PHYSICAL ANTECEDENTS OF SCIENCE 121

s thus the special, the only, intermediary between our

consciousness and the external world, and the only bridge

between the subjective and all that is objective besides

itself. It both receives the various effects to which the world

about us and our own body can give rise to within it, and

which result in sensations ; and it also causes all the move-

ments which take place in response to stimuli. But it is

necessary to note that it not only acts as an intermediary

between each organ and its environment, through the sensa-

tions to which it gives rise, but also that it so acts without

the intervention of sensations. When acted on by external

influences it may, and constantly does, excite corresponding

activities in our body without giving rise to any feeling

of which we are conscious. The special consideration of

sensation itself, its various forms, and their other mental

accompaniments and effects, will be considered in our next

chapter on the psychical antecedents of science ; but sensa-

tion in its physiological aspect, in so far as it is related to

different portions and diverse conditions of parts of the

nervous system, concerns us here and now.

As everyone knows, different parts of the nervous system

have different functions, and the special functions of different

nerves are partly learned by the study of their distribu-

tion, and partly by the simplest observations. Thus an

irritation of the nerve which goes to the eye (to the retina)

or to the internal ear, does not produce feeling in the

ordinary sense of that word, but only certain sensations

of light or of sound. The nerves which, as before said, are

connected in pairs with the spinal cord, minister either to

sensation or to motion according to their distributions and

connexions.

If one of these nerves be divided, and the part cut off

from the spinal cord be irritated, then motion ensues in

the muscles to which such nerve is distributed, but no pain

s thus the special, the only, intermediary between our

consciousness and the external world, and the only bridge

between the subjective and all that is objective besides

itself. It both receives the various effects to which the world

about us and our own body can give rise to within it, and

which result in sensations ; and it also causes all the move-

ments which take place in response to stimuli. But it is

necessary to note that it not only acts as an intermediary

between each organ and its environment, through the sensa-

tions to which it gives rise, but also that it so acts without

the intervention of sensations. When acted on by external

influences it may, and constantly does, excite corresponding

activities in our body without giving rise to any feeling

of which we are conscious. The special consideration of

sensation itself, its various forms, and their other mental

accompaniments and effects, will be considered in our next

chapter on the psychical antecedents of science ; but sensa-

tion in its physiological aspect, in so far as it is related to

different portions and diverse conditions of parts of the

nervous system, concerns us here and now.

As everyone knows, different parts of the nervous system

have different functions, and the special functions of different

nerves are partly learned by the study of their distribu-

tion, and partly by the simplest observations. Thus an

irritation of the nerve which goes to the eye (to the retina)

or to the internal ear, does not produce feeling in the

ordinary sense of that word, but only certain sensations

of light or of sound. The nerves which, as before said, are

connected in pairs with the spinal cord, minister either to

sensation or to motion according to their distributions and

connexions.

If one of these nerves be divided, and the part cut off

from the spinal cord be irritated, then motion ensues in

the muscles to which such nerve is distributed, but no pain