616 THE EXCITO-MOTORY NERVOUS SYSTEM OP 
extent, without inducing any expression of pain or contraction 
in the muscular system. 
The spinal marrow is to be regarded as a process from or pro- 
longation of the cerebrum, or as a large nerve emanating from it; 
and hence the reason why the nerves of the body, generally speak- 
ing, are derived conveniently from the spine, instead of the brain 
itself. 
Formerly it was believed that the properties of sensation and 
volition were conducted to and from the brain and different parts 
of the body by the same cord of nervous filaments : the double 
roots the spinal nerves possessed were not understood or explained 
until their utility and distinctness in function were shewn and 
proved beyond all dispute by the beautiful experiments of Sir 
Charles Bell. It is now known that the anterior nerves are 
those conveying the motive mandate or power, while the pos- 
terior simply endow parts with sensibility; and it is believed 
that the corresponding columns of the spinal marrow possess 
similar endowments ; though this latter is, according to some, a 
doctrine requiring further confirmation. A rabbit being struck 
behind the ear so as to deprive it of sensibility. Sir C. Bell ex- 
posed the spinal marrow. On irritating the 'posterior roots of 
the nerves, he could perceive no motion consequent in any part 
of the muscular frame ; but on irritating the anterior roots of the 
nerves, at each touch of the forceps there was a corresponding 
motion of the muscles to which the nerve was distributed. Every 
touch of probe or needle on the threads of this root was attended 
by a muscular motion, as distinct as the motion produced by 
touching the keys of a harpsichord. “ These experiments,” con- 
cludes Sir Charles, “satisfied me that the different roots and dif- 
ferent columns whence these roots arose were devoted to distinct 
offices, and that the notions drawn from the anatomy were 
correct.” 
Although this experiment, with other similar ones, sufficiently 
shewed that through the anterior nerves, and them alone, muscular 
motion was excited, still there was no direct proof that the same 
nerves were the conductors of voluntary motion. This disput- 
able point was afterwards cleared up in the following beautiful 
manner by Professor Muller: — “ If we divide,” says the Pro- 
fessor, “ in the same frog all the three posterior roots of the in- 
ferior extremities on the left side, and all the three anterior roots 
of the nerves on the right side, sensation in the left leg, and 
motion in the right leg, are destroyed. If we then cut off the 
foot from the right leg, which retains sensation but not motion, 
the frog manifests, by motion in all parts of its body, the utmost 
pain ; but the right leg, which is the seat of pain, is immoveable. 
