122 



HUMAN BIOLOGY 



Reflex Action, Consciousness, and Will. - -Usually not all 

 of the force of the impulse is transferred to the motor cell. 

 The sensory cell by means of another of its many branches 

 may transfer part of tJie impulse to a cell which sends it to 

 the brain. Hence a reflex act is not necessarily an uncon- 

 scious one. If you unintentionally touch the hand to a 

 hot stove pipe, you may be conscious of the pain and the 

 involuntary jerking away of the hand at the same time. 

 Reflex Action and the Will. - The will may inhibit, or 

 prevent, an expected reflex act. Yet many reflex acts 



occur in spite of the effort of the will to 

 prevent them. One cannot always keep 

 from closing the eyes before a threatened 

 blow even if from the other side of a plate 

 glass window, and it is known there is no 

 danger. Sneezing is a reflex act and can- 

 not always be prevented. The forming of 

 saliva and other secretions are reflex acts. 

 Reflex acts are quicker than voluntary acts. 

 An eighth of a second is about the time 

 required for a person to press an electric 

 button after seeing a signal ; a reflex act 

 may occur in a shorter time. 



The Brain consists of Three Chief Parts. 

 -(i) There is an enlargement at the top 

 of the spinal cord called the medulla, or 

 the medulla oblongata. It may be re- 

 garded as the part of the spinal cord 

 within the skull (see Figs. 109, no, 114). 

 (2) Above the medulla is the cerebellum, 



or little brain. (3) The cerebrum, or large 



tfa gkull except the sma n 



CORD. part occupied by the medulla and cere- 



I 



K 



H 



FIG. I0 9 . BRAIN brai fin 



AND SPINAL 



