the location of other functions has been
secured. Much of the cerebral surface, however, is still undetermined
(Fig. 142).

NERVOUS CONTROL OF IMPORTANT PROCESSES

*Circulation of the Blood.*--1. _Control of the Heart._--The ability to
contract at regular intervals has been shown to reside in the heart
muscle. Among other proofs is that furnished by cold-blooded animals, like
the frog, whose heart remains active for quite a while after its removal
from the body. These automatic contractions, however, are not sufficient
to meet all the demands made upon the circulation. The needs of the
tissues for the constituents of the blood vary with their activity, and it
is therefore necessary to vary frequently the force and rapidity of the
heart's contractions. Such changes the heart itself is unable to bring
about.

For the purpose of controlling the rate and force of its contractions, the
heart is connected with the central nervous system by two kinds of fibers:

_a._ Fibers that convey _excitant_ impulses to the heart to quicken its
movements.

_b._ Fibers that convey _inhibitory_ impulses to the heart to retard its
movements.

The cell-bodies of the excitant fibers are found in the sympathetic
ganglia, but fibers from the bulb connect with and control them. The
cell-bodies of the inhibitory fibers are located in the bulb, from where
their fibers pass to the heart as a part of the vagus nerve.

In addition to the fibers above mentioned, are those that convey impulses
_from_ the heart to the bulb. These connect with neurons that in turn
connect with blood vessels and with them act reflexively, when the heart
is likely to be overstrained, to cause a dilation of the blood vessels.
This lessens the pressure which the heart must exert to empty itself of
blood. These fibers serve, in this way, as a kind of safety valve for the
heart.

2. _Control of Arteries._--Changes in the rate and force of the heart's
contractions can be made to correspond only to the _general_ needs of the
body. When the blood supply to a particular organ is to be increased or
diminished, this is accomplished through the muscular coat in the
arteries. The connection of the arterial muscle with the sympathetic
ganglia and the method by which they vary the flow of blood to different
organs has already been explained (pages 311 and 49), so that only the
location of the controlling neurons need be noted here. These, like the
controlling neurons of