REFLEX ACTION 591 



from cerebral concussion and the absorption of toxic agents, such as picrotoxin, 

 morphin, quinin, potassium bromid, and others. 



Acceleration and Conditioning of Reflexes. Certain conditions 

 may arise at times which will tend to augment reflex action in such 

 a degree that it becomes difficult to differentiate these responses from 

 those previously described under the heading of spreading of impulses. 

 The causes to which this acceleration may be assigned are twofold. 

 Thus, it may be caused either by an increase in the strength of the 

 stimulus or by a heightened irritability of the nervous tissue. If an 

 irritant is applied to the nasal mucosa of a strength just sufficient to 

 incite merely a slight tendency to sneeze, this primary stimulus may be 

 reinforced by the act of sniffing. Clearly, as this augmentation is de- 

 pendent upon volition, it must be attained with the help of the cere- 

 brum. We are also in a position to strengthen those reflexes which 

 ordinarily result in consequence of cutaneous impressions, either by 

 the application of cold water or by stimuli involving the optic or 

 auditory mechanism. In a similar manner, the corneal reflex may be 

 accelerated by gently blowing a current of air across the surfape of the 

 conjunctiva. On the whole, however, it must be conceded that reflex 

 acceleration cannot be effected so easily as reflex inhibition. 



By far the largest number of reflexes are not conditioned. A 

 particular kind of stimulus gives rise to a particular reaction with 

 almost mechanical exactitude. This is true of coughing, sneezing, 

 yawning and other acts with which we are familiar. It is possible, 

 however, to subject these reflexes to other influences so that they 

 assume an elaborated or conditional character. l Thus, we are able to 

 incite a flow of saliva quite readily by the introduction of a drop of 

 dilute acetic acid into the mouth of the subject. If this stimulation 

 is repeated a number of times at intervals and if this stimulation is 

 accompanied by a visual impression, such as may be effected by a 

 receptacle filled with colored water, the primary stimulus may be 

 dispensed with in time, because the retinal stimulus alone will then 

 suffice to produce the aforesaid result. While many of our reflexes 

 may be conditioned in one way or another, it is true that this cannot 

 be done without the help of perception. In other .words, the condi- 

 tioned reflexes require training or education. This conversion of a 

 simple reflex into an association reflex, however, does not necessitate 

 the participation of volition; in fact, it precludes this modification 

 for the reason that the reflex would then lose its primitive character 

 and become a reaction. 



Classification of Reflexes. In accordance with their qualitative 

 peculiarities, reflexes are divided into: 



(a) Simple reflexes, in which a single muscle or glandular unit is involved. As 

 an example of this kind of response might be mentioned the corneal reflex. The 

 afferent arc is formed by the nervi ciliares trigemini #nd the efferent arc by the 



1 Pawlow, Livre jubil. du Prof. C. L. Richet, 1912. 



