444 A MANUAL OF VETERINARY PHYSIOLOGY 



opposite side, the creature descends. It is beyond the belief of 

 a layman that such acts are not of a purposive character. As a 

 matter of fact, they fall within the definition of a true reflex 

 act. 



If a headless frog be lightly stimulated on one hind-leg, a 

 reflex contraction of the limb occurs ; if the stimulus is made 

 stronger, both limbs react. By increasing the strength of the 

 stimulus the fore-limbs may be involved, so that there is a pro- 

 portion between the magnitude of the reflex acts produced and 

 the strength of the stimulus. There is a latent period in a reflex 

 act (see p. 459), and with each successive increase in the strength 

 of the stimulus the latent period becomes longer in consequence 

 of a greater length of cord being involved. If the frog be injected 

 with strychnine and stimulated, convulsive movements occur, 

 the sensitiveness to touch is greatly increased, so that even a 

 current of air will cause muscular contraction. In this case it 

 is supposed that there is an overflow, as it were, into all motor 

 paths, the strychnine breaking down the usual barriers which 

 direct the impulse. The strychnine does not open new paths, 

 it uses the old ones, but it converts inhibitory effects into 

 excitatory, so that contractions appear in muscles at the moment 

 they should under normal conditions relax. Incidentally, the 

 sensitiveness of the strychnine preparation demonstrates that a 

 sensory path leads to all the motor neurones of the body. 



The inhibition of reflexes is an extremely important question, 

 which will receive separate consideration. 



The reflex acts exhibited by the higher animals are more 

 complex than those of the lower. The normal actions of the 

 latter are almost entirely reflex, whereas those of the former 

 are normally reflexes controlled, modified, or set free, as the case 

 may be, by cerebral centres. If in the dog the cord be divided in 

 the anterior thoracic region, the animal becomes paralysed, 

 there is loss of sensation as well as of motion ; yet in process 

 of time recovery occurs, and the isolated portion of cord 

 is capable of carrying on the reflex function of micturition, 

 defalcation, impregnation, parturition, scratching, and stepping, 

 without the knowledge of the animal. Such a dog may even 

 be able in course of time to walk or run. In the first instance it 

 learns to stand, then takes a few steps before subsiding, and 

 gradually the reflex paths are educated to the new condition 

 which has arisen. Locomotion, we have stated, is not normally 

 a true reflex act, yet under purely reflex conditions it may be 

 carried out for hours. The horse has not to think of the order 

 in which he uses his four legs, and how each step is to be 

 taken ; the fact is that this is carried out by the spinal cord, with 

 which the animal is not concerned ; the work is done for him. 



