NERVOUS SYSTEM. 181 



knotted fibres composing the white matter of the hu- 

 man brain, (b) represents the coarser beaded fibres composing 

 the human auditory nerve, and (c) exhibits those of the hu- 

 man optic nerve. This structure is observed on pressing 

 fine sections of these parts between plates of mica or of 

 glass, and viewing them through the microscope. This 

 knotted structure of the minute filaments is seen also in the 

 olfactory nerve, and in the sympathetic. The ordinary sym- 

 metrical nerves of sensation and of motion, throughout 

 the body, are composed of tubular filaments of more equal 

 calibre throughout their course, and which are filled with 

 minute white globular particles, as seen in those of the hu- 

 man facial nerve (Fig. 80. d } ) where the globular minute par- 

 ticles filling the neurilematous tubes are seen escaping from 

 the cut ends of the filaments. This knotted appearance of 

 the cerebral and sensitive filaments has been considered by 

 some as resulting from the external aggregation of clusters 

 of minute particles along the surface of chains of similar 

 globules composing the ultimate nervous fibrils. These ex- 

 tremely minute fibrils, variously aggregated together by en- 

 veloping sheaths of condensed cellular tissue, but without 

 anywhere anastomosing, constitute the nerves of sensation 

 and of motion throughout the animal kingdom, and the 

 spino-cerebral axis of the vertebrata. The nervous system is 

 developed, like other organs of the body, from the periphery 

 to the centre, and it presents great uniformity of plan in its 

 adult conditions in the inferior classes, and in its transient 

 embryo-forms throughout all the higher orders of animals. 



SECOND SECTION. 



Nervous System of the Cyclo-Neurose, or Radiated Classes. 



Many of the poly gastric animalcules, as the cercarite, are 

 distinctly sensitive to light, and organs of vision, in form of 

 minute red points, are seen in almost every genus. They 

 appear also to possess an acute sense of taste, they distin- 

 guish, pursue, and seize their prey, they avoid impinging on 

 each other while swimming, crowded in myriads, in a drop of 



