observed, the hand of Nature has prepared for us these very experiments without that drawback. The animal series, as we advance upward from its lowest members, proves to us what is the effect of the addition of new parts in succession to a nervous system, as also does any individual thereof in its successive periods of development. It is one of the most important discoveries of modern physiology that, as respects their nervous system, we can safely transfer our reasonings and conclusions from the case of the lowest to that of the highest animal tribes.

First introduction of governing ganglia.

The articulata present structures and a mode of action illustrating in a striking manner the nervous system of man. Lengthwise upon their ventral region is laid a double cord, with ganglia, like a string of beads; sometimes the cords are a little distance apart, but more generally they are coalesced, each pair of ganglia being fused into one. To every segment of the body a pair is supplied, each pair controlling its own segment, and acting toward it automatically, each also acting like any of the others. But in the region of the head there is a special pair, the cephalic ganglia, receiving fibres from the eyes and other organs of sense. From them proceed filaments to the ventral cord, establishing communications with every segment. So every part has two connexions, one with its own ventral ganglia, and one with the cephalic.

But thus far actions are

It is not difficult to determine experimentally the functions of the ventral ganglia and those of the cephalic. If a centipede be decapitated, its body is still capable of moving, the motion being evidently of a reflex kind, originating in the pressure of the legs against the surface on which they rest. The ventral cord, with its ganglia, is hence purely an automatic mechanism. only instinc- But if, in making the decapitation, we leave a portion of the body in connexion with the head, we recognize very plainly that the cephalic ganglia are exercising a governing power. In the part from which they have been cut off the movement is forward, regardless of any obstacle; in that to which they are attached there are modifications in the motions, depending on sight or other special senses; obstacles are avoided, and a variety

tive.

of directions pursued. Yet still the actions are not intelligent, only instinctive. The general conclusion therefore is, that the cephalic ganglia are of a higher order than the ventral, the latter being simply mechanical, the former instinctive; but thus far there is no trace of intelligence.

In man these typical parts are all present, and discharge the functions specified. His spinal cord answers Nervous anato the ventral cord of the articulates. It has its tomy of vertelateral communications in the same way, and brates, as man, each segmental portion presents the same reflex action. Toward its upper part it dilates to form the medulla oblongata, sending forth nerves for respiration and deglutition. Of these the action is still reflex, as is proved by the involuntary movements of respiration and deglutition. A portion of food being placed in the pharynx, Their autocontraction instantly occurs, the will having matic appano kind of control over the act of swallowing. Above or in front of this enlargement is a series of ganglia, to which converge the nerves of special sense-of hearing, sight, smell; these are, therefore, the equivalents Their instinc of the cephalic ganglia of insects, their function tive apparabeing also the same. In the lowest vertebrates, as in the amphioxus, the nervous system consists of nothing more. It may therefore be said to have only two parts-the cord and the sensory ganglia, and to have two functions-the automatic, attributable to the former, and the instinctive, attributable to the latter.

ratus.

tus.

But as we advance from the low vertebrates upward in the animal scale, we begin to detect new organs; on the medulla oblongata a cerebellum, and on the Their intelsensory ganglia a cerebrum. From this moment lectual appathe animal displays reasoning powers, its intel- ratus. ligence becoming more strikingly marked as the development of the new organs is greater.

It remains to determine with exactness the function of one of these new parts, the cerebrum; the other Functions of portion, the cerebellum, being of minor interest, the brain. and connected, probably, with the locomotive apparatus. For the same reason it is unnecessary to speak of the sympathetic nerve, since it belongs to the apparatus of

tive and auto

tions.

lobes.

organic life. Confining our attention, therefore, to the
true brain, or cerebrum, we soon recognize that the intel-
ligence of an animal is, in a general manner, proportional
to the relative size of this organ as compared with the
sensory ganglia. We are also struck with the fact that
the cerebrum does not send forth to other portions any
independent fibres of its own, nor does it receive any from
them, its only means of communication being through the
parts that have been described-that is to say, through
Its relations the sensory and automatic apparatus. The
to the instin- cerebrum is therefore a mechanism of a higher
matic por- order, and its relationship with the thalami
optici and corpora striata indicate the conditions
of its functions. It can only receive impressions which
have come through them, and only act upon the body
through their intermedium. Moreover, as we ascend the
animal scale, we find that these cerebral parts not only
increase in size, but likewise, in their turn, give
Its secondary rise to offshoots; secondary lobes emerging pos-
teriorly on the primary ones, and, in due season,
tertiary lobes posteriorly on the secondary. To these, in
human anatomy, the designations of anterior, middle, and
posterior lobes have been respectively given. In propor-
tion as this development has proceeded, the intellectual
qualities have become more varied and more profound.
The relation of the cerebrum to the cranio-spinal axis is
manifested by the circumstance that the latter
can act without the former. In sleep the
alone. cerebrum is, as it were, torpid, but respiration,
deglutition, and other reflex actions go on. If we touch
the palm of a sleeping infant our finger is instantly
Conjoint ac- grasped. But, though the axis can work with-
out the cerebrum, the cerebrum can not work
without the axis. Illustrations of these truths
may be experimentally obtained. An animal from which
the cerebrum has been purposely removed may be observed
to perform actions automatic and instinctive, but never
intelligent; and that there is no difference between
animals and man in this respect is demonstrated by the
numerous instances recorded in the works of medicine and
surgery of injuries by accident or disease to the human

Action of the spinal cord

tion of the

nervous system, the effects corresponding to those artificially produced in experiments on animals. This important observation, moreover, shows that we may with correctness use the observations made on animals in our investigations of the human system.

Three distinct

tem of man.

the instinc

In the nervous system of man our attention is therefore especially demanded by three essentially distinct parts-the spinal cord, the sensory ganglia, and parts of the the cerebrum. Of the first, the spinal cord, the nervous sysaction is automatic; by its aid we can walk, from place to place, without bestowing a thought on our movements; by it we swallow involuntarily; by it we respire unconsciously. The second portion, the They are the sensory ganglia, is, as we have seen, the counter- automatic, part of the cephalic ganglia of invertebrates; it tive, the is the place of reception of sensuous impressions intellectual. and the seat of consciousness. To these ganglia instinct is to be referred. Their function is not at all impaired by the cerebrum superposed upon them. The third portion, the cerebrum, is anatomically distinct. It is the seat of ideas. It does not directly give rise to motions, being obliged to employ for that purpose its intermediate automatic associated apparatus. In this realm of ideas thoughts spring forth suggestively from one another in a perpetual train or flux, and yet the highest Dominating branch of the nervous mechanism still retains control of the traces of the modes of operation of the parts from latter. which it was developed. Its action is still often reflex. Reason is not always able to control our emotions, as when we laugh or weep in spite of ourselves, under the impression of some external incident. Nay, more; the inciting cause may be, as we very well know, nothing material-nothing but a recollection, an idea-and yet it is enough. But these phenomena are perhaps restricted to the first or anterior lobes of the brain, and, accordingly, we remark them most distinctly in children and in animals. As the second and third lobes begin to exercise their power, such effects are brought under control.

There is, therefore, a regular progression, a definite improvement in the nervous system of the animal series, the plan never varying, but being persistently carried

out, and thus offering a powerful argument for relationship Progressive among all those successively improving forms,

nervous development in the animal

an observation which becomes of the utmost interest to us in its application to the verteseries. brates. In the amphioxus, as has been said, the cranio-spinal axis alone exists; the Cyclostome fishes are but a step higher. In fishes the true cerebrum appears at first in an insignificant manner, a condition repeated in the early embryonic state both of birds and mammals. An improvement is made in reptiles, whose cerebral hemispheres are larger than their optic lobes. As we advance to birds, a further increase occurs; the hemispheres are now of nearly sufficient dimensions to cover over those ganglia. In the lower mammals there is another step, yet not a very great one. But from the anterior lobes, which thus far have constituted the entire brain, there are next to be developed the middle lobes. In the Rodents the progress is still continued, and in the Ruminants and Pachyderms the convolutions have become well marked. In the higher carnivora and maximum in quadrumana the posterior or tertiary lobes appear. The passage from the anthropoid apes to man brings us to the utmost development thus far attained by the nervous system. The cerebrum has reached its maximum organization by a continued and unbroken process of development.

It attains its

man.

opment occurs

This orderly development of the nervous system in the The same pro-animal series is recognized again in the gradual gressive devel- development of the individual man. The priin each indi- mitive trace, as it faintly appears in the germinal vidual man. membrane, marks out the place presently to be occupied by the cranio-spinal axis, and, that point of development gained, man answers to the amphioxus. Not until the twelfth week of embryonic life does he reach the state permanently presented by birds; at this time the anterior lobes are only perceptible. In four or six weeks more the middle lobes are evolved posteriorly on the anterior, and, finally, in a similar manner, the tertiary or posterior ones are formed. And thus it appears that, compared with the nervous system of other animals, that of man proceeds through the same predetermined succes