Book - The Nervous System of Vertebrates (1907) 5
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Chapter V. The Functional Divisions of the Nervous System
The nervous system can be understood only by studying it from the standpoint of the work which it does. It must be insisted that a knowledge of mere structure is of little value and may be misleading without a knowledge of function. For this reason in speaking of the nervous system the term mechanism is preferable to architecture; for mechanism implies work as well as structure. It implies structure in action. The work of the nervous system is to adapt the activities of the animal to the conditions of its life and of the perpetuation of the species. As the necessary adaptations and correlations are successfully carried out the organization of the nervous system is perfected through experience. In all vertebrates the same general plan of organization is seen; the degree of organization is a measure of adaptation and is correlated with the survival of the best adapted. The higher animals are adapted to more complex and changing conditions of life and have more highly organized nervous systems.
We may distinguish two main groups of activities in the vertebrate organism which have determined the general plan of organization of the nervous system: actions in relation to the external world, and internal activities having to do with the processes of nutrition and reproduction. The actions toward the external world consist of the finding and capturing of food, fighting with other animals, preparing nests or homes for protection against the physical elements, and many minor reactions to changes in heat, light, moisture, etc. The internal activities include all the processes related to metabolism, the distribution of nutritive material to various parts of the organism, and the various processes connected with the formation of the reproductive elements and the nutrition of the embryo.
With regard to each of these groups of activities the work of the nervous system is two-fold. On the one hand it receives stimuli from the external world or the internal organs; on the other hand it directs the responses to those stimuli. All actions are in response to stimuli ; directly or indirectly. The stimuli usually affect the organs only through the nervous system. It is noticed that certain characteristic responses habitually follow upon certain stimuli, either as the result of heredity or of individual experience. For example, a frog will jump to catch an object which moves in a manner characteristic of its usual food. So a hellbender will pay no attention to a living earthworm so long as it is quiet, even although the worm be lying on the animal's nose. The moment the worm wriggles vigorously it is snapped up. This is because these animals habitually judge their food by its movements. In other animals the reactions necessary for capturing food are called forth by stimuli received through the organs of smell or taste. In any case the reactions toward the external world are in the nature of reflex acts, simple or complex. As far as the nervous processes are concerned the internal reactions are of the same character, although their reflex nature is not so readily seen as in the case of external activities.
With regard to the stimuli, then, those which affect the bodily welfare of the animal in its surroundings must be distinguished from those which affect the internal activities. A morphological distinction has long been made between the soma and the viscera. The soma comprises those organs (skin, muscle, skeleton, etc.) by which the animal deals with its environment. The viscera comprise those organs (alimentary canal and appendages, circulatory, excretory and reproductive apparatus) concerned with the processes of metabolism by which the organism is built up and the reproductive elements are formed. A part of the soma, the skin, comes into contact with the external world and offers the medium by which stimuli reach the nervous system. The remainder of the soma (muscles and skeleton) responds to these stimuli by appropriate movements under the direction of the nervous system. The peripheral nerve fibers and the central mechanisms which have to do with stimuli affecting the welfare of the animal in its surroundings are arranged on a common plan in all*segments of the body in all vertebrates, and constitute a distinct portion of the nervous system both structurally and functionally. These structures are best called collectively the somatic afferent division of the nervous system.
The stimuli having reference to food and the visceral activities include those arising in the viscera and those produced by chemical changes in the surrounding medium. Chemical changes in the surroundings do not stimulate the somatic sense organs. The special organs of the sense of taste, which in fishes lie in the skin as well as in the branchial and mouth cavities, and the olfactory organs are affected by chemical stimuli and are used in finding food. All the nervous structures concerned with impulses arising in the viscera, hi the taste organs and hi the olfactory organ are closely related and constitute the visceral afferent division of the nervous system.
FIG. 45. A diagram of the component elements in the spinal cord and the nerve roots in a trunk segment, to illustrate the four functional divisions of the nervous system, s. s., somatic sensory; v. s., visceral sensory; v. m , visceral motor; s.m., somatic motor.
The movements of the soma are aroused chiefly by somatic afferent impulses and have to do with the relation of the animal to its surroundings. Thus, all the usual movements of locomotion, of offense and defense, and so forth, are directed ordinarily in response to stimuli from without. Somatic movements are also performed in response to gustatory and olfactory stimuli and have for their object the capturing of food. It is shown by experiments on fishes and by clinical observations on man that very strong gustatory stimuli can be localized without the help of tactile stimuli, but the responses given by fishes to such stimuli are less constant and precise than the reactions to combined gustatory and tactile stimulation. Usually the localization of objects detected by the gustatory or olfactory organs requires concurrent somatic stimuli. This is easily illustrated in our own experience. We can not tell with certainty the location of an object which we smell. If a wind brings the odor to us we can tell the direction of the object, but not its distance. The direction is known only by the pressure (somatic) stimuli due to the wind. If we are blindfolded in a room and wish to find the position of a boquet of flowers which we detect by its odor we must go about and sniff until we find the place where the odor is strongest. In general, while somatic movements may be called forth by visceral stimuli they are more typically called forth by somatic stimuli and are more precise when they are directed in response to somatic stimuli. The nerve centers and peripheral nerves which direct somatic movements constitute a distinct portion of the nervous system called the somatic efferent (motor) division.
FIG. 46. Diagrams to represent the extent and arrangement of the functional divisions in the brain of a selachian. A, projection upon the mesial plane; B and C, sections taken at the levels indicated by the arrows. C. g., corpus geniculatum. ., superior secondary gustatory nucleus. Other legends and symbols are explained in the figure. Compare with Figure n.
FIG. 47. A diagram to show the arrangement of the two afferent divisions in the brain of man. Compare with Fig. 46. A cutaneous mesencephalic root of V is hypothetical in man.
The visceral activities consist of contractions of visceral muscles, secretory processes, vaso-motor regulation, etc. These all contribute directly or indirectly to the processes of nutrition in the widest sense, or of reproduction. These activities are aroused chiefly by visceral afferent impulses, including gustatory and olfactory impulses. How far they may be called forth by somatic stimuli is not fully understood. It is a fact of common experience that the concept of food aroused by sight or tactile perception may cause salivary secretion. Vaso-motor regulation is in large part the result of heat and cold stimuli in the skin. Perspiration is called forth by rise in temperature within the body. The nerve centers and fibers which control visceral activities constitute the visceral efferent division of the nervous system.
In the life of any vertebrate animal four kinds of nervous activities are called for: (i) the reception of somatic stimuli; (2) the direction of somatic movements; (3) the reception of visceral stimuli; (4) the direction of visceral activities. Corresponding to these four sorts of activities there are four anatomically distinct divisions of the nervous system: somatic afferent and efferent, visceral afferent and efferent divisions (Fig. 45). With exceptions to be noted in their proper places, the four kinds of activities are called for in all segments of the body, and consequently each of the functional divisions is represented in each segment of the body and all the segments of a given division are serially homologous with one another. These longitudinal divisions of the nervous system are therefore the most fundamental and important divisions both structurally and functionally. The segmentation of the nervous system is to be regarded as a segmentation of each of the functional divisions. It is probable that the functional divisions of the nervous system are more fundamental than the metamerism of the body.
One point of contrast should be noted between the two somatic divisions on the one hand and the two visceral divisions on the other. Although somatic afferent impulses may produce somatic reflexes directly without sensation, very commonly sensations are produced. When present the sensations are definitely localized and the responses may be consciously directed. Visceral afferent impulses, on the other hand, usually produce reflexes without sensation. When present the sensations are vague, general, poorly localized, and consciously directed visceral activities are very exceptional if not abnormal. The somatic activities are par excellence related to the conscious life.
There is given here for reference a table of the four functional divisions with the structures included in each. In addition to the structures included in this outline there are certain brain centers which with their fiber tracts serve functions of correlation between the four primary divisions. These will be treated in later chapters (Chap. XIV and following).
A. Somatic sensory division.
i. General cutaneous subdivision. Consists of: free nerve endings in the skin, general cutaneous system of components,
dorsal tracts of the cord, spinal V tract in the medulla oblongata,
together with their accompanying nuclei : the dorsal horn, nucleus
funiculi, nucleus spinalis trigemini, acusticum and cerebellum,
secondarv tracts and centers: internal and external arcuate fibers forming the tractus spino- and bulbo-tectalis (lemniscus), tecturn mesencephali, and other nuclei, tertiary tracts to motor nuclei and coordinating centers.
2. Special cutaneous subdivision. Consists of:
neuromasts (acustico-lateral sense organs), neuromast components,
spinal VIII tract and nucleus, nucleus funiculi, acusticum, cerebellum, secondary tracts and centers and tertiary tracts as in i, the cochlea, its nerve and centers in higher vertebrates.
3. Special sense organs belonging to the somatic sensory division.
Lateral eyes. Consist of:
retina, which includes the equivalent of sensory ganglion, nerve
component, and primary brain center, optic tract and tectum opticum corresponding to the secondary
tracts and centers of the cutaneous subdivision. Pineal eyes. (Compare Chap. VIII below.)
B. Visceral sensory division.
1. General visceral subdivision. Consists of:
free nerve endings in the mucosae,
fasciculus communis system of components,
CLARKE'S column or its equivalent, nucleus commissuralis CAJAL,
lobus vagi, and lobus facialis, secondary visceral tract and its continuation in the cord,=the
direct cerebellar tract in higher forms, secondary visceral nucleus,=end nucleus of direct cerebellar tract
in the vermis of higher forms, tertiary tracts to the hypothalamus in lower vertebrates.
2. Special visceral subdivision. Consists of: end buds (taste buds) : components and central nuclei and tracts not yet distinguished from those of the general visceral subdivision.
3. Special sense organ belonging to the visceral sensory division. Consists of: olfactory epithelium and nerve, bulbus olfactorius, tractus olfactorius, area olfactoria, and tertiary tracts to coordinating centers in the diencephalon.
4. Sympathetic system, afferent portion. An outgrowth or offshoot from visceral sensory ganglia which reaches a high specialization in the vertebrate series. Consists of simple visceral sensory component fibers and of ganglion cells which together with the efferent portion control contraction of smooth muscles and glandular secretion.
C. Somatic motor division. Consists of: ventral horn of the cord, nuclei of origion of Nn. XII, VI, IV, III, and nucleus of somatic motor fasciculus, motor components in ventral roots supplying musculature derived from the somites.
D Visceral motor division. Consists of:
1. Motor nuclei in lateral horn or intermediate zone of the cord and corresponding region of the medulla oblongata, motor component in dorsal and ventral roots supplying musculature derived from lateral mesoderm.
2. Sympathetic system, efferent portion. Consists of ganglion cells and fibers concerned with glandular secretion, and the contractions of smooth muscle. They receive impulses from visceral efferent fibers.
To illustrate the positions of the four divisions in the brain of vertebrates two diagrams are given, Figures 46 and 47. These show by conventional symbols the areas occupied by the functional divisions in the brain of a fish and of man. In these figures as in others the shading by right lines, either vertical or horizontal, indicates somatic sensory areas; the oblique lines indicate visceral sensory areas. The figures should be compared with Figs. 2, 3, n, and with figures of the human brain in a text-book of anatomy.
Demonstration of Laboratory Work
- Dissect the cranial nerves of a fish, a frog and a mammal with especial reference to the cutaneous and visceral rami.
- Dissect the brain of a large dogfish or skate and of a large Ameiurus, or other bony fish which has large vagal or facial lobes. Examine the form relations of the functional divisions in the medulla oblongata, and of the cerebellum, tectum mesencephali, inferior lobes and olfactory lobes and bulbs by means of hemisections and dissections of the brain and by sections under low power of the microscope.
- Compare the brain of some mammal with those of the fishes with regard to each of these points.
The importance of dissections of the brain cannot be over-estimated. Very satisfactory results in demonstrations can be obtained by no more elaborate means than dissections and well prepared sections stained by Delafield's haematoxylin.
Allis, E. P., jr.: The Anatomy and Development of the Lateral Line System in Amia calva. Jour. Morph., Vol. 2. 1889.
Cole, F. J.: On the Cranial Nerves of Chimaera monstrosa Linn., etc. Trans. Roy. Soc. Edinb., Vol. 38. 1896.
Ewart, J. C.: On the Cranial Nerves of Elasmob ranch Fishes, Proc. Roy. Soc., Vol. 45. 1889.
Ewart, J. C.: The Lateral Sense Organs of Elasmobranchs. I. The Sensory Canals of Laemargus. Trans. Roy. Soc. Edinb., Vol. 37. 1893.
Gaskell, W. H.: On the Structure, Distribution and Function of the Nerves which innervate the Viscera and Vascular Systems. Jour, of PhysioL, Vol. 7. 1886.
Herrick, C. J.: The Cranial and First Spinal Nerves of Menidia; a Con tribution, etc., Jour. Comp. Neur., Vol. 9. 1899.
Herrick, C. J.: The Doctrine of Nerve Components and some of its Applications. Jour. Comp. Neur., Vol. 14, 1904.
Johnston, J. B.: An Attempt to Define the Primitive Functional Divisions of the Central Nervous System. Jour. Comp. Neur., Vol. 12. 1902.
Johnston, J. B.: Das Gehirn und die Cranialnerven der Anamnier. Merkel u. Bonnet's Ergebnisse, Bd. n. 1902. Bibliography.
Johnston, J. B.: The Morphology of the Vertebrate Head from the Viewpoint of the Functional Divisions of the Nervous System. Jour. Comp. Neur. and Psych. Vol. 15. 1905.
Osborn, H. F.: A Contribution to the Internal Structure of the Amphibian Brain. Jour. Morph., Vol. 2. 1888.
Strong, O. S.: The Cranial Nerves of Amphibia. A Contribution, etc. Jour. Morph., Vol. 10. 1895.
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