238 THE NERVOUS SYSTEM 



Habits, which form so large a share of our daily life, are 

 thus seen to be nothing more nor less than the customary 

 way in which our brain centres act. We do a thing 

 in a given way to-day because we have done it so often 

 in just that way in the past that our brain cells are 

 used to no other way of doing it. We can thus see why 

 it is that habits once formed are hard to break. To 

 change our habits means to change the way in which 

 nerve cells have been trained by constant repetition to 

 work together. If our habitual way of doing things is 

 the best way, habits are our powerful servants, in that 

 through them we can do things easily and quickly. If 

 the opposite is true, they may be evil masters, wasting 

 our time and energy. 



EXPERIMENTS AND DEMONSTRATIONS 

 Anatomy of the Nervous System 



Materials : Frog ; ether ; fresh calf's or sheep's head ; piece of 

 spinal cord of lamb, pig, calf, or beef; alcohol, 50 per cent; 

 small saw; stout scissors; bone forceps; microscope. 



1 ) Kill frog with ether ; open abdomen and remove viscera. 



a) Note at back of abdominal cavity a bundle of white cords 

 (nerve trunks) passing to each hind leg. 



b) Trace sciatic nerve into which they unite and dissect it 

 out along its course until it ends in fine branches in muscles. 



2) With stout scissors cut carefully bodies of vertebra} 

 (which will be seen projecting in middle line at back of ab- 

 dominal cavity), until neural canal is laid open and spinal cord 

 exposed. Note a) Origin of nerves from spinal cord. 



b) Their division into ventral and dorsal roots before they 

 join cord. 



c) Ganglionic enlargements on posterior roots. 



3) Turn frog upon abdomen and remove skin and muscles on 

 dorsal side of spinal column. Carefully cut away upper -f 

 of neural arches of vertebrae. Then remove upper half of 



