned, i.e. the part headed Nervous System and Sense Organs. What these are, and why they should exist, cannot be understood without reference to another fundamental property of protoplasm, which we may broadly term Sensitiveness and Spontaneity, there being, unfortunately, no briefer way of putting it. The surroundings of an animal are constantly changing; all sorts of external agents are continually acting upon it to varying extents; and life wholly depends upon successful adjustment or adaptation to this perpetually altering Environment. Alternations of day and night, succession of seasons, tidal flow and ebb, variations of foodsupply, the diminution or increase in number of enemies, may be taken as examples of changes which have much to do with the preservation or extinction of old species and the evolution. of new ones. That protoplasm is sensitive means that it is not inert to its surroundings, but reacts, in ways which tend to the preservation of life, to the influences which are constantly affecting it. If, when you are not looking, someone touches your hand with a red-hot poker, the member thus treated is drawn back without the exercise of will-power, and immediately after a painful sensation is experienced. This practically illustrates the fact that human protoplasm is sensitive to one external agent, i.e. heat, and the usefulness of reaction is sufficiently obvious. If animals were not sensitive to heat many of them would very quickly perish in an untimely manner. And a little consideration will make it apparent that Sensitiveness to a great variety of external agents is absolutely necessary to existence. All actions, however, are not the direct results of external agents acting for the time being. Protoplasm is spon taneous, i.e. it performs actions which find their starting-point within the body itself, as in the case of many voluntary human actions. Any change in the surroundings which brings the sensitiveness of an organism into play is technically known as a stimulus (L. stimulus, an ox-goad), and stimuli may broadly be classified as mechanical, chemical, thermal, photic, and electrical. The corresponding stimulating agents are pressure, change in chemical nature of the surroundings, heat, light, and electricity, which are scientifically defined as different forms of energy, or, to use the old expression, "force". Protoplasm, like every other kind of matter, may be regarded as made up of excessively minute particles or molecules, much too small to be seen with even the most powerful microscope, which are in a state of constant vibration, throbbing, or to-and-fro movement. The pendulum affords a simple example of vibratory movement. It may further be said that every sort of stimulus is of the nature of a vibration, e.g. in a sound-wave transmitted through air the particles of air move in a particular way and at a rate depending upon the pitch of the sound. All the changes that take place in living matter result from modifications in the movement of its molecules, but we are profoundly ignorant of what exactly takes place when, say, a muscle-fibre contracts or an impulse passes along a nerve. The adjustment to surroundings that is necessary for the maintenance of life results from these molecular changes in the body, which take place in response to the action of pressure, heat, light, &c., these themselves being of a vibratory nature, as has already been stated. So far as an animal is "sensitive" to its surroundings it is comparable to a complex musical instrument capable of playing all sorts of tunes with all kinds of variations, in response to external influences of different kind. The reaction of an animal to its environment at any given moment depends upon how external agents are acting upon it at that moment: it is they which "call the tune". If the supposed musical instrument could also play tunes of its own accord, independently of the direct action of the surroundings, such tunes might be taken to represent the "spontaneous actions of an animal. That Sensitiveness and Spontaneity, as above defined, are essential properties of living matter, may best be realized by studying a very simple organism, such as the Proteus Animalcule (Amaba), which is a particle of comparatively pure protoplasm (fig. 1006). That this creature is sensitive to mechanical stimuli is easily proved by tapping the glass slide on which one is crawling about under the microscope. The protruding lobes of the body (pseudopods) by which creeping is effected will be drawn in, and the animal will assume a spherical form (fig. 1006, a). 1 250 A B C 2 2 A good example of chemical stimulation is afforded by the addition of very weak caustic potash to the water in which the slug-shaped species of Amoeba (A. limax) is moving along, the reaction consisting in this case of the protrusion of long pointed pseudopods (fig. 1006, B). The same kind of Amoeba reacts in a marked way to changes of temperature (fig. 1006, c). At freezing point (0° C.) it is spherical and inert; as the temperature increases from 0° C. to 35° C. it moves about with ever-increasing activity; above this the activity gradually diminishes; and at about 40° C. the animal has assumed a spherical form, and dies in a condition of "heat-stiffening" or coagulation. This illustrates very well the fact that any particular stimulus has only a certain range of action, the range in this case being between o° C. and 40° C., which are known as the minimum and maximum points of heat-stimulation for this particular animal. Between these two points is an optimum one (35° C. in this instance), at which the stimulus exerts its greatest effect by way of promoting activity. Light does not appear to affect the creeping movements of Amoeba, but is said to check the taking in of food, which process goes on most actively at night. If a constant current of electricity is passed through the body of an Amoeba which is protruding pseudopods in all directions it will begin to creep against the current, and all those pseudo D Fig. 1006. Proteus Animalcules (Amaba`, much enlarged, showing effect of various stimuli. 1 and 2, Appearance before and after application of stimulus. See text. pods will be drawn in which are not at the front end for the time being (fig. 1006 D). Since a hungry Amoeba creeps actively about for a long time we are probably justified in concluding that some of its movements are spontaneous, and these are probably initiated by chemical changes which take place within its body, and may be called internal stimuli. NERVOUS SYSTEMS OF INVERTEBRATES The Amoeba, like most other animalcules, is a single cell or structural unit, which has to discharge all the functions of life, and does not exhibit the principle of division of labour to the same extent as animals belonging to the higher groups, which are collectively termed Metazoa, as contrasted with the Animalcules or Protozoa. Every member of the former group is made up of more or less numerous cells, and may therefore be styled a cell-community. It is clear that in such a case advantageous adjustment to the surroundings is best secured on the principle of division of labour, by which the vital activities are shared among the members of the community. Evolution on these lines has resulted in the development of Digestive Organs, Respiratory Organs, Organs of Movement, &c., the complexity of which is very great in some of the higher groups of animals. Hence the need for some means of central control, some way of correlating the diverse parts of the body, and at the same time of adjusting the body to its environment. These duties are discharged by the Nervous System, with the aid of Sense Organs, which keep it in touch with external agents. The Sensitiveness and Spontaneity of a Metazoon, in fact, are more or less centred in the Nervous System and Sense Organs, and this is true to an increasing extent as we consider animals higher and higher in the scale. At the same time it must not be forgotten that every cell in the body is endowed with all the primary properties of protoplasm, though cells specialize as it were in different directions, according to the nature of the organs of which they form a part. NERVOUS SYSTEMS OF ZOOPHYTES (CELENTERAta). The members of this primitive group, comprising Freshwater Polypes (Hydra), Hydroid Zoophytes, Jelly-Fish, Sea - Anemones, and |