Imágenes de páginas
PDF
EPUB

ASSOCIATION FIBRES.

39

of the cortex constitute the first part of the central motor path.

[ocr errors]

Association Fibres. The third class of connecting fibres are called association fibres. They connect one part of the cortex with another. Says Edinger: "They extend everywhere from convolution to convolution, connecting parts which lie near each other as well as those which are widely separated." They are called association fibres because it is supposed to be by means of them that we are able to associate one experience with another.

[graphic][subsumed][subsumed][subsumed][subsumed]

FIG. 7.-Lateral view of a human hemisphere, showing the bundles of association fibres. (Starr.) A, A, between adjacent gyri; B, between frontal and occipital areas; C, between frontal and temporal areas, cingulum; D, between frontal and temporal areas, fasciculus uncinatus; E, between occipital and temporal areas fasciculus longitudinalis inferior; C.N., caudate nucleus; O.T., optic thalamus. (Donaldson.)

[ocr errors]

Commissural Fibres. The fourth class of connecting fibres are those which connect identical parts of the two hemispheres of the cerebrum with each other. They are called commissural.

QUESTIONS ON THE TEXT.

I. What is the difference between nerve cells and nerve fibres?

2. What is a neuron?

3. What are the two functions of the nervous system?

4. What are afferent nerves?

5. Mention the parts of the brain in which Psychology is espe cially interested.

6. Describe the four classes of fibres which connect one part of the cortex with another, and with the various parts of the body.

LESSON V.

THE FUNCTIONS OF THE NERVOUS SYSTEM.

Functions of the Fibres. After this brief survey of

snatch your you

the nervous system we are ready to consider its functions. It is evident that the office of the fibres is to conduct nervous excitations. When hand away from a hot stove, the pain is not in the hand; for, if the nerve which connects the hand with the spinal cord is divided, you will feel no pain. The brain has caused a change in the ends of the nerve that terminate in the injured part, and this change has been transmitted along the nerve to the spinal cord. The same kind of evidence shows that the motor nerves running from the spinal cord to the muscles have the same office. For, if the nerves extending to the muscles of your arm be divided, you can not snatch your hand away when you feel the sensation of pain. You will be like an animal shot by an arrow which has been dipped in the poison called curari — a poison which renders the motor nerves incapable of action, while it does not affect the sensory nerves. You will feel the pain, but will be unable to move your hand.

[ocr errors]

Nature of a Nervous Impulse. As to the nature of the change which takes place during the passage of a nervous impulse, physiologists and psychologists are almost

entirely ignorant. Says Professor Martin: "Since between sense organs and sensory centres, and these latter and the muscles, nervous impulses are the only means of communication, it is through them that we arrive at our opinions concerning the external universe and through them that we are able to act upon it; their ultimate nature is therefore a matter of great interest, but one about which we unfortunately know very little." 1

Nerve centres also conduct nervous excitations, but this Perhaps the best way to realize what this is, is to contrast reflex with voluntary actions, as many physiologists understand it.

is not their most characteristic work.

We all know what is meant by voluntary actions. They are actions which seem to be the result of our volitions. For certain conscious reasons, we will to act in a certain way, and the action follows. If, however, the act takes place as the result of the stimulation of an afferent nerve, without the intervention of consciousness, it is called reflex.

Voluntary, Reflex and Semi-reflex Actions. - Professor James gives a clear illustration of the difference between voluntary and reflex actions and a kind of action intermediate between the two. "If I hear the conductor calling All aboard!' as I enter the depot," he says, "my heart first stops, then palpitates, and my legs respond to the air waves falling upon my tympanum by quickening their movements. If I stumble as I run, the sensation of falling provokes the movement of the hands towards the direction of the fall, the effect of which is to shield the body from too sudden a shock. If a cinder enter my eye,

1 Martin's Physiology, p. 203.

MECHANICAL NATURE OF REFLEX ACTIONS. 43

its lids close forcibly, and a copious flow of tears tends to wash it out." 1

ness.

In this illustration we have examples of three different kinds of action. The quickening of the pace in consequence of the conductor's "All aboard!" is an example of voluntary action. It is an action following upon a distinct volition, or at least upon a definite state of consciousWith the closure of the eye, on the other hand, and the flow of tears, consciousness had nothing to do. The nervous impulse caused by the cinder passed along an afferent nerve leading from the eye to a certain nerve centre, and that centre imparted an impulse to an efferent nerve connected with the muscles whose contraction results in the closure of the eye, and the result was the closure of the eye without the intervention of consciousness. actions are called reflex.

Such

The movement of the hands illustrates what is sometimes called semi-reflex actions, and sometimes acquired reflexes. The last term is the better because it marks the two essential facts in the case: (1) The action so characterized is now performed without the intervention of consciousness. In that respect it is like reflex actions, so called. (2) Such actions were not originally so performed. They are therefore said to be acquired reflexes.

[ocr errors]

Mechanical Nature of Reflex Actions. That the actions described as reflex are mechanical, there can be no manner of doubt. Certain afferent and efferent nerves, with the nerve centres of which they are outgrowths or prolongations, with the muscles with which the efferent nerves are connected, are the mechanical contrivances for

1 James's Psychology, p. 12.

« AnteriorContinuar »