308 THE HUMAN BODY. 



muscles of respiration. When this centre is cut off from 

 all sensory nerves it still acts, and its activity goes on even 

 against the Will. We can voluntarily hold the breath for 

 a short time, but not long enough to kill ourselves by suffo- 

 cation. Although automatic nerve-centres act indepen- 

 dently of impulses carried to them by nerve-trunks, they 

 are nevertheless usually more or less subject to control by 

 them. For example, stimulating the branch of the pneu- 

 mogastric nerve (p. 293) which goes to the automatic heart 

 nerve-centres slows the beat of the organ; and a dash of 

 cold water on the skin makes us draw a deep breath. 



Reflex Centres are aroused to activity by nervous im- 

 pulses conveyed to them through afferent nerves: they 

 then excite efferent nerves and produce a movement or a 

 secretion. Such nerve-centres do all the routine of the ad- 

 ministrative control of the organs of the body, without 

 troubling the psychic centres. They frequently act with- 

 out the intervention of consciousness at all, and often in 

 spite of the Will. When sugar is placed in the mouth it 

 excites its sensory nerves ; these stimulate a centre from 

 which nerves go to the salivary glands, and these nerves, 

 aroused by the centre, make the gland-cells secrete and pour 

 saliva into the mouth; no effort of the Will can stop this 

 reflex action, so called because a nervous impulse sent to a 

 centre by one set of nerve-fibres is turned back or reflected 

 from it along another set. When a morsel of food enters 

 the pharynx it excites the sensory nerves of the mucous 



Can a man commit suicide by holding his breath? Are the 

 automatic centres entirely free from control? Illustrate? 



How are reflex centres excited? What is the consequence of 

 their stimulation? "What sort of work in the body is executed by 

 reflex nerve-centres? Are we always conscious of their action? Can 

 the Will always control them? What happens when sugar is placed 

 on the tongue? Why is it called a _H reflex action"? 



