CHAPTEE XVI 



NERVE-CENTRES 



THE nerve-centres consist of the brain and spinal cord; they are 

 characterised by containing nerve-cells, from which the nerve-fibres 

 of the nerves originate. Small collections of nerve-cells are found 

 also in portions of the peripheral nervous system, where they are 

 called ganglia. The spinal ganglia on the posterior roots of the 

 spinal nerves, and the sympathetic ganglia are instances of these. 



The general arrangement of the cerebro-spinal axis is given in 

 the accompanying diagram. The nerves which take origin from the 

 brain are called cranial nerves ; there are twelve pairs of these ; some 

 of them, such as the olfactory, optic, and auditory nerves, are nerves 

 of special sense ; others supply the region of the head with motor 

 and sensory fibres. One pair (the tenth), called the pneumogastric 

 or vagus nerves, are mainly distributed to the viscera of the thorax 

 and abdomen, and a part of another pair (the eleventh), called the 

 spinal accessory nerves, unites with the vagus prior to such distribu- 

 tion. We shall in our subsequent study of the heart, lungs, stomach, 

 and other organs have frequently to allude to these nerves. The 

 first two pairs of cranial nerves (the olfactory and the optic) arise 

 from the cerebrum. The remaining ten pairs are connected with the 

 district of grey matter called the floor of the fourth ventricle or its 

 immediate neighbourhood ; this tract of grey matter is situated at 

 the lower part of the brain where it joins the spinal cord; this 

 portion of the brain is called the Bulb or Medulla dblongata. 



The spinal nerves are arranged in pairs, 31 in number. Their 

 general structure and functions we have already studied (pp. 155-158). 



The more intimate structure of the brain and spinal cord we shall 

 consider at length in subsequent chapters. For the present we shall 

 deal with some of the general aspects of the nerve-centres, both as 

 regards structure and function. 



The brain and spinal cord consist of two kinds of tissue, easily 

 distinguishable by the naked eye. They are called respectively white 

 matter and grey matter. 



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