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THE NERVOUS SYSTEM. : 263 :

and receive names to correspond with this fact. All the foramina are
not constant in their existence, and they vary in size. Named in the
order of their size and constancy, they are as follows
The Mastoid, which empties into the lateral sinus by passing through
the mastoid foramen in the temporal bone.
The Parietal, which empties into the longitudinal sinus by passing
through the parietal foramen in the parietal bone.
The Condylar, which runs through the cervical plexus, and empties
into the lateral sinus by passing through the posterior condyloid foramen
in the occipital bone.
The Occipital, which is (][uite inconstant, extends from the structures
near the external occii)ital ])rotuberance and empties into the torcular
Herophili by passing through a small foramen in this situation.
There are several other small emissary veins which pass through
different foramina, such as the ovale, middle lacerated, anterior con-
dyloid, and the carotid canal.

THE NERVOUS SYSTEM.
The Nervous System consists of all that portion of the body
engaged in the generation and transmission of nerve-force, through
whic-h sensation, volition, and vital influence are conveyed to or from
the brain. It is made up of several organs known as nerve-centres,
nerves, and jjerij^heral end-organs. These are arranged in two great
systems, the Cerebrospinal and the Sympathetic; the former is frequently
described as the nervous system of animal life, the latter of organic or
vegetative life.
The Nerve-centres are found within the gray matter of the cerebro-
spinal centres, the ganglia of the roots of the spinal, and some of the cra-
nial nerves, also in the various ganglia of the sympathetic system. They
are composed of gray matter, white fibrous structure, and intercellular
substance known as neuroglia. Within the gray matter are found
numerous nerve- or ganglion-cells (Figs. 116, 117, 118). These cells
are apolar, unipolar, bipolar, or multipolar in form ; some investigators
claim those of the apolar variety to be undeveloped nerve-cells which
eventually become polar.
The function of the nerve-cells is to generate nerve-force in a manner
analogous to that of a galvanic cell or battery in the generation of
electricity.
The Nerves are white fibrous cords of various sizes extending between
nerve-centres and between nerve-/3entres and peripheral end-organs.
They do not generate nerve-force, but act as conductors, similarly to the
wires of a galvanic cell or battery in the transmission of electricity.
The nerves of the cerebro-spinal system are divided into three classes
(1) those which conduct nerve-force from the nerve-centres outward to
the muscles, known as motor or centro-peripheral or centrifugal nerves ;