180 



NERVOUS SYRTEM, 



narrow and extended form in the more lengthened trunks of 

 the articulated animals, and the vertebrata. The great cen- 

 tral portion of the nervous system is perforated by the ali- 

 mentary canal in the invertebrated classes, either in its mid- 

 dle, as in the radiata and the mollusca, or at its anterior ex- 

 tremity, as in the articulata ; but in the vertebrata the spi- 

 no-cerebral axis lies wholly above the digestive cavity, by 

 which it is nowhere pierced. The nerves of sensation and 

 motion closely accompany each other, forming, by their 

 union, chords or columns, or a spino-cerebral axis ; but the 

 sympathetic nerves, appropriated to the more slow and re- 

 gular movements of organic life, form a more isolated sys- 

 tem, and these three systems are developed together, almost 

 from the lowest animals. The nervous system consists 

 of very fine tubular filaments, generally containing white- 

 coloured opaque particles, much smaller than the globules 

 of the blood ; these minute fibres do not ramify like blood- 

 vessels, but continue uninterrupted from their peripheral ex- 

 tremity in the textures of the organs, to their proximal end 

 in the cineritious substance of ganglia, or of the brain. The 

 cineritious, or cortical matter is composed of a vascular 

 plexus, in the meshes of which is an irregular granular 

 pulp, and the fibrous arrangement becomes more obvious 

 and regular at its junction with the white medullary portion. 

 In the white portion of the brain, and in the nerves of the prin- 

 cipal senses the ultimate component tubular filaments have 

 a knotted or beaded appearance, from their numerous small 

 dilations, and they appear to be empty, or to contain only a 

 transparent homogenous fluid, as seen in the annexed figure 

 of Ehrenberg (Fig. 80,) where (a) is a magnified view of the 



