NERVE-CENTRES IN THE SPINAL CORD 559 



absent ; nor are there any true voluntary movements, as the organ of 

 the will is destroyed. Still, in decapitated animals, the sensory nerves 

 are for a time capable of conducting impressions, and the motor nerves 

 can transmit impulses to. the muscles; but the only part capable of 

 receiving impressions or of generating motor impulses is the gray mat- 

 ter of the cord. If, in addition to the removal of all the encephalic 

 ganglia, the cord itself is destroyed, all muscular movements are abolished, 

 except as they may be produced by direct stimulation of the muscular 

 tissue or of individual motor nerves. 



The gray matter of the brain and spinal cord is a connected chain of 

 ganglia, capable of receiving impressions through the sensory nerves 

 and of generating motor impulses. The cerebro-spinal axis, taken as 

 a whole, has this general office ; but some parts have separate and 

 distinct properties and can act independently of the others. The cord, 

 acting as a conductor, connects the brain with the parts to which the 

 spinal nerves are distributed. If the cord is separated from the brain 

 in a living animal, it may act as a centre independently of the brain ; but 

 the encephalon has no communication with the parts supplied with 

 nerves from the cord, and it can act only on the parts which receive 

 nerves from the brain itself. 



When the cord is separated from the encephalon, an impression 

 made on the general sensory nerves is conveyed to its gray substance, 

 and this gives rise to a stimulus, which is transmitted to the voluntary 

 muscles, producing certain movements that are independent of sensa- 

 tion and volition. This impression is said to be reflected back from the 

 cord through the motor nerves; and the movements occurring under 

 these conditions are called reflex. As they are movements excited by 

 stimulation of sensory nerves, they are sometimes called excitomotor. 



The term " reflex," as it is now generally understood, may properly be 

 applied to any generation of impulses that occurs as a consequence of 

 an impression received by a nerve-centre ; and it is evident that reflex 

 phenomena are by no means confined to the action of the spinal cord. 

 The movements of the iris are reflex, and yet they take place in many 

 instances without the intervention of the cord. Movements of the 

 intestines and of the involuntary muscles usually are reflex, and they 

 involve the action of the sympathetic system of nerves. Impressions 

 made on the nerves of special sense, as those of smell, sight, hearing, 

 etc., give rise to certain trains of thought. These involve the action of 

 the brain, but still they are reflex. In this last example of reflex action, 

 it sometimes is difficult to connect the operations of the mind with 

 external impressions as an exciting cause ; but it is evident, on a little 

 reflection, that this often occurs. 



