26 



THE MECHANISM OF LIFE 



are " the way out." When the animal does anything in response 

 to some change that occurs in its environment — (1) a receptor 

 organ is stimulated by the external event, (2) an impulse is 

 propagated along an afferent nerve into the central nervous 

 system, (3) some change occurs in the latter, (4) an impulse is 

 propagated out along an efferent nerve, and (5) the motor organ 

 is stimulated, and responds by some kind of movement. 



The Minute Structure o£ the Nervous System.— Complicated as 

 this is, its general scheme is easily understood. In a receptor 

 organ there is always an essential part which we call the nerve 



Node 



Jrri rn'ifi ve 

 sheaih 



Jde'dulla. 



Fig. 8. — Forms of Neurones. All Highly Magnified. 



1, Transverse section of a nerve fibre; 2, a nerve fibre; 3, nerve cell from 

 the spinal cord; 4, nerve cell from the cerebellum; 5, nerve cell from 

 a sympathetic ganglion. 



termination; this is the retina in the eye, the auditory hairs in 

 the ear, and so on. Between the receptor and the brain or spinal 

 cord there are stretched a series of nerve fibres, and it is along 

 these that the impulse passes. In the central nervous system 

 there are collections of nerve cells and nerve tracts. Between 

 the centre and the motor organ there are other fibres, and in the 

 former there are, again, nerve terminations. 



All this structural detail is built up of nervous elements called 

 neurones. 



Neurones are structures that vary greatly in appearance and 

 size, but they have all the same general form. Each of them 

 consists of a nerve cell — that is, a minute fragment of specialised 



