ite
velocity, so in a nerve filament the influence advances progressively at
a rate said to be dependent on the temperature of the animal examined.
It seems in the cold-blooded to be much slower than in the hot. It has
been estimated in the frog at eighty-five feet per second; in man at two
hundred feet--an estimate probably too low.

The fibres thus described are of the kind designated by physiologists as
the cerebro-spinal; there are others, passing under the name of the
sympathetic, characterized by not possessing the investing medullary
substance. In colour they are yellowish-gray; but it is not necessary
here to consider them further.

[Sidenote: Structure of a nerve vesicle.] 2nd. The other portion of the
nervous structure is the vesicular. As its name imports, it consists of
vesicles filled with a gray granular material. Each vesicle has a
thickened spot or nucleus upon it, and appears to be connected with one
or more fibres. If the connexion is only with one, the vesicle is called
unipolar; if with two, bipolar; if with many, multipolar or stellate.
Every vesicle is abundantly supplied with blood.

[Sidenote: Function of a nerve vesicle.] As might be inferred from its
structure, the vesicle differs altogether from the fibre in function. I
may refer to my "Physiology" for the reasons which have led to the
inference that these are contrivances for the purposes of permitting
influences that have been translated along or confined within the fibre
to escape and diffuse themselves in the gray granular material. They
also permit influences that are coming through many different channels
into a multipolar vesicle to communicate or mix with one another, and
combine to produce new results. Moreover, in them influences may be long
preserved, and thus they become magazines of force. Combined together,
they constitute ganglia or nerve centres, on which, if impressions be
made, they do not necessarily forthwith die out, but may remain
gradually declining away for a long time. Thus is introduced into the
nervous mechanism the element of time, and this important function of
the nerve vesicle lies at the basis of memory.

It has been said that the vesicular portion of the nerve mechanism is
copiously supplied with blood. Indeed, the condition indispensably
necessary for its functional activity is waste by oxydation. Arterial
vessels are abundantly furnished to insure the necessary supply of
aerated blood, and veins to carry