484 THE NERVOUS SYSTEM. 



canal. So, also, the spinal cord ; for a decapitated animal 

 lies motionless 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 centri- 

 petal nerve-fibres. In nerve-fibres, as already said, im- 

 pressions 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 

 ganglia 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 impression may be conducted 

 through the spinal cord to the brain, where the mind may 

 perceive it. In the opposite direction, mental influence 

 may be conducted from the brain through a succession of 

 nervous centres the spinal cord and ganglia, and one or 

 more ganglia of the sympathetic to produce the influence 



