. Elements of biology; a practical text-book correlating botany, zoology, and human physiology. Biology. 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,
. Elements of biology; a practical text-book correlating botany, zoology, and human physiology. Biology. 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 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 quadr
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