408 



HUMAN PHYSIOLOGY 



may cause nausea or even vomiting. The glands of the stomach 

 will not secrete unless a proper stimulation has been given to 



them by food entering the stomach. 

 When food reaches the gland, the 

 message is sent from the nerve endings 

 in the gland, to the reflex nervous 

 center. The gland begins to secrete 

 its fluid on the receipt of a return 

 message from that center. Each set 

 of glands in the digestive tract acts in 

 a similar manner, and thus digestive 

 fluid is poured out, and digestion is 

 accomplished. 



Automatic Acts. — Some acts, how- 

 ever, are learned by conscious thought, 

 as writing, walking, running, or swim- 

 ming. Later in life, however, these 

 activities become automatic. The ac- 

 tual performance of the action is then 

 taken up by the cerebellum, medulla, 

 and spinal ganglia. Thus the think- 

 ing portion of the brain is relieved of 

 part of its work. 



Habit Formation. — The training of 

 the different areas in the cerebrum to 

 do their work well is the object of edu- 

 cation. When we learned to write, 

 we exerted conscious effort in order to 

 make the letters. Now the act of forming the letters is done with- 

 out thought. By training, the act has become automatic. In 

 the beginning, a process may take much thought and many trials 

 before we are able to complete it. After a little practice, the same 

 process may become almost automatic. We have formed a habit. 

 Habits are really acquired reflex actions. They are the result of 

 nature's method of training. The conscious part of the brain has 

 trained the cerebellum or spinal cord to do certain things that, at 

 first; were taken charge of by the cerebrum. 



Diagram of the path of a nervous 

 impulse which results in the hand 

 reaching to seize an object seen 

 with the eye; C, cerebrum; Cb, 

 cerebellum; M, medulla oblon- 

 gata ; MC, motor center in brain ; 

 OC, optic center in brain; P, 

 pons Varolii; Q, corpora quadri- 

 gemina; S, spinal cord; T, optic 

 thalami. 



