208 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



hence there is no iiMiim in at which the nervous system is not stimulated, and 

 no moment at which the effectiveness of this stimulus is not varied. The 

 response t<> this continuous and ever-varying stimulation i- not necessarily 

 always evident, but occasionally intensification of the stimuli renders them so 

 Strong that an evident reaction follows. 



Though the foregoing statements suggest that the chief variable is 

 that represented by the stimulus, the strength of which changes, yet as 

 a matter of fad the variations in the physiological (chemical) condition 

 of the nerve-cells are equally important; but neither factor can be studied 

 independently. 



The term "central stimulation" has been sometimes employed. For ex- 

 ample, the spasmodic movements of the young child, when there is no change 

 noticeable in the external stimuli acting upon it, are sometimes attributed to 

 this cause ; but these, although doubtless due to central changes, altering the 

 irritability of the cells, are most properly classed with the reactions which 

 follow the external stimulus. The misconceptions here to be avoided are 

 those of supposing that the nervous system is at any time unstimulated, and 

 that the evident responses follow a change of the external stimulus only. 



When the impulse in one cell-element is used to arouse an impulse in 

 another, as in all experiments where the nerve-cells are examined in a physi- 

 ological series, the strength of the impulse from the second is not easily pre- 

 dicted. This is explained as due to variations in the ease with which the 

 impulse in one element stimulates the next, and also to the variations in the 

 second cell of those conditions which determine the intensity with which it 

 shall discharge. 



When an impulse has once entered the central system by way of a dorsal 

 nerve root, it is found to follow the course of the afferent axones within the 

 central system, and thus must be distributed almost simultaneously to a 

 length of cord coextensive with that of the branches of the afferent axones. 



The arrangement makes possible the stimulation of a large number of 

 central cells, and thus greatly increases the distribution of the initial disturb- 

 ance. In the case of some of the cells about which the branches of the axone 

 end, the impulse will not be adequate to cause in them a discharge, although 

 it may still produce a certain amount of chemical change in them. The im- 

 pulse thus tends to disappear within the system by producing, in part, chemi- 

 cal changes strong enough to cause a discharge of the next clement in the 

 series and. in an increasing number, similar changes of a less intensity. 



Diffusion of Central Impulses. — If the previous description has been 

 correct, two very important events occur: in the firsl place, the impulse 

 reaches a far greater number of cell- than evidently discharge, and in the 

 second, the pathway followed by the impulses which do produce the discharge 

 is by no means the only pathway over which the impulses can or do travel. 



Simple Reflex Actions. — We turn next to an examination of these groups 

 of neurones in action. 



'flic simplest and most constant of the co-ordinated reactions of the nerv- 



