COMPARATIVE NERVOUS SYSTEM. Feeling, think ing, and acting depend upon the nervous system. In man the sense organs, the central nervous organs, and the muscles form a combination by which we feel, think, and act. It is usual to limit the term nervous system to the first two of these three sets of organs. All three, however, are linked together as a unit in performance.
The sense organs, such as the eye and the ear, receive external stimuli, light and sound, and originate nervous messages or im pulses that are sent over the appropriate nerves to the brain. In this central organ these impulses awaken the sensations of sight and of hearing and are switched over outgoing nerves to the ap propriate muscles whereby a specified act may be performed. Thus if very strong light is thrown on the eye we experience the sensation of great brightness and certain muscles respond in that the eye winks.
Since sense organs are the means of receiving external changes they are commonly called receptors. They include in addition to the eye and the ear, the organs of taste and of smell, of touch and of pain, of cold and of heat, and of a score or more other senses. The central organs, such as the brain, are not only the seat of the sensations, but, like the central office in a telephone system, they switch the impulses to the proper terminals. Hence they are often called adjustors. The terminals of the various animals may be not only muscles but glands, electric organs, luminous organs, colour organs and the like. They are the effective parts of the combination by which a given animal responds to changes about it. Hence they have been called effectors. By the combined working of receptors, adjustors, and effectors all animal behaviour is carried out. It is by means of these three sets of organs that the worm retreats from danger into its burrow, the ant carries food to its hill, the frog jumps for a fly, the bird builds its nest, and man writes a page. In short these are the organs that under lie all animal behaviour.
Many of the acts of the higher animals are carried out with almost invariable uniformity, e.g., the withdrawal of the foot when it is pricked or the watering of the mouth when food is smelled. Such acts are called reflex, for the sensory impulse seems to be turned back to the exterior by the central organ in almost the mechanical way in which light is reflected from a mirror. Other acts, e.g., the crying of a newborn child or the suckling of a young mammal, are more complicated than reflexes and yet are inborn in that they are not learned and need only an appropriate stimulus to start them going. Such acts are called instinctive and rest upon a structural basis preformed in each individual. Finally there is the large body of daily performance acquired by learning and including those plastic activities that, under the influence o f memory and other stored experience, are moulded into the acts of everyday life. These range from such simple performances as walking, running, or flying, to the enor mously complex operations in the intellectual life of the higher animals including man. Here belong the capacity for memory, for imagination, for voluntary action with its moral implications, and the like. Here too occur those important modifications, which, essentially pathological, lead to abnormal mental conditions and insanity.
When an attempt is made to arrange normal animal activities under the three classes of reflexes, instincts, and higher acts, it is found that they are not always conveniently so placed. Walk ing is not only learned, but is in part instinctive if not reflex. The so-called reflexes themselves are of ten open to profound change. Thus the sneeze of a yokel, an almost purely reflex act, can through the social training of the well-bred be brought under control and even completely extinguished. Hence reflexes, in stincts, and habits are really not classes of acts in themselves but rather phases of behaviour that qualify almost all perform ances. In the vertebrates, from fishes to man, the nervous organi zation includes a large variety of plastic operations which thus favour active habit formation. In the insects and other like creatures, on the contrary, the nervous organization, though in cluding plasticity, leans strongly toward the instinctive side. Bees like men may learn to find their way about and are helped in this by their fellows, but they build their comb and do a thou sand other things instinctively and without training. Much of the life of an insect is thus semi-automatic, and in this respect it is in strong contrast with that of a vertebrate, where freedom of action and learning play predominant parts.
The structure of the various types of nervous systems by which the behaviour of the higher animals is carried out shows great individuality, and yet each such type exhibits a certain uniformity in that it is made up of a combination of nerve-cells or neurones. A neurone commonly consists of a cell-body and one or more lengthy processes or nerve-fibres. Ordinarily the cell-bodies mass together and thus establish what are called nerve-centres whereas the nerve-fibres are assembled in the form of nerve tracts by which one centre is put into connection with another. These rela tions can be easily seen in the nervous system of such an animal as the earthworm.
The nervous system of this creature consists of a small brain in its head from which a long strand of nervous material passes backward along the under side of its body to the tail. In each of the hundred or more segments in the worm this strand enlarges into a nerve-centre from which nerves pass out to the adjacent parts, muscles and skin. In the skin are lodged a large number of nerve-cell bodies whose outer ends reach the external surface of the worm as fine bristle-like terminals and whose inner ends be come attenuated into delicate nerve-fibres that make their way as constituents of a nerve to their terminations in the ventral nerve-centre. Each of these peripheral cells with its attached fibre is a neurone and since these neurones are concerned with the reception of stimuli that impinge upon the skin they are called receptor neurones. In the ventral nerve-centre these neurones come into contact with the cell-bodies of a second set of neurones whose nerve-fibres make their way out from the centre to termi nate in the worm's muscles. Neurones of this kind, in conse quence of their connection with muscles, are known as effector neurones. When the skin of an earthworm is stimulated and the animal responds by muscular movement, receptor and effector neurones are active in carrying out such a simple nervous operation.
Beside these two classes of neurones there is in the nervous system of the earthworm a third class, the internuncial neurones. These are neurones the whole of whose extent lies within the central organs and which serve as a means of uniting one nerve centre with another. They extend for the most part up and down the length of the central nervous organ.
In worms generally the three types of neurones already de scribed are about equally abundant, but in the crabs, insects and other higher invertebrates the internuncial neurones increase dis proportionately till finally in the vertebrates they make up a very large part of the central nervous organs. In these animals the central organs consist of a brain in the head and a spinal cord extending backward through the body. These nervous parts are located on the dorsal aspect of the animal, not on its ventral face as in insects, crabs, and worms. The cord and brain in the verte brates include receptor and effector neurones as in the inverte brates but these two classes of elements together do not make up more than a few per cent of the central organs, the great bulk of which is composed of internuncial neurones. In fact some of the most important organs in man and the higher vertebrates, such for instance as the cerebral hemispheres, are composed exclusively of this type of neurone.
In all animals from the worms and insects to the higher forms the brain is a conspicuous and very characteristic part of the central system. It is always located at the anterior end of the creature and is the nervous centre with which the chief sense organs of the body are connected. In man and in other higher animals the brain is thus associated with the organs of taste and of smell, with the ears and the eyes. The development of these important receptors and of the brain has progressed hand in hand in an evolutionary way. This progress has involved the change of several of the head sense organs from what may be called surface-receptors to distance-receptors. The nature of these two kinds of receptors may be illustrated by certain human sense organs. We refer our sensations of touch to the spot on the outer surface of the skin where the foreign body impinges. We believe we taste our food where it is in contact with the surface of our tongue. Receptors that are thus concerned with the surfaces of our bodies are called surface-receptors and represent a very primi tive type of such organ. There is good reason to assume that all the receptors of an earthworm or other lowly creature are of this type. But in higher animals there is a second kind of receptor. We see the surrounding landscape through the eye but we do not think of seeing it on the sensitive surface of the eye where the image is really formed but in the distant exterior. In a similar way the sound of the noon bell does not seem to be in the ear but comes from the distant clock tower. With such sense organs as the eye and the ear we project the disturbance that excites us to a distant point and think of it as being far away from us and not in contact with our bodies. Hence such sense organs are appropriately called distance-receptors.
In the evolution of sense organs there are ample grounds to be lieve that distance-receptors have evolved from the relatively more primitive and simpler surface-receptors. Those sense organs in the skin of the earthworm that are concerned with the recep tion of odours, of light, and of vibrations are without doubt surface-receptors of the kind from which have evolved the dis tance-receptors of the higher animals.
In the evolutionary changes whereby surface-receptors have been converted into distance-receptors there has gone on hand in hand in the higher animals a corresponding growth of the brain. Thus in a way the transformations undergone by the sense organs have made possible a corresponding transformation of the brain. In this growth, however, the brain has assumed a role of its own and has developed peculiar and characteristic functions that have carried it far beyond what is merely needed for increased sensory action. This growth has placed the brain at the apex of nervous development. For the brain of the higher animals and especially of man is not only a most complicated switching station for nerve impulses, but has become a repository of past experiences, and a centre for all those higher nervous activities that make up mental life, and that we regard as most characteristic of our personal selves. In this way the brain has come to serve as the seat of those remarkable performances of memory, volition, imagination and the like that are looked upon as the distinguishing features of the individual man.
As already shown the ordinary nervous activities of the higher animals depend upon the three-fold organization of sense-organ, central organ, and effector. In animals simpler than crabs and worms this degree of nervous organization is greatly reduced or even lacking. In many lowly creatures, especially in jelly-fishes and sea-anemones, the nervous system is represented chiefly by receptors which are often directly connected with the muscles. These receptors are located in the outer skin of the animal but are seldom specialized enough to be easily distinguishable as such. They connect with the muscle below them either immediately or through a delicate nervous net-work. This net-work spreads over much of the animal's body following the distribution of receptors and muscles. It is nowhere sufficiently concentrated to form a central organ and the system may therefore be described as diffuse. It is an arrangement of receptors and effectors without an adjustor.
Since this diffuse system very generally permeates the bodies of these lower animals their parts show remarkable independence of action. Thus the tentacles of a sea-anemone which surround the mouth of the animal and are very effective in the appropria tion of its food, will respond to bits of meat with great efficiency even of ter they have been cut from the animal. The creeping foot of the sea-anemone will continue to creep after the rest of the animal has been cut away. This autonomy of the parts demonstrates the all-sufficiency of the nerve and muscle con tained in each particular portion and shows that a given activity is not dependent upon some distantly located central organ. The leg of a crab, an insect, or a frog, if cut off, shows no such powers. It no longer moves as it formerly did because it lacks nervous connection with a central adjustor. The tentacle of the sea-anemone contains within itself the nerve and muscle neces sary for its own type of response.
The receptors of the sea-anemone and other such animals serve as delicate triggers to set their muscles in action. As this seems to be their one function they are more appropriately designated receptors than sense organs for they are apparently not concerned with sensations.
Collectively the receptors and underlying nerve-net of these simple creatures constitute a type of nervous system quite dis tinct from that seen in the higher forms. Not only does such a nervous system lack centralization but its nerve-net has the remarkable property of transmitting nervous impulses in any direction. Its diffuse character is not only structural but also functional. In the centralized nervous system of one of the higher animals it is possible to send a nervous impulse over a succession of neurones from the receptor to the effector side. When the attempt is made to send the impulse in the reverse direction the transfer fails. In the region where one neurone touches the next there is a valve-like device which will allow the impulse to pass in one direction but not in the reverse. No such impediment occurs in nerve-nets where impulses spread with the utmost freedom, and in any direction. It is interesting to observe that although the nerve-net is a type of nervous organization charac teristic of the lower animals, it has been retained in part at least in many higher forms. Thus the vertebrate digestive tube has a nervous organization closely resembling that of a sea-anemone, and many details in the movements of the vertebrate digestive organs are repetitions of those found in animals whose nervous systems are wholly of the nerve-net type.
Simple receptors and nerve-nets such as those described ap parently mark the first step in the evolution of true nervous organs, for in animals lower in the scale than sea-anemones, e.g., sponges, no nervous structures at all have been identified nor do the activities of these animals suggest such parts. Sponges grow almost plant-like from a submerged base and show scarcely any movements. By means of a system of minute lashes they draw through their bodies a current of water from the floating contents of which they extract their food. Their chief movements are the opening and closing of inlet and outlet pores for this system of water circulation. These movements are accomplished by very simple muscles which apparently act through direct stimulation and without the intervention of nerves. Such muscles since they act of themselves may be designated independent effectors.
In the protozoans, the simplest of all animals, each individual as a rule is a single cell. Within the bodies of some of these forms fibrous systems have been discovered which connect the central part of the cell with locomotor lashes on the exterior. These systems have been regarded by some as nervous in char acter but it is by no means certain that such is the case.
Apart from these possible traces of nervous origins in the pro tozoans, true nervous elements appear first in the trigger-like receptors of the sea-anemones and jelly-fishes. Organs of this type were doubtless the forerunners of the true sense organs of the higher animals. To these in the course of time were added central nervous parts, especially the brain, that organ of para mount importance in the life of the higher animals and especially of man.