ORIGIN AND CIRCULATION OF THE NERVOUS INFLUENCE. 357 
the circulation of the blood ; and, on the other hand, changes 
in the circulation of the blood must have a powerful influence 
in modifying the electric current. There is thus a constant 
circulation of nervous fluid dependent upon the circulation 
of the blood; and just as the heart does not generate, but 
only distributes the blood, so the brain and the other nervous 
centres do not produce, but serve to accumulate, modify, 
and distribute the nervous influence. 
This constant circulation of nervous fluid, in union with 
the circulation of the blood, is sufficient to give us an idea of 
organic life; and, perhaps, in some of the lower forms of 
animals this is ail that exists. But as we rise in the scale of 
organisation, the animal system becomes a much more com- 
plicated machine, having a variety of functions. It is there- 
fore necessary to inquire how these may be performed. 
The experiments of various inquirers have rendered it 
probable that there are distinct nervous centres, and that to 
each of these belongs its own separate function. It is pro- 
bable that these functions are performed, not as has generally 
been explained, by electric shocks communicated from or 
transmitted to the central organs, but by sustained and con- 
stant circuits of electricity produced by the changes of the 
blood in the capillaries, and by the modification of these 
currents by external or internal agencies. 
On the supposition that the different nervous centres have 
separate functions, and that they all receive their electricity 
from the chemical changes of the blood, there comes the 
question whether each central organ has a completely dis- 
tinct set of nerves, and that thus there are distinct nervous 
extremities, as well as distinct centres, for each function ; or 
whether there is only one afferent and one efferent fibril 
accompanying each capillary of the arteries and veins, which, 
receiving the electricity, conveys it to the nearest ganglion, 
from whence it is partly returned to the same point, thus 
maintaining the life of the part ; but that a portion of the 
electricity is transmitted along other nerves to the more 
distant central organs, thus forming a longer circuit of motion 
or sensation. 
It seems to be an universally received opinion that sensation 
results from impressions transmitted by the nerves to the 
central organ ; that motion, on the other hand, arises from 
an influence sent from the central organ to the extremities of 
the nerves, thus conveying the idea that these centres are 
more sentient and more capable of originating motion than 
the other parts of the nervous system. This idea of impres- 
sions being conveyed to and from the brain is contrary to 
xxvii. 47 
