Picture - Cosmic Polymath

134 DESIGN IN NATURE
PLATE LVII {continued)
groove in which the posterior sensory nerve (6) disappears ; (5), anterior motor nerve sinking into antero-lateral depression of cord.
These fibres display a ganglionic swelling (6') in their course ; (7), the united compound or mixed nerve ; (7'), the posterior primary
branch of same. This nerve is derived partly from the anterior and partly from the posterior roots of the spinal nerves (after Allen
Thomson).
Fig. .5.— Illustrates the comparative anatomy of the brain and spinal cord iu the fish, reptile, and mammal. The brain and
spinal cord are divided longitudinally into two portions, and are bilaterally symmetrical.
A. The brain and spinal cord of cod-fish, a, Olfactory lobes, two in number ; h, cerebral lobes or hemispheres, also called
cerebrum ; c, middle brain, giving rise to the optic nerves ; d, cerebellum ; e, spinal cord, with expansion of cord (medulla oblongata).
B. Brain of hammer-headed shark. The lettering is the same as in A (after Dallas).
C. Brain of alligator, a, Olfactory ganglia; h, cerebral ganglia or hemispheres ; c, optic tubercles ; d, cerebellum ; e, expansion of
the spinal cord into the medulla oblongata.
D. Brain of rabbit, a, Olfactory bulbs or ganglia; h, cerebral ganglia or hemispheres, separated to show the corpora striata (t),
optic thalami {d), and tubercula quadragemina, situated behind the optic thalami (d) ; /, cerebellum ; g, spinal cord expanding into the
medulla oblongata (after Dfilton).
Fig. 6.~Vertical mesial line section of the human brain and spinal column (semi-diagraiiiniatic), showing the situation of the
several great ganglia at the base of the brain and the course of the conducting nerve fibres.
A. Olfactory ganglion ; B, cerebral lobes or hemispheres (cerebrum) ; C, corpus striatum ; D, optic thalamus ; E, tubercula
quadragemina ; F, cerebellum ; G, ganglion of the tuber annulare ; H, ganglion of the medulla oblongata.
In considering A, B, C, and D of Fig. .5 it will be seen that the great ganglia forming the brains of the fish, reptile, and mammal
are arranged in the same plane as the spinal cord, of which they are mere expansions. In Fig. 6 the great ganglia (the cerebral
hemispheres excepted) are bent slightly forwards. These ganglia are situated on a different plane to the spinal cord, of which, as in
the lower animals, they are mere expansions. The peculiarity of the human brain is the comparatively very great size of the cerebral
lobes (cerebrum) which have grown upwards, forwards, and backwards, so as to cover in and conceal the ganglia at the base of the
brain. The brains in Fig. 5 are seen from above. Fig. 6 gives a vertical mesial line section of the human brain (after Dalton).
In the aplysia, one of the molluscs, a rudimentary brain makes its appearance. In this case there is a
double row of ganglia, with their sensory and motor nerves in the cephahc portion of the animal ; the two ganglia
which form the brain being more or less completely fused. The nervous system is symmetrical in the upper part
of the body, but unsymmetrical in the lower part, where the respiratory nerve centre occurs (Plate Ivii., Fig. 1, B).
In the centipede the nervous system has made a considerable stride. It consists of two longitudinal commissural
tracts of nerve matter with a double series of ganglia, each segment of the animal being provided with two ganglia
and with sensory and motor nerves extending between the skin and ganglia on the one hand, and between the
ganglia and muscles on the other. The ganglia and nerves are united longitudinally and transversely, so that all
parts of the body are capable of receiving sensory impressions, and are under control.
The two cephalic or highest ganglia are larger than the others. They are also united to each other, and more
or less fused. A fairly well-developed brain can now be detected. The centipede is quick to perceive and flee from
danger. Its power of volimtary independent movement is very remarkable. That the centipede feels by its
antennae and other parts cannot be doubted, and that it is aware of matter dead and living outside of itself is
equally certain. If, however, the centipede feels and is capable of controlhng and directing its own movements,
and is aware of matter, however Kmited, outside of itself, then the question of self and conscious self, in however
rudimentary a form, is raised : the question of a low form of cognition and a rudimentary power of reasoning is
also raised. This subject will be discussed further on. It is enough to state here that in the nervous system of the
centipede the Unes of communication for receiving sensory impressions and sending out motor impulses are well
marked, there being distinct sensory and motor nerves, and a rudimentary brain (Plate Ivii.).
The nervous system of the centipede is the harbinger of similar systems in the vertebrata up to man. In man
the brain is an expansion of the upper part of the spinal cord. The spinal cord itself is composed of two bilateral
halves, with longitudinal and transverse nerve commissures and symmetrical groups of ganglia on either side associated
with sensory and motor nerves. The brain, hke the cord, is bilaterally symmetrical. It too is connected by longi-
tudinal and transverse commissural and sensory and motor nerves, with their concomitant groups of ganglia and nerve
centres. The brain and spinal cord vnth their groups of gangha and sensory and motor nerves form an elaborate
and complicated whole : the lines of communication and of force are all definitely laid down, and may be readilv
traced by the trained anatomist and mioroscopist. The nervous system in the higher animals, and in man verv
closely resembles the telegraphic system of a great empire, where the brain corresponds to the capital and re-
presents the central telegraphic station ; the ganglia in the spinal cord corresponding to the towns and villaees
with their lesser and intermediate stations, and the sensory and motor nerves to the several telegraphic wes alonff
which messages are sent.
The sensory nerves transmit impressions or messages from without, as from the sldn and sense organs •
motor nerves transmit messages from within, as from the brain to the muscles.
the