122 THE GROUNDWORK OF SCIENCE

accompanies such irritation. If the part which remains

attached to the spinal cord be irritated, then pain is caused

but not motion. If the so-called posterior root* of a spinal

nerve be alone severed, the parts supplied with twigs from

such nerve only, lose their power of feeling, but their power

of motion remains. If the anterior root of such a nerve

be alone divided, then the parts supplied by such nerve are

paralysed as to motion, but, nevertheless, retain their sensi-

bility their power of feeling. If the spinal cord itself be

cut or broken through, it is impossible for a man thus injured

to feel any irritation which may be applied to those portions

of his body which are supplied with nerves which are

connected with any part of the spinal cord below the point

of' injury. Neither can he move such parts by any act of

his will, try as he may. Nevertheless, movements of those

very parts may be produced by stimuli applied to them,

of which he remains entirely unconscious, or which, if by

observation he is aware that they are applied, he has none

the less no feeling whatever, nor can he possibly withdraw

any such part out of reach of the stimulus so being applied.

A man so injured, though he may have entirely lost the

power of feeling any pricks, cuts, or burns applied to such

parts, will, none the less, execute movements, often in an

exaggerated manner, in response to such stimuli, just as if

he did feel them. He will withdraw his foot if it be tickled

just as if he felt the tickling, which he is incapable of feeling.

Such unconscious movement in response to stimuli which

are not felt is called reflex action, for the following reason :

Under ordinary circumstances stimulations of the surface

of the body convey an influence inwards which produces

sensation, and gives rise to an outwardly proceeding influence

passing to the muscles, and resulting in definite appropriate

motions. The influence inwards appears to travel upwards

* See ante, p. 1 14.

accompanies such irritation. If the part which remains

attached to the spinal cord be irritated, then pain is caused

but not motion. If the so-called posterior root* of a spinal

nerve be alone severed, the parts supplied with twigs from

such nerve only, lose their power of feeling, but their power

of motion remains. If the anterior root of such a nerve

be alone divided, then the parts supplied by such nerve are

paralysed as to motion, but, nevertheless, retain their sensi-

bility their power of feeling. If the spinal cord itself be

cut or broken through, it is impossible for a man thus injured

to feel any irritation which may be applied to those portions

of his body which are supplied with nerves which are

connected with any part of the spinal cord below the point

of' injury. Neither can he move such parts by any act of

his will, try as he may. Nevertheless, movements of those

very parts may be produced by stimuli applied to them,

of which he remains entirely unconscious, or which, if by

observation he is aware that they are applied, he has none

the less no feeling whatever, nor can he possibly withdraw

any such part out of reach of the stimulus so being applied.

A man so injured, though he may have entirely lost the

power of feeling any pricks, cuts, or burns applied to such

parts, will, none the less, execute movements, often in an

exaggerated manner, in response to such stimuli, just as if

he did feel them. He will withdraw his foot if it be tickled

just as if he felt the tickling, which he is incapable of feeling.

Such unconscious movement in response to stimuli which

are not felt is called reflex action, for the following reason :

Under ordinary circumstances stimulations of the surface

of the body convey an influence inwards which produces

sensation, and gives rise to an outwardly proceeding influence

passing to the muscles, and resulting in definite appropriate

motions. The influence inwards appears to travel upwards

* See ante, p. 1 14.