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medullary sheath, a white, fatty substance, isolating and protecting the nerve-axis; (3.) A thin thread of gray

matter called the axis. The axis is composed of minute fibrils. Illustrate with a common lead-pencil-the paint representing the external sheath; the wood, the medullary sheath; the lead, the nerve-axis.

2. Office. Nerves transmit vibrations. Their sole office is to transmit sensor and motor molecular waves. As the nerve-axis conducts the vibrations, it may be considered the essential part of the nerve. Like telegraphwires, nerves simply carry messages.

3. Classes. Nerves that convey impressions from sensor organs to sensor ganglia are called sensor nerves; as, the optic nerves are the sensor nerves that convey impressions from the eyes to the optic ganglia. Nerves that convey motor impulses from motor ganglia to motor organs are called motor nerves.

The following classification of the cerebro-spinal nerves, by Dr. S. S. Laws, is simple and complete :

Cerebro-Spinal Nerves.—

SENSORY NERVES IN PAIRS:

1. Cranial.-1. Olfactory.

2. Optic.

3. Trifacial.

4. Gustatory.

5. Auditory.

6. Glossopharyngeal.

7. Pneumogastric.

MOTOR NERVES IN PAIRS: 1. Cranial.-1. Oculomotorius. 2. Patheticus.

3. Small root of V.

4. Abducens.

5. Facial.

6. Spinal accessory. 7. Hypoglossal.

2. Spinal.-31 pairs of posterior 2. Spinal.-31 pairs of anterior

roots.

roots.

38 pairs of sensory nerves.

38 pairs of motor nerves.

4. Nerve-fibers are continuous. Sensor nerves extend without break from sensor organs to sensor ganglia. Motor nerves are continuous from motor ganglia to motor organs. Let silk threads represent sensor nerves and cotton threads motor nerves. Trace these threads through all their windings. You will find each continuous. Nerves do not divide or unite.

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Nerve Currents. Touch a warm surface. stimulus in some unknown way starts nerve-currents which move through tactile nerves to tactile ganglia. You feel the dangerous warmth. You will the withdrawal of your hand, and thus start currents in the motor ganglia. The motor currents move through motor nei ves to muscles. The muscles contract and thus withdraw your hand. The nature of the change produced in nerve-fiber by stimulus is quite unknown. How matter affects mind or mind matter must be classed with the many unsolved problems of science. But science now claims to have demonstrated that (1) sensor nerve-currents move at the rate of 140 to 150 feet per second, and motor nerve-currents about 100 feet per second. (2) Stimuli excite vibratory nerve currents. A wave of molecular movement passes through the nerve. These nerve-currents are the only media of communication between the mind and the outer world. (3) Sensation takes place only in the sensor ganglia found in the gray matter of the cerebrum.

A Sensor Organ is a vital mechanism capable of receiving and transmitting sensor vibrations. Each sensor organ is connected by sensor nerves with its sensor ganglia in the surface of the brain. Take, for example, the optic apparatus:

Objective World - Eyes +

Optic
Nerves

+

Optic
Ganglia

- Mind.

Striking the retina of the eye, light-vibrations in some unknown way excite sensor vibrations, which move in molecular waves through the optic nerves to the optic ganglia. The nerve-currents agitate the optic ganglia, and the mind feels and interprets the vibratory signals -sees the rising sun. The ear does not change soundwaves into sensor waves, but in the ear sound-waves excite sensor waves.

A Special Sense receives extra organic messages. The world of color and form comes to us through the eye; the world of sound through the ear; the world of odor through the nose; the world of flavor through the mouth; the world of touch through the skin. As each of these senses opens to us a special world, they are called the five special senses.

A General Sense transmits organic sensations. Conditions of the organs of the body come to us through the general senses. Sensations of indigestion are messages from the stomach. Toothache is a message from a nerve. Pain and comfort, hunger and satisfaction, temperature, and so forth, are some of the messages received through the fifteen general senses.

The Brain. Organism reaches its climax in the human brain. A human brain, it is estimated, embraces not less than one billion nerve-cells, nor less than five billion nerve-fibers. To produce an imperfect brainmap has required ages of toil. Much remains for other ages to discover. The brain and its connections must continue to be the most absorbing field of scientific

research. The brain includes the lower, middle, and higher nerve-centers. The cuts on pages 40, 42, and 46 give different views of the brain.

The lower nerve-centers are the medulla oblongata and the cerebellum. Like the spinal cord, these are reflex and distributing centers. Some claim that the cerebellum is a relay-battery to enforce nerve-currents. Others claim that it is connected with the co-ordination of movements. The following eut represents a perpendicular section of the brain on the median line.*

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The middle nerve-centers are the pons Varolii, the cerebral peduncles, the corpora quadrigemina, the optic thalami, the corpora striata, etc. These ganglia are geographically central, and, as all messages between the outer and inner worlds seem to pass through these cen

*The above cut is taken by permission from Bastian, p. 452.

ters, they may be considered telegraphic headquarters. Who can tell what changes take place in these mysterious centers? Destroy these centers, and you render sensation as well as voluntary action impossible. Sever the nerve-connections between the tubercula quadrigemina and the cerebrum, and vision is wholly reflex. The animal is utterly unconscious of seeing.

Remarks. The spinal cord, the lower nerve-centers, and the middle nerve-centers, with their nerve-connections, make a wonderful organism for reflex action; but it is only a machine. When stimulus falls upon the appropriate sensor surface, a wave of molecular movement is sent up the attached sensor nerves to a nervecenter, which thereupon issues another wave of molecular movement down a motor nerve to the group of muscles over whose action it presides. When the muscles receive this wave of nervous influence, they contract. This kind of response to stimuli is purely mechanical, or non-mental, and is termed reflex action. Thus far we fail to find mind. Remove the cerebrum: the animal may still show reflex action, but all traces of mind will have disappeared. All activity below the cerebrum is unconscious activity, is non-mental.*

The higher nerve-centers are the cerebral hemispheres. Here is the border-land where mind and matter meet. The soul is embodied; it dwells in and works in connection with a physical organism. In man the cerebrum is so large that it completely fills the arch of the skull as far down as the level of the eyebrows. The two hemispheres of which it consists meet face to face in the middle line of the skull, which runs from the top of the nose backward. The cerebrum is composed of two conspicuously distinct parts, called respectively the gray matter and the white matter. The gray matter is external, enveloping the white matter

* Holbrook.

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