Elementary Biology. 



afferent nerve to a nerve-centre ; (2) a metabolic change 

 taking place in the centre, which results in (3) the trans- 

 mission of a motor or secretory impulse along an efferent 

 fibre to the muscle or gland concerned. For example, the 

 olfactory nerve is a sensory nerve ; stimulations of the ter- 

 mination of that nerve in the organ of smell, say by means 

 of the fumes of acetic acid, produces an effect on the nerve- 

 centre (in this case the olfactory lobes) which we term a 

 sensation something perceived. In ordinary circumstances, 

 i.e. when the brain proper does not interfere, an impulse 



FIG. 174. DIAGRAM TO ILLUSTRATE REFLEX ACTION. (Landois and Stirling.) 



A 



s, sensitive surface ; G, ganglion on sensory nerve ; af, afferent or sensory 

 nerve ; ef, efferent or motor nerve ; M, muscle fibre ; N, nerve-cell ; 

 A, P, anterior and posterior aspects of the spinal cord. The arrows in- 

 dicate the direction of the nerve-impulse. 



originating involuntarily in the nerve-centre is carried along 

 a secretory neive (the glosso pharyngenl) to the salivary 

 glands, causing them immediately to secrete a large quantity 

 of saliva. Numerous instances might be given of reflex 

 actions in different parts of the body ; that just given may 

 suffice to illustrate what is meant by the term. That these 

 reflex actions are possible without the intervention of the 

 brain is proved by the fact that even in a decapitated Irog 

 stimulation of the sensory-nerve terminations in the foot 

 is at once followed by the withdrawal of the foot from the 

 source of irritation. 



