IRKITABILITY 119 



11 a throwing off of conventionality so that a person 

 iay do things which in a normal moment seem impos- 

 ble because of the influence of habit. If larger quan- 

 ties are taken, all strength is inhibited and we find 

 ie peculiar vagaries of the drunken man. Control of 

 ind and body is lost ; speech becomes thick and unin- 

 telligible ; muscular action becomes loose and ineffective ; 

 and in addition to the loss of voluntary control, involun- 

 tary physical control is lost. 



Alcoholic indulgence is particularly bad for young 

 people because it stunts growth and because the danger 

 of forming the habit is infinitely greater before the 

 age of thirty than it is afterwards. 



Summary. — Irritability manifests itself in the cir- 

 culation of protoplasm within the cell, in ciliary motion, 

 and in muscular activity. In higher animals the move- 

 ments of the body are due to muscular contraction. 

 The contraction takes place in response to an outside 

 stimulus, either physical or chemical, which is con- 

 veyed to the muscle through a nerve that is very sensi- 

 tive and able to convey stimuli quickly and easily. 



The variety of movements that we are capable of mak- 

 ing is due to the number, shape and arrangement of 

 muscles and their relation to the bones. The bones are so 

 arranged that they maintain the shape of the body and 

 act as levers to aid the motion. 



Neither nerves nor bones can initiate a motion. The 

 muscles are responsible. 



Muscular activity is important because it is inti- 

 mately connected with every function of the body. Dur- 

 ing activity, disintegration of complex substances in the 



