THE TISSUES 



73 



II. NEEVE 



It is through the nerves that our surroundings act upon us, 

 and through nerves that our muscles are made to respond 

 appropriately to the surrounding conditions. 



1. Structure and Development 



In unicellular organisms changes in the surroundings act 

 directly on the cell protoplasm, e.g. an amoeba, when touched, 

 draws itself together. But, even in these simplest organisms, 

 certain kinds of external conditions will produce one kind of 

 change, while others will produce a different one, as has been 

 shown in considering unilateral stimulation (p. 15). Even 

 among unicellular organisms e.g. among the infusoria animals 

 are found in which the cell is differentiated into a receiving 



o 



and reacting part. Poteriodendron, a little 

 infusorian sitting in a cup-like frame, con- 

 sists of a long process or cilium extending 

 up from a cell while a contractile myoid 

 attaches the cell to the floor of the cup. 

 When the cilium is touched the myoid con- 

 tracts, and draws the creature into the pro- 

 tection of its covering. 



In more complex multicellular organism, 

 e.g. in medusa, the different parts are con- 

 nected to one another by a network of 

 protoplasmic? strands, which bring each part 

 into relationship with the others, and thus 

 secure co-ordinate reaction to any stimulus. 

 A similar network exists and performs im- 

 portant functions in the wall of the aliment- 

 ary canal of vertebrates. 



In more highly organised animals, where the reaction has to 

 be more definitely appropriate to the surrounding conditions, 

 and where the complexity of the mechanism involved is greater, 

 there is a development by which special conditions at special parts 

 of the surface each lead to special reactions. This is brought 

 about by the establishment of a nervous system a mechanism 

 which may be compared to a series of shunting stations between 

 the receptive mechanism on the surface and the reacting mechan- 



FIG. 34. Poterio- 

 dendron to illus- 

 trate the first stage 

 in the evolution of 

 a neuro - muscular 

 system. 



