FUNCTIONS OF NERVE-CENTRES. 383 



centres has a proper range of functions, the extent of which 

 bears a direct proportion to the number of nerve-fibres that 

 connect it with the various organs of the body, and with other 

 nervous centres ; but they all have certain general properties 

 and modes of action common to them as nervous centres. 



It is generally regarded as the property of nervous centres 

 that they originate the impulses by which muscles may be ex- 

 cited to action, and by which the several functions of organic 

 life may be maintained. Hence, they are often called sources 

 or originators of nervous power or force. But the instances in 

 which these expressions can be used are very few, and, strictly 

 speaking, do not exist at all. The brain does not issue any 

 force, except when itself impressed by some force from within, 

 or stimulated by an impression from without ; neither without 

 such previous impressions do the other nerve-centres produce 

 or issue motor impulses. The intestinal ganglia, for example, 

 do not give out the nervous force necessary to the contractions 

 of the intestines, except when they receive, through their cen- 

 tripetal nerves, the stimuli of substances in the intestinal canal. 

 So, also, the spinal cord ; for a decapitated animal lies motion- 

 less so long as no irritation is applied to its centripetal nerves, 

 though the moment they are touched movements ensue. 



The more certain and general office of all the nervous centres 

 is that of variously disposing and transferring the impressions 

 that reach them through the several centripetal nerve-fibres. 

 In nerve-fibres, as already said, impressions are only conducted 

 in the simple isolated course of the fibre ; in all the nervous 

 centres an impression may be not only conducted, but also 

 communicated ; in the brain alone it may be perceived. 



Conduction in or through nerve-centres may be thus simply 

 illustrated. The food in a given portion of the intestines, 

 acting as a stimulus, produces a certain impression on the 

 nerves in the mucous membrane, which impression is conveyed 

 through them to the adjacent ganglia of the sympathetic. In 

 ordinary cases, the consequence of such an impression of the 

 ganglia is the movement of the muscular coat of that and the 

 adjacent part of the canal. But if irritant substances be 

 mingled with the food, the sharper stimulus produces a stronger 

 impression, and this is conducted through the nearest ganglia 

 to others more and more distant ; and from all these, motor 

 impulses issuing, excite a wide-extended and more forcible 

 action of the intestines. Or even through all the sympathetic 

 ganglia, the impression may be further conducted to the gan- 

 glia of the spinal nerves, and through them to the spinal cord, 

 whence may issue motor impulses to the abdominal and other 

 muscles, producing cramp. And yet further, the same morbid 



