140 



THE STUDY OF ANIMAL LIFE CHAP. 



from the outer world and con- 

 trols its own activities. It is 

 like a telegraphic system in a 

 way, receiving messages from 

 the outer world and sending out 

 tidings to distant parts ; it also 

 integrates or co-ordinates the 

 activities of all the organs of the 

 body. 



/ 



The unit in nervous reaction 

 in any highly organised animal 

 is called the reflex, and if we can 

 in some measure understand it 

 we have the key to the whole. 

 Three structures are involved, a 

 receptor (or end-organ), a con- 

 ductor., and an effector (muscle). 

 The receptor consists of sensory 

 cells which receive stimuli from 

 the outer world, and may be 

 combined in a sense-organ. The 

 conductor consists of nerve-cells 

 or neurones which connect the 

 receptor with the effector. Mes- 

 sages from the sense-cells travel 

 along sensory or afferent nerve- 

 fibres to the conductor cell or 

 cells, whence excitation is con- 

 veyed to other motor or efferent 

 neurones. From these a nervous 

 impulse passes along motor 

 nerve-fibres to the muscle, which 



FIG. 42. DIAGRAM OK A NERVE-CELL 



AND ITS FIBRES. (Alter Stohr.) 

 A is the nucleus of the cell, C the central 

 cytoplasm or cell-body. B is one of the 

 dendrites, processes by which this nerve- 

 cell communicates with adjacent nerve- 

 cells. D is a nerve-fibre or neurite, 

 enclosed in an envelope or sheath (). It 

 divides terminally into branches (F) going 

 to muscles. G is a terminal organ on a 

 muscle. Above D is given oil what is 

 called a collateral. 



