v SPINAL COED AND NEEYES 317 



not an injurious factor from which the animal must escape, but a 

 condition favourable to the normal development of useful functions. 

 Thus the plantar reflex is a reflex which the animal usually carries 

 out in walking or leaping. It represents the extensor reaction 

 of the limb applied to the ground for the purpose of raising the 

 body. 



The reflexes of the second category do not for the most part 

 spread to different muscles, like the reflexes of the first class, when 

 the strength or duration of the stimulus is increased ; they behave, 

 on the contrary, more as if they conformed to the " all or nothing " 

 law of the heart. 



Finally Baglioni has brought out the fact that reflexes of the 

 first class are usually produced by injurious electrical or chemical 

 stimuli such as Pfliiger employed. To evoke reflexes of the second 

 category it is necessary to use adequate stimuli, and to apply them 

 to the peripheral sense organs which normally receive them, and 

 not to exposed nerve trunks. 



VIII. The nature of any reflex movement is determined by 

 the quality, intensity, and seat of the stimulation, and lastly by 

 the state of the centres that participate in the reflex. 



All the different modes of cutaneous stimulation (electrical, 

 mechanical, thermal, chemical) are capable, even when they induce 

 painful sensations, of evoking spinal reflexes. The form of the 

 movement may differ, however, with the nature of the excitation. 

 For instance, the tail of the eel, according to Pfliiger's experiments, 

 moves towards a tactile stimulus, and away from a painful 

 stimulus. Certain special reflexes only come off with specific 

 stimuli ; gentle patting of the skin of a dog's flank may cause a 

 rhythmical scratching movement. 



It is easier to evoke a reflex by weak mechanical stimulation 

 of the skin than by strong induction shocks applied directly to a | 

 nerve trunk. Faradisation of the central end of a muscular nerve, 

 for instance, has much less effect on respiratory rhythm and on 

 blood pressure than the stimulation of a cutaneous nerve : in the 

 first case there is a fall, in the second a rise, of blood pressure. 

 Excitation of the dorsal roots induces reflexes more easily than 

 stimulation of the peripheral nerve-endings in the skin ; but in 

 the second case the reaction is more like an ordinary co-ordinated 

 movement, while in the first it resembles a reflex spasm. This 

 shows that in mammals the spinal roots are less a functional than 

 a purely morphological complex ; the functional combinations of 

 the root filaments are first formed in the nerve plexuses. 



The character of the reflex is also influenced by the intensity 

 of the stimulation, independently of any change in the nature of 

 the afferent impulse. A w r eak stimulus evokes a reflex reaction 

 that is transmitted to a few efferent fibres ; a stronger stimulus 

 causes the reflex to spread to many efferent fibres. But there is 



