ON THE NERVOUS SYSTEM. 361 



CHAPTER IX. 



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 sympa- 

 thetic 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, or rather fasciculi, which in some situations 

 are ranged in a longitudinal direction, and in others are interlaced at va- 

 rious angles by cross fibres. The fasciculi are connected and held together 

 by a delicate areolar web, wnich forms the bond of support to the entire 

 organ. It is also observed that the cerebro-spinal axis presents two sub- 

 stances differing from each other in density and colour : a grey or cineri- 

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

 grey substance forms a thin lamella over the entire surface of the convolu- 

 tions of the cerebrum, and of 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, 

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



Two kinds of grey substance are described by Rolando as existing in 

 the spinal cord ; the one (substantia cinerea spongiosa vasculosa) is the ordi- 

 nary grey matter of the cord, and the other (substantia cinerea gelatinosa) 

 forms part of the posterior cornua. The former resembles in structure the 

 grey matter of the brain, while the latter is composed of small bodies re- 

 sembling the blood corpuscules of the frog. 



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

 verging and converging. The diverging fibres proceed from the medulla 

 oblongata, and diverge to every part of the surface of the brain ; while the 

 converging fibres commence at the surface and proceed inwards towards 

 the centre, so as to connect the diverging fibres of opposite sides. In cer- 

 tain parts of their course the diverging 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 and mutual relations of 

 these fibres and ganglia may be best explained by reference to the mode 

 of development of the cerebro-spinal axis in animals and in man. 



The centre of the nervous system, in the lowest animals possessed of a 



