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NERVE REFLEXES IN ANIMALS 259
performance of certain important functions intimately connected with the life and necessary to the well-being of the
individual. They are, more or less, rhythmic in character, and resemble in some respects, although not identical
with, the lower forms of mental acts performed by animals with rudimentary brains. They are supposed to be the
outcome of irritability and of external or internal stimulation, but in a healthy organism irritability and stimulation
in all their forms are contra-indicated. It is only in the diseased or abnormal animal that irritability and artificial
stimulation come into play. A reflex movement is defined by Professors Landois and Stiriing ^ as follows : "By the
term reflex movement is meant a movement caused by the stimulation of an afferent (sensory) nerve. The stimulus,
on being applied to an afferent nerve, sets up a state of excitement (nervous impulse) in that nerve, which state of
excitement is transmitted or conducted in a centripetal direction along the nerve to the centre (spinal cord in this
case), where the nerve cells represent the nerve centre ; in the centre, the impulse is transferred to the motor, efferent,
or centrifugal channel. Three factois therefore are essential for a reflex motor act—a centripetal or afferent fibre,
a transferring centre, a centrifugal or efferent fibre ; these together constitute a reflex ad. In a purely reflex act,
all voluntary activity is excluded." The so-called reflex function represents the natural or enforced working of an
important part of the nervous system, to which is delegated the co-ordination and supervision of the involuntary
movements, and all those operations which, for the wisest of purposes, are removed from the immediate ken and
supervision of the individual. It is important to draw a distinction between what I designate natural reflex acts
in a healthy body, and induced mechanical reflex acts in a diseased or unhealthy one. That the reflex nerves act
spontaneously and purposively seems certain from the following experiment. If a frog be decapitated and a patch
of moist mustard be placed, say on the inside of the right thigh, it draws up the left leg and scrapes the mustard
off with the left foot. If the mustard be then placed on the inside of the left thigh, it draws up the right leg and
scrapes it off with the right foot. This result is due to irritation and stimulation, but as the head of the frog has
been removed, it is an abnormal experiment. The experiment, however, is of considerable value, as it shows that
the spinal cord, and its gangUa and nerve cells, have an independent power of interpreting sensory impressions
and of sending out purposive motor impulses.
The ganglia and nerve cells of the spinal cord hold the same relation to the cord which similar cells in the
brain hold to that, the most august of all the organs. It is permissible to assign to the ganglia of the brain and
the cord a variety of fimctions, and even to designate them by special names.
The functions discharged by the brain and spinal cord are ultimately molecular in character, and the ganglia,
nerve cells, and nerve fibres, as units of these structures, have an independent and separate existence, and are entitled
to the most attentive examination and consideration.
Assuredly all parts of the nervous system can act spontaneously as apart from irritability and stimulation ;
in other words, the nervous system, like all the other systems of the body, is self-acting. It is this power of spontaneous
self-action which distingiiishes the living from the dead thing. It is the prerogative of life to use and
abuse extraneous dead matter, and to be very Uttle affected by environment. Environment, and the stimuli which it is
supposed to supply, neither originate hfe nor keep it going. Every hving organism has the mainsprings of Hfe in itself,
and these mainsprings embrace all the systems and organs, indeed all the parts of even the most complex beings.
The reflex acts are mainly concerned in the vegetative functions of the body— that is, those functions which
are in no way dependent on volitions or efforts of will. They are necessary to life, as they regulate, as explained,
the highly important functions of alimentation, respiration, circulation, urination, defsecation, parturition, &c. They
hold an intermediate position between the involuntary rhythmic movements on the one hand, and the voluntary
purposive movements on the other.
From one point of view, the involuntary reflex acts are quite as important as the voluntary purposive acts.
They depend principally on the spinal cord with its auxiliaries, the ganglia, and the sensory and motor nerves. As,
however, the brain is a mere expansion of the spinal cord, and the spinal cord has within it the potentiaUties of the
brain, both as regards substance and function, it follows, that while the reflex acts cannot, strictly speaking, be
regarded as voluntary and inteUectual, neither can they be regarded as wholly devoid of intention and purpose.
They are, in fact, the bhnd instruments of the Creator, and produce intelligent results. In compound animals, of
which man is the best representative, a great many different systems have to be co-ordinated, regulated, and kept
going. Thus there is the ahmentary system, the respiratory system, the circulatory system, the lymphatic system,
the involuntary muscular system, the glandular system, the reproductive system, &c. These have all to be har-
monised and the substances forming them to be duly fed and nourished. This intricate double duty of co-ordinating
and feeding is largely handed over to the reflex nerve centres in the spinal cord. It would be impossible personally
to superintend the extensive and complicated machinery of life. This, for the wisest of purposes, is placed in
higher hands.
1 " A Text-Book of Human Physiology," by Dr. L. Landois and William Stirling, M.A., Sc.U., vol. ii. ii. 904.