Keith, _Human Embryology and Morphology_ (London, 1904); O. Hertwig,
_Handbuch der vergleichenden und experimentellen Entwickelungslehre
der Wirbeltiere_, Bd. 2, part 3 (Jena, 1902-1906); _Development of the
Human Body_, J.P. McMurrich (1906). (F. G. P.)

2. PHYSIOLOGY

The nervous system has as its function the co-ordinating of the
activities of the organs one with another. It puts the organs into such
mutual relation that the animal reacts as a whole with speed, accuracy
and self-advantage, in response to the environmental agencies which
stimulate it. For this office of the nervous system there are two
fundamental conditions. The system must be thrown into action by
agencies at work in the environment. Light, gravity, mechanical impacts,
and so on, which are conditions significant for animal existence, must
find the system responsive and through it evoke appropriate activity in
the animal organs. And in fact there have been evolved in the animal a
number of structures called receptive organs which are selectively
excitable by different environmental agencies. Connected with these
receptive organs lies that division of the nervous system which is
termed _afferent_ because it conducts impulses inwards towards the
nervous centres. This division consists of elongated nerve-cells, in man
some two million in number for each half of the body. These are living
threads of microscopic tenuity, each extending from a receptive organ to
a central nervous mass. These central nervous masses are in vertebrates
all fused into one, of which the part which lies in the head is
especially large and complex, because directly connected with
particularly important and delicate receptive organs. The part of the
central nervous organ which lies in the head has, in consequence of its
connexion with the most important receptive organs, evolved a dominant
importance in the nervous system, and this is especially true of the
higher animal forms. This head part of the central nervous organ is
sufficiently different from the rest, even to anatomical examination, to
have received a separate name, the _brain_. But the fact of its having
received a separate name ought not to obscure the singleness and
solidarity of the whole central nervous organ as one entity. The
functions of the whole central nervous organ from region to region are
essentially similar throughout. One of its essential functions is
reception, via afferent nerves, of nervo