substance. Within the
brain, each artery being more or less terminal, there is no free
anastomosis between adjacent vessels, with the result that if any
individual artery is obstructed the vitality of the area supplied by
it is seriously impaired. The venous arrangements are also peculiar in
that the veins are thin-walled and valveless, and open into the rigid,
incompressible sinuses which run between the layers of the dura mater.
Most of the blood passes to the internal jugular vein, and any
increase in the pressure of this vessel is immediately transmitted
back to the cerebral veins. As the blood vessels project into a rigid
case filled with incompressible material, and as the total _volume_ of
blood in the brain is constant (Munro and Kelly), any alteration in
the supply of blood to the cerebral tissue must be due to an increased
_velocity_ of flow, and this in turn depends upon changes in the
aortic and vena cava pressure. Thus, if the aortic pressure rises,
more blood will enter the cerebral vessels and will move along more
rapidly; while if the pressure in the vena cava rises there is
obstruction to the passage of blood in the arteries and diminished
velocity of flow. The ebb and flow of cerebro-spinal fluid in and out
of the spinal canal may also help to control the pressure.

#Nerve Elements.#--The nervous system is composed of a multitude of
units, called _neurones_, each neurone consisting of a nucleated cell,
with branching protoplasmic processes or _dendrites_ and one
_axis-cylinder_ or _axon_. The nutrition of an axis cylinder depends
on its continuity with a living cell. If the cell dies, the axis
cylinder degenerates. If the axis cylinder is severed at any point, it
degenerates beyond that point, and the nucleus of the nerve-cell
disintegrates--chromatolysis.

The axis cylinder of one cell ends in a number of fine filaments which
arborise around another nerve-cell, thus bringing it into
physiological, if not anatomical, relationship with the first cell.
The termination is called a cell-station or _synapsis_. In this way
the various sections of the nervous system are kept in association
with one another and with the rest of the body.

_Motor Functions and Mechanism._--The nerve centres, which together
make up the motor area, and govern the voluntary muscular movements of
the body, are situated in the grey matter of the praecentral or
ascending frontal gyrus, and of the frontal aspect of the central
sul