50 



THE ELEMENTS OF STRUCTURE 



But in Hydra there are superficial sensory cells, whose basal pro- 

 l(jngations are connected either directly with contractile cells, or with 

 deeper ganglion-cells, some of which give off motor processes to the 

 contractile cells. 



In sea-anemones and some other Coelentera there is a more sharply 



defined division of labour. Super- 

 ficial sensory cells are connected 

 with subjacent nerve- or ganglion- 

 cells, from which fibres pass to the 

 contractile elements. 



In higher animals the sensory 

 cells are mostly integrated into 

 sense organs, the ganglionic cells 

 into ganglia, while the delicate 

 fibres which form the connections 

 between sensory cells and gang- 

 lionic cells, and between the latter 

 and muscles, are compacted to 

 form well-developed nerves. 



So far as we know, nervous tissue 

 always arises from the outer or 

 ectodermic layer of the embryo, as 

 we would expect from the fact that 

 this is the layer which, in the course 

 of history, has been most directly 

 subjected to external stimulus. 



I-et us consider first the gang- 

 lionic cells which receive stimuli 

 and shunt them, which regulate the 

 whole life of the organism, and are 

 the physical conditions of " spon- 

 taneous " activity and intelligence. 

 They are of very varied shape, but 

 consist always of a cell-body which 

 gives off one or more processes. 

 One of these processes is long, 

 branches very sparingly, and is 

 known as the axis-cylinder. There 

 are usually present other processes 

 which ramify like the branches of 

 a tree and are called dendrites. 

 The cell-body contains a nucleus, 

 distinct granules, and a network of 

 fine fibrils. The nervous system is 

 built up of such "neurones." In 

 the ganglia they are supported and 

 held apart by much-branched neuroglia cells. 



In all but a few of the simplest Metazoa, the nerve fibres (axis- 

 cylinders) are surrounded by a sheath called the neurilemma, said to be 

 formed by adjacent connective tissue. Several nerve fibres may com- 

 bine to form a nerve, but each still remains ensheathed in its neuri- 



5TR 



Fig. 24. — A piece of striped muscle 

 fibre with its nerve-endings. 



STR., Striations of tlie muscle fibre ; 

 N., nuclei of the muscle fibre ; 

 M.N., a motor nerve giving off 

 motor nerve fibres (M.F.), which 

 lead to branching motor-endings 

 (M.A.). 



S.A. is a sensory nerve-ending, from 

 which impulses are carried by 

 sensory fibres (S.F.) to a sensory 

 nerve. 



