THE ORGANIC SEAT OF CONSCIOUSNESS.

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THE NERVOUS SYSTEM.

84. § 4. Structure of Nervous System. Since the nervous system is thus in a peculiar manner related to our mental life it is important to have made a study of it before coming on to the consideration of mental phenomena. This the reader is supposed to have done by aid of one of the easily accessible text-books of physiology.1 All that is offered here is a résumé of some of the more important points which appear to have a psycho-physical significance.

The nervous system is primarily divisible into two portions. The one is a group of compact masses known as nerve-centres lying within the bony covering of the skull and vertebral column, and constituting the cerebro-spinal axis or brain and spinal cord; the other consists of thread-like ramifications running from these nerve-centres to all parts of the body and known as nerves.

§ 4a. Nerves. The nerves are found to consist of bundles of minute white fibres. The more important class of these fibres have as their essential element a central pith (axiscylinder), enclosed in a sheath (medullary sheath), which probably serves to insulate the fibres.

The nerves fall into two classes, which, though they appear to have the same structure, are marked off one from another by their mode of attachment at the periphery and at the centre, and as a consequence of this subserve distinct functions. Of these the first class are connected at their peripheral termination with some sensitive structure, as the skin, the nervous membrane of the stomach, and so forth. They are put into a state of activity at their peripheral end by a process of stimulation, and have as their function to convey nervous action to the centre. Hence they are called afferent or in-carrying and also sensory nerves.

The more important of these afferent nerves for the psychologist are the nerves of special sense which connect the peri

1 The reference here is principally to that higher and more important group of structures known as the Cerebro-Spinal System. The lower sympathetic system which regulates the vegetative functions, though involved in certain psychical phenomena, e.g., the organic sensations and emotions, affects mind indirectly only through its connexion with the higher system.

pheral organs of sense, the skin, the retina, and so forth, with the nerve-centres. The fibres of these nerves tend to separate towards the peripheral termination, and each fibre has its own terminal appendage, the several terminal appendages making together a sort of mosaic work. These appendages, which differ greatly in the case of the different organs, constitute the proper "end organ" of the sense. It is these, as we shall see, that are acted upon by the outer stimulus (as mechanical pressure, light) which excites the organ to activity.

The second class are, for the most part, attached peripherally to the muscles-those bundles of fibres by the contraction of which movements of the limbs, the heart, etc., are brought about-and have as their function to convey nervous excitation from the centres to these organs. Hence they are known as efferent or out-carrying and also as motor nerves.1 The most important of these motor nerves, again, for the psychologist are those which run to the striated or "voluntary muscles," as those of the limbs.2

While these two classes of nerves are thus differentiated by their peripheral attachments they appear also to have a different mode of attachment at the nerve-centres. Thus it has been known since the researches of Sir Charles Bell that of the two branches or roots which go to form a spinal nerve the one issuing from the anterior half of the column (anterior root) consists of efferent or motor fibres, the other starting from the posterior half (posterior root), of afferent or sensory fibres.3

§ 4b. Nerve-centres. The chain of nerve-centres or centrospinal axis consists of masses of greyish and of white substance arranged in a very intricate manner. The essential element in the grey matter is the nerve-cell, or more correctly "ganglionic nerve-cell," a minute sac-like structure with neck-like projections or "processes". With these cells or corpuscles are mixed

1 The term "motor" has been objected to on the ground that the efferent impulse does not always issue in muscular contraction, but sometimes, e.g., in the case of the salivary and other glands, in a change in epithelial cells. See Foster, Text-Book of Physiology, pt. i., p. 184.

The striated and voluntary muscles do not strictly coincide: but the exceptions need not here be considered.

These classes of nerves are also distinguished as centripetal and centrifugal. But the terms afferent and efferent seem the most convenient.

NERVOUS STRUCTURES.

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fibrous elements, and these last constitute the main constituent of the white substance of the nerve-centres.

Recent anatomical, investigation renders it probable that nerve-cells are connected by their processes with nerve fibres, and that in this way structural continuity is maintained between one nerve-cell and another, and one region of the nerve-centres and other regions. The fact that motor fibres are attached to the anterior portion of the grey matter of the spinal cord, sensory fibres to the posterior portion, suggests that the central substance is throughout divisible into motor and sensory regions. Experiment tends on the whole to support this conjecture. Anatomical examination, too, appears to favour the idea of certain differences in the size of the nerve-cells corresponding to this division of sensory and motor regions in the nervecentres.1

This chain of nerve-centres falls into a number of divisions, easily distinguishable by their shape, size, and the arrangement of the grey and white substance. The most obvious division is that of the narrow cylindrical spinal cord, and the bulbous globular mass known as the brain. In the cord the grey

matter constituting the central organ forms the pith or axis, being surrounded by strands of nerve-fibre. The cord thus serves both as a centre to connect the sensory and the motor fibres of spinal nerves one with another, and also as a prolongation of these fibres towards the higher centres of the brain.

The transition from the cord to the brain is formed by an expansion known as the medulla oblongata. Then follow the different organs of the encephalon or brain itself. These are roughly divisible into (1) a group of inferior organs, viz., the cerebellum or little brain, and certain smaller masses called the basal ganglia, and (2) the cerebral hemispheres forming the larger part of the brain. In these last we have the reverse arrangement of grey and white substance to that found in the cord. The grey matter forms the rind or cortex, and is arranged somewhat after the manner of foliage about a branching system of nerve-fibres.

1 This distinction between sensory and motor cells is however denied by certain physiologists, e.g., Münsterberg (see Die Willenshandlung, p. 141), and cannot be said as yet to be fully established.

These highest nerve-centres in the cortex are connected by means of intricate arrangements of nerve-fibre with the lower centres, basal ganglia, medulla, and cord, but the precise mode of these connexions is not yet fully ascertained. So far, anatomists have failed to trace the exact course of the fibres of the cord up to the cortex. One thing however is known, that they undergo a more or less complete crossing or decussation, so that fibres coming from one of the right limbs pass to the left hemisphere. The same thing is true of the cranial nerves, those which enter the skull and attach themselves directly to one of the lower centres of the brain, and which. include the nerves of special sense whose end-organ is in the head, viz., the eye, ear, organ of taste, and smell. The two hemispheres are moreover closely connected one with another and with the intermediate centres (basal ganglia).

It is to be added that the nerve-centres are richly furnished with blood-vessels. More particularly the brain is surrounded by a minute network of vessels by which its substance is amply supplied with arterial blood.

§ 5. General Plan of Nervous Structures. It is evident from this slight sketch of the Nervous System that it is a system of closely conjoined parts by means of which action at any one point, say of a sensory nerve, may be propagated in a number of definite directions so as to affect other and distant regions of the system itself, and the end-organs connected with this system. Not only so, we see from the arrangement of the nerve-centres that they form a series of organs of growing complexity, admitting of more and more intricate and varied connexions between one point of the organism and other points. Thus the grey matter of the cord is a meeting point for comparatively few paths afferent and efferent, and consequently its actions are marked by a high degree of simplicity and invariability. The higher centres on the contrary contain meeting points for a much larger system of nervous paths, and consequently provide a field for more intricate and varied actions.1 § 6. Function of Nerve-fibres. The nerve-fibres are, we are told, pure conductors. Their sole function is to transmit

1 Mr. Spencer has illustrated in a very interesting way this effect of growing complexity of structure by comparing the higher and lower types of nervous system in Man and Animals. See Principles of Psychology, pt. i. chap. ii.

FUNCTION OF NERVE-FIBRES.

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nervous excitation from one point of the nervous system to another. But of the exact nature of this nervous activity little is known beyond the fact or common assumption that it is some form of molecular movement. It is found to have some important affinities with electrical action, but it must not be confounded with this. For one thing, the process of transmission is relatively slow, being about 100 feet per second.1

As already pointed out, the two classes of nerves marked off as afferent and efferent have a marked difference of function. Under normal circumstances afferent nerves are only excited by way of their peripheral attachments (sensitive structures, endorgans), and have to conduct the state of nervous excitation or 'nerve-commotion' from the periphery to the centres. Efferent nerves, on the other hand, are stimulated or 'innervated' by way of their central connexions, and have to transmit the process of excitation outwards to the muscles.3

§ 6a. Specific Energy of Nerves. It was formerly supposed that each nerve had its own peculiar and unalterable function. This view is known as the doctrine of the specific energy of the nerves. The doctrine was supported, not only by such facts as these just mentioned, but by experiments which show that when a sense-organ is stimulated by an unaccustomed stimulus, as when by pressing on the eye-ball we mechanically irritate the retina, the psychical consequent is still a sensation proper to that particular sense. Nevertheless recent investigation has tended to show that the function of nerve-fibres is not unalterable. Thus it seems to have been proved by experiment that under certain exceptional circumstances afferent and efferent nerves may exchange functions. Further, there is, as we shall see later on, every reason to believe that in the case of the nerves of

1 For an account of what is known of the process of conduction along nervefibres, see Ladd, Elements of Physiol. Psychology, pt. i. chap. i. § 32 and following, and chap. iii.; and Wundt, Physiol. Psychologie, ier Abschnitt, 6er cap. 2. and 3. Cf. Foster, Text-Book of Physiology, part i. chap. ii.

This generalisation as we shall see later when dealing with the psychophysical process in attention is not perfectly certain. There may be a reverse central stimulation in certain cases.

Or, in the case of the glands, to the epithelial cells which certain efferent fibres are known to influence directly, and independently of any action on the muscular fibres of the arteries.

'See G. H. Lewes, Physical Basis of Mind, p. 280 ff.

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