LETTERS TO A STUDENT. 
185 
which error usually commences. Some means must be found of 
carrying the foreign body to the brain. Some contend that there 
is a fluid in the nerves, which oscillates backward and forward, 
from the skin to the brain, and from the brain to the muscles; 
according to others there is no such fluid, but a vibration in the 
texture of the nerves every time they receive an external impres- 
sion. A third hypothesis has been invented, which converts the 
nervous system into a galvanic apparatus. After a great deal of 
argument for and against these conjectural explanations, you 
are probably dismissed with a reflection, that this subject is one 
of great obscurity ; that it has hitherto baffled human inquiry, 
and seems likely to do so for ever. 
Here you have a very simple process rendered as intricate as 
error and bad reasoning can well make it. There are not less than 
three attempts to carry something to the brain, to bring the im- 
pressing agent in close contact with the distant organ by which 
it is supposed the impression is received. The inventors of these 
hypotheses seem to have thought that there is no possibility of 
acting upon the brain without striking it, or sending something 
to it, or taking something from it. 
It is almost unnecessary to observe, that there is no direct 
proof in support of any of the assertions. 
Such explanations would never have been offered, or they 
would not have been received, if men had not been in the con- 
stant habit of associating action with contact. Let it be granted 
that the nerves do vibrate, or that there is a nervous fluid tra- 
versing the cords of sensation, from one extremity to another, is 
the process of sensation in any degree less wonderful than before ? 
The brain and the nerves form but one organ. Considered in 
relation to their functions, they are as inseparable as the air- 
vessels and the bloodvessels of the lungs. Each part has a task 
of its own ; but that task cannot be performed without the 
assistance and integrity of other parts. That the nerves may 
perform their function, it is an essential condition that they be 
continuous with the brain. But this condition neither proves 
nor implies the transmission of a fluid, nor of an impulse. 
Either, it is possible, may exist, but at present its existence is 
entirely conjectural and unnecessary. 
The hypotheses now, or lately, current regarding conception, 
afford another illustration of our tendency to suppose a vital pro- 
cess explained so soon as it is shewn to bear some analogy to 
the phenomena with which we have been long familiar. Here, 
as in sensation, there are the same efforts to produce contact, 
when we are far from being sure that contact is necessary. After 
the uterus has received the semen, a change takes place in the 
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