BRAIN AND NERVE 



91 



spinal nerves go mainly to the muscles of the trunk and limbs, 

 or come from sense organs in the muscles, joints, tendons, and 

 skin. The nerves which go out from the sympathetic ganglia 

 are connected mainly with the bloodvessels and viscera. 



Examining these structures with the aid of the microscope, we 

 find two main kinds of tissue. The central part of the spinal 

 cord, the superficial part of the cerebrum and cerebellum, much 

 of the deeper part of the brain and the sympathetic ganglia are 

 all called " grey matter," and here the substance of the central 

 nervous system is made of up nerve cells with their dendrites. 

 The superficial parts of the spinal cord, the deeper parts of the 

 cerebrum and cerebellum, and much of the other parts of the 

 brain are composed of " white matter," and this consists of 

 nerve fibres possessing a medullary sheath. The nerves issuing 



Dorsa.1 



J)o re, a. I 



~i j ^~^i yi.li "; ^visax- /lerj/e 



Trunk (Ta.ng/jcL 



B 



FIG. 22. THE NERVOUS SYSTEM OF THE EARTHWORM. 

 I A y The head parts; B, a section through the body. 



from the brain and spinal cord are also composed of white matter, 

 but those coming from the sympathetic ganglia are rather 

 different in appearance, since they do not possess medullary 

 sheaths. Two kinds of tissue nerve cells and nerve fibres 

 therefore make up the nervous system, central and peripheral. 



The Arrangement of the Ganglia. Consider first such a very 

 simple nervous system as that of an earthworm. This animal 

 possesses a segmented or jointed body, and every segment is, on 

 the whole, similar to the one in front or behind it. In each 

 segment there is a pair of nerve ganglia connected together 

 across the middle line of the body by a commissure. The ganglia 

 are also connected together, each with the one in front and that 

 behind. In the first or " head " segment of the worm, the 

 ganglia are bigger than those in the body segments, and they are 

 arranged to form a nerve collar round the oesophagus. These 



