CHAPTER VIII. 



ON THE NERVOUS SYSTEM. 



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

 spinal centre of 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 

 lengthened 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 

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

 ranged side by sid# or collected into bundles or fasciculi, and in 

 other situation&rjtjx) interlaced at various angles by cross fibres. 

 The fibres are connected and held together by a delicate cellular 

 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 gray or cineri- 

 tious or cortical substance, and a white or medullary substance. 

 The gray substance forms a thin lamella over the entire surface of 

 the convolutions of the cerebrum, and the laminae of the cerebellum : 

 llifice .;jit has been named cortical; but the gray substance is not 

 oonfijtfed^k) 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 bd|henceP traced through the medulla oblongata, crura 

 cerebri, thalamj optici, and corpora striata ; it enters also into the 

 composition of the-lobus perforatus, tuber cinereum, commissura 

 mollis, pineal gland, and corpus rhomboideum. 



The fibres of me 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 pro- 

 ceed inwards towards the centre so as to connect the diverging 

 fibres of opposite sides. In certain parts of their course the 

 diverging fibres are separated by the gray substance, and increase 



