elow.

The progress of Physiology has been so great in all its branches that it
is difficult to decide which most deserve mention; perhaps the most
important advances are those connected with the nervous system and with
internal secretions. Little or nothing was known fifty years ago of the
minute structure of the nervous system, nor of the special functions of
its different parts. Now the main functions of the various parts of the
brain, and the relation of these parts to the activities of the other
organs of the body, are well known, although much remains to be
discovered with regard to the more detailed localization of function.
The study of the microscopic structure of brain and nerve, and
experiment on the conduction of nervous impulse, have given us some
insight into the mechanism of the nervous system, but the fundamental
nature of nervous action still remains unsolved.

The nervous system is the chief co-ordinating link between the various
organs of the body, but in recent years it has been discovered that the
relations of the different parts to one another are greatly influenced
by substances known as internal secretions or 'hormones'. These
substances are produced by ductless glands (the thyroid, suprarenals,
&c.), from which they diffuse into the blood-stream and exercise a
remarkable influence either on particular organs or systems, or on the
body as a whole. Some of these secretions act specifically on the
involuntary muscles of the body, others control growth, others the
development of the secondary sexual characters, such as the distinctive
plumage of male birds, and also greatly influence the sexual instinct.
Much still remains to be discovered with regard to them, but it seems
clear that they are of immense importance in the economy of the body. It
has been suggested, without much experimental support, however, that if
a part of the body becomes modified by use or environment, it may
produce a modified hormone, and that so, by the action of this on the
germ-cells, the modification may be transmitted to subsequent
generations.

Before leaving the subject of physiology in the more special or
technical application of the term, reference must be made to another
science the growth of which has been largely under the influence of
medicine. This is bacteriology, one of the newest branches of biology,
and yet one which both from its practical importance and from the
theoretical interest of its discoveries is