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CHAPTER VIII. 



ON THE NERVOUS SYSTEM. 



THE nervous system consists of a central organ, the cerebro-spinal 

 centre or axis, and of numerous rounded and flattened white cords, 

 the nerves, which are connected by one extremity with the cerebro- 

 spinal centre, and by the other are distributed to all the textures of 

 the body. The sympathetic system is an exception to this description ; 

 for in place of one it has many small centres which are called ganglia, 

 and which communicate very freely with the cerebro-spinal axis and 

 with its nerves. 



The cerebro-spinal axis consists of two portions, the brain, an organ 

 of large size, situated within the skull, and the spinal cord, a length- 

 ened portion of the nervous centre continuous with the brain, and 

 occupying the canal of the vertebral column. 



The most superficial examination of the brain and spinal cord shews 

 them to be composed of fibres, which in some situations are ranged side 

 by side or collected into bundles or fasciculi, and in other situations 

 are interlaced at various angles by cross fibres. The fibres are con- 

 nected and held together by a delicate areolar web, which forms the 

 bond of support to the entire organ. It is also observed that the 

 cerebro-spinal axis presents two substances differing from each other 

 in density and colour ; a grey or cineritious or cortical substance, and 

 a white or medullary substance. The grey substance forms a thin 

 lamella over the entire surface of the convolutions of the cerebrum, and 

 the laminae of the cerebellum : hence it has been named cortical ; but 

 the grey substance is not confined to the surface of the brain, as this 

 term would imply ; it is likewise situated in the centre of the spinal 

 cord its entire length, and may be thence traced through the medulla 

 oblongata, crura cerebri, thalami optici, and corpora striata ; it enters 

 also into the composition of the locus perforatus, tuber cinereum, com- 

 missura mollis, pineal gland, pituitary gland, and corpora rhomboidea. 



The fibres of the cerebro-spinal axis are arranged into two classes, 

 diverging and converging. The diverging fibres proceed from the 

 medulla oblongata, and diverge to every part of the surface of the 

 brain ; while the converging commence upon the surface, and proceed 

 inwards towards the centre so as to connect the diverging fibres of op 

 posite sides. In certain parts of their course the diverging 1 fibres are 

 separated by the grey substance, and increase in number so as to form 

 a body of considerable size, which is called a ganglion. The position 



