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 



