NERVE-CKNTRES. 79 



cular, glandular, or other organ is called into activity or 

 arrested in consequence of the excitation of an afferent 

 nerve. 



The most important reflex actions are muscular. These 

 are of two kinds, which may be called normal and 

 abnormal. Normal reflex processes spring from the 

 excitation of one or more peripheral sense-organs, by 

 usually feeble stimuli. They are characterized by the fact 

 that excitations of the same kind, originating from the 

 same end-organs always lead to the same results, that is, 

 occasion the same combinations or series of co-ordinated 

 muscular actions. This fact justifies the hypothesis that 

 in the centres there are channels of propagation, by which 

 the excitatory process is guided, notwithstanding that 

 our present knowledge of anatomy affords no clues by 

 which they may be traced. All normal reflex processes 

 are adapted to the accomplishment of useful purposes in 

 the animal economy. 



Abnormal, incoordinate, or convulsive reflex processes, 

 do not occur in the healthy body, excepting in consequence 

 of injury. The excitatory state which here as in the other 

 case is communicated to the centre by an afferent nerve, 

 spreads from it to other centres by mere continuity of 

 structure, irrespectively of channels of propagation. Con- 

 sequently those centres which are nearest, are as a rule 

 first affected, and in their turn the motor nerves which 

 spring from them and the muscles to which such nerves are 

 distributed, without distinction of function. In the abnormal 

 state, whether induced by loss of blood, by interference 

 with respiration, by disease, or by poisons, incoordinate 

 reflexes may be excited by the action of ordinary stimuli 

 on sensory end-organs, but much more readily by injuries 

 of nerve trunks. 



The time occupied by normal reflexes varies according 

 to their complexity, and to the remoteness of the centres 

 concerned, from a twentieth to a tenth of a second, or even 



