102 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



The nerve-fibres, considered from the physiological standpoint, 

 are either afferent or efferent, transmitting molecular impulses to 

 and from the ganglion-cells. These are confined to the ganglia, 

 which are consequently the central organs. Most of the afferent 

 nerves are sensory, connecting sense-organs with the ganglia, while 

 a majority of the efferent ones are motoi', running from the ganglia 

 to muscles. Each ganglion has more especially to do with the 

 segment it supplies, but can also work conjointly with others. 

 An abdominal ganglion, for example, if the connectives which 

 unite it with its neighbours are cut, still causes regular move- 

 ments in the corresponding swimmerets. These movements are 

 undoubtedly caused by efferent nerve impulses from the ganglion, 

 as they cease on its extirpation. 



If the nerve-cord is cut at the end of the thorax, the abdominal 

 ganglia, working as a whole, cause all the swimmerets to work 

 regularly together. And by allowing more and more ganglia to 

 remain, connected movements of increasing complexity are 

 rendered possible. The addition of the cerebral ganglion, how- 

 ever, makes far more difference than the addition of any other. 

 Without this the movements are for the most part irregular 

 and lacking in purpose. The cerebral ganglion, in fact, controls 

 the body at large. 



The actions may be reflex or spontaneous. A reflex action 

 starts with an external stimulus, by which one or more afferent 

 fibres are affected. These carry impulses to the central organ, 

 from which other impulses pass along efferent fibres to the parts 

 they supply, causing them, if muscular, to contract. Suppose, 

 for instance, the abdomen, when nervously isolated from the 

 rest of the body, be irritated, say, by tapping, it will " flap 

 vigorously." Here the tapping causes afferent fibres to carry 

 impulses to the abdominal ganglia (central organs). These 

 " reflect " the impulses into motor fibres, down which they pass 

 to the flexor and extensor muscles, and cause these to contract. 



Spontaneous actions are not directly dependent on external 

 stimuli. A crayfish from which the cerebral ganglion is removed 

 exhibits constant movements, to all appearance "spontaneous," 

 as above defined. They are, however, not voluntary, the effect 

 of "will," spontaneous actions of this sort being probably the 

 result of impulses passing from the cerebral ganglion. This, too, 

 is the seat of consciousness and intelligence in so far as these exist.. 



