598 NERVOUS SYSTEM. 



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 excito-motor. 



The term reflex, as it is now generally understood by physiologists, may 

 properly be applied to any generation of nerve-force which occurs as a con- 

 sequence 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 ye.t they take place in many in- 

 stances without the intervention of the cord. Movements of the intestines 

 and of the involuntary muscles generally are reflex, and they involve the 

 action of the sympathetic system of nerves. Impressions made upon the 

 nerves of special sense, as those of smell, sight, hearing etc., give rise to cer- 

 tain trains of thought. These involve the action of the brain, but still they 

 are reflex. In this last example of reflex action, it is sometimes difficult to 

 connect the operations of the mind with external impressions as an exciting 

 cause ; but it is evident, from a little reflection, that this is often the case. 



Reflex Action of the /Spinal Cord. Simple reflex action involves the 

 existence of an afferent (sensory) nerve, a collection of nerve-cells, and an 

 efferent (motor) nerve, the nerves being connected with the nerve-cells. In 

 a decapitated animal, not only are the movements independent of sensation 

 and volition, but no movements occur if the sensory nerves be protected 

 from any kind of impression or stimulation (Marshall Hall, 1832 and 1833). 

 If the cord be destroyed, however, no movements follow stimulation of the 

 surface ; and if either the afferent and the efferent nerves be divided, no 

 reflex movements can take place. Experiments upon decapitated animals 

 are in accord with the results of observations upon acephalous foetuses and 

 in cases of complete paraplegia from injury to the cord. 



In the simplest form of a reflex movement, the muscular contraction is 

 confined to the muscle or muscles which correspond, in their nervous supply, 

 to the afferent nerve stimulated ; but when the stimulus is sufficiently power- 

 ful or when the cord is in a condition of exalted excitability, the impression 

 is disseminated throughout the gray matter, and the entire muscular system 

 may be thrown into action. With feebler stimulation, one side only of the 

 muscular system may respond. When the reaction extends to the opposite 

 side, it is called crossed reflex. The extension of a stimulus conveyed by a 

 single afferent nerve throughout the cord is called irradiation. 



When a feeble stimulus applied to an afferent nerve is repeated frequently 

 and at short intervals, general muscular movements are produced. This fol- 

 lows stimuli applied three times in a second, and the effect is increased up 

 to sixteen shocks in a second, but not beyond this number (Rosenthal). 



In studying the paths of conduction in the cord it has been seen that 

 sensory conduction takes place through the gray matter and possibly through 

 the columns of Goll, that motor impulses are conducted by the direct and 

 the crossed pyramidal tracts, and that the columns of Burdach are connected 

 with muscular co-ordination. The fibres of the cord that are specially con- 



