114 THE COMMON FROG. [chap. 



inwards the effect of a stimulus applied towards their 

 ultimate termination, and are therefore called afferent, 

 or sensory. 



The fibres which come ultimately from the ventral 

 aspect of the spinal marrow, are those which carry an 

 influence outwards, and produce a contraction in the 

 muscles, and are therefore called efferent or motor. 



It is the nervous system of the frog, rather than 

 any other set of its organs, which has especially 

 excited interest and attention. It is especially to the 

 relations inter se, of the parts of this system that 

 inquiry has been directed. The relations, that is, of 

 its central or axial portion (the brain and spinal 

 column) to its peripheral or appendicular portion (the 

 nerves of the body and limbs). 



In the ever memorable year 1789, Galvani acci- 

 dentally discovered in the separated legs of certain 

 frogs, prepared for broth, those motions produced by 

 irritation of the exposed great nerve of the thigh, 

 nov/ so familiar to most. This action was long called 

 galvanism, after this observer, not, however, that he 

 was absolutely the first to notice a fact of which he 

 was but a re-discoverer — Swammerdam as long ago 

 as 1658 having observed such motions. 



They are generally considered as demonstrating 

 the purely " reflex action " of the nervous system — 

 the responsive action, that is, upon muscles, of nervous 

 centres acted on by external stimuli without the inter- 

 vention of sensation. 



It is affirmed that not only will a decapitated frog 

 endeavour to remove an irritating instrument by 

 means of its hind legs and feet ; but that if a caustic 



