ties is unknown. Some
portions, however, are of less importance than others. Large portions
of the cortical, or outer part, are frequently removed without
affecting the functions of this organ. Pieces of the medullary, or
central parts, have been removed by injuries without impairing the
intellect or destroying life. This organ, although it takes cognizance
of every sensation, is, of itself, but slightly sensible. It may be
cut, or parts may be removed without pain, and the individual, at the
same times retain his consciousness.

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771. What is said of the relation of the brain to all of the organs of
the body? 772. Are the involuntary functions of different parts of the
system influenced by the brain? 773. Where is sensation perceived? By
what agency are the impressions of external objects conveyed to the
brain? What is the difference between sensations and impressions? 774.
Is it known what part of the brain has the most intimate relation with
the intellectual faculties?

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775. The brain is the seat of the _will_. It superintends the physical
as well as the mental movements, and the medium of communication from
this organ to the muscles, or the parts to be moved, is the motor
nerves. If the brain is in a quiescent state, the muscles are at rest;
if, by an act of the will, the brain sends a portion of nervous
influence to a voluntary muscle, it immediately contracts, and those
parts to which the muscle is attached move. There is no perceptible
interval between the act of the will and the motion of the part.

776. Some physiologists assert, that the medulla oblongata is the
point at which excitement to motion commences, and sensation
terminates; and also, that it possesses the power of originating
motion in itself.

_Observation._ The medulla oblongata, unlike the brain, is highly
sensitive; if slightly punctured, convulsions follow; if much injured,
respiration, or breathing, immediately ceases.

777. It is remarkable that the nerves which arise from the right side
of the spinal cord communicate with the left hemisphere of the
cerebrum, and _vice versa_; this results from the crossing of the
fibres in the medulla oblongata. It follows from this, that if the
right side of the brain receives an injury, the parts of the opposite
side of the body lose their sensibility and motion.

_Observations._ 1st. If the cranial nerves which are connected by a
single root