Picture - Cosmic Polymath

DEVELOPMENT OF BLOOD, &c., IN MAN AND MAMMALS 413
vesicle (anterior part of hind-brain), the cavity of which becomes the anterior (superior) part of the fourth
ventricle ; and finally, the medulla oblongata is developed as a thickening of the wall of the fifth vesicle, the
cavity of which expands from the central canal of the spinal cord to form the calamus scriptorius of the fourth
ventricle.
" While these changes are going on in its walls the embryonic brain does not remain straight as at fh-st, with
its axis in a line with that of the spinal cord, but undergoes certain flexures, the general result of which is to bend
the anterior end towards the ventral surface."
As the nervous system is, in a sense, inoperative in the embryo and foetus it is not necessary to pursue the
subject further here. It is, however, fully treated further on, both in its simple and complex condition, as it occurs
in the lower animals and in man. It is only necessary to state that the nerve-cells, ganglia, spinal cord, brain,
sensory, motor and sympathetic nerves which constitute it, and which furnish an unparalleled display of the
highest forms of organic matter, are all prepared in advance of the complicated functions to be discharged
by it. The nervous system, more than any other system in the body, bears eloquent testimony to design and a
Designer. In no other system is there such extraordinary elaboration ; such a magnificent array of well-chosen
means to definite and obvious ends. Nor is the labour expended in any sense squandered or wasted, when it is
remembered that on the quantity, quality, and integrity of the nervous system all mental and conscious
manifestations depend.
Fig. 80.—Nervous systems of the centipede and o£ man.
A. Centipede {Scolopendra). The nervous system of the centipede consists of a
Hnear series of nearly equal and similar ganglia arranged in pairs, situated upon the
median line, along the ventral surface of the alimentary canal. Each pair of ganglia is
connected longitudinallj' and transversely by commissural nerve fibres, and distributes
sensory and motor nerves to its own segment or articulation. In the first articulation
which forms the head (a), the ganglia are larger than elsewhere, and send nerves to the
antennae and to the organs of special sense. This pair is termed the cerebral ganglion,
or the " brain." c, c'. Longitudinal commisural nerve fibres, which with their ganglia
form what is virtually a spinal column (compare with c, c' of Fig. B).
B. Cerebro-spinal system of man (Homo sa^nens). (i, Cerebrum ; h, cerebellum
c, c', spinal cord giving off sensory and motor nerves (compare with c, c' of Fig. A )
d, brachial nerves ; e, sacral nerves (after Dalton).
The nervous systems of the centipede and of man bear a striking general resemblance
to each other ; allowance being made for the greater development and complexity of
the latter. The cerebro-spinal nervous system in man is seen on an enlarged scale at
Plate cxxxii. Fig. 1.
^
FiQ. 80.
The evidence of design so conspicuous in the construction of the heart and brain is equally apparent in the
development and construction of the lungs. The heart, the brain, and the lungs may be said to form the tripod of
life in the higher animals and in man.
The brain, as already explained, is an expansion and modification of the spinal cord, and hterally consists of
millions of nerve cells connected directly and indirectly with nerve fibres. It is simple in its inception, and becomes
complex in the adult and as the animal rises in the scale of being. Here again there is community of structure
and proof of type as between the lower and higher animals. In the centipede, one of the articulata, the nervous
system consists of a brain and what is virtually a spinal cord, with sensory and motor nerves given ofi at regular
intervals, as shown at A of Fig. 80, above. The body of the centipede is segmented, that is, divided transversely
the sensory and motor nerves being distributed to the segments. The nervous system is divided longitudinally,
the portions representing the quasi-cord and brain being composed of two equal halves. The quasi-cord is com-
posed of a double chain of gangha united by nerve commissures longitudinally and transversely ;
the cephaUc gangUa
being enlarged and approximated to form a rudimentary brain. Here there is longitudinal and transverse division
and a repetition of parts indicating general plan and type. In the human brain and spinal cord a precisely similar
arrangement of parts obtains, as shown at B of Fig. 80, above.
Thus the human spinal cord and brain are divided longitudinally into two symmetrical halves, and the ganglia
are arranged in pairs, or in duphcate both in the cord and brain. The sensory and motor nerves, moreover, are