NERVE TISSUE 



51 



central elements of the nervous system; in them the impressions 

 which have come from the sensory cells become sensations which can 

 be transformed into will impulses; from them proceed the stimuli 

 which cause the muscle fibres to contract. 



The nervous system becomes complicated as low down as among 

 the Ccehntffrata, but more especially in animals of a higher grade. 

 Between the central parts, i.e. the ganglion cells on the one side, and 



Fig. 51.— Ganglion cell from tlie anterior oomu of tlie 

 human spinal cord (after Gegsnbauer). y. Pigment ; /i. 



Fia. 62.— Ganglion cell 

 from a human spinal 

 ganglion (after Gegeu- 

 Jjauer). n. Nuclei of the 

 neurilemma. 



the sensory and muscle cells on the other, nerve cells are interposed, 

 which, being stretched like fibres, become nerve fibres. These under- 

 take exclusively the function of the transmission of sense impressions 

 from the sensory cells to the ganglion cells, and the transmission of 

 stimuli from the ganglion cells to the muscle fibres. 



Besides this, the glanglion cells no longer appear scattered, 

 arranged in a plexus, but they unite into masses, which are defined 

 as the central organs of the nervous system, e.g.^the. brain. The 

 nerve fibres also unite into nerves. There are two sorts of nerve 



