216 AN AMERICAN TEXT- BO OK OF PHYSIOLOGY. 



from precise ; and although the movements of the leg, when the stimulus i.- 

 applied far up on the rump, differ from those which follow the application 

 of the stimulus to the lower part of the thigh, yet in either case they are very 

 wide, and in both cases the fool is brushed across a large part of both the 

 rump and leg. Considering, therefore, the rather general character of these 

 movements, and the fact that the movements of the opposite leg only follow- 

 after a continued stimulus to the leg of the same side has produced an inef- 

 fective response, it is besl to explain the result by the diffusion of the impulses 

 within the cord, leaving quite to one side the psychical clement. Such reflex 

 actions are in a high degree predictable, but in reality this has little signifi- 

 cance, since there i> but one general movement that a frog in such a condition 

 can make, whether the stimulus be applied to the toes or the rump— namely, 

 the flexion of' the leg — so that under these circumstances the prediction of the 

 kind of movement is a simple matter. The extent of the contraction is related 

 to the intensity of the stimulus, and is in turn dependent on the excitability 

 of the central system, which can be increased or diminished in various ways. 

 The mollification of the reaction as dependent on the location of the stimulus 

 can be in a measure predicted, but the modification is wanting in precision 

 just in so far as the movements themselves are wanting in this quality. 



Reflexes in Man. — In the normal individual reflexes involving striped 

 muscles are found in the tendon reflexes, of which the knee-kick is an exam- 

 ple, in winking, and the whole series of reflex modifications of respiration, 

 such as coughing, sneezing, and the like. 



The activities of the alimentary tract are examples of reflex actions involv- 

 ing the contraction of muscles in deglutition, defecation, and similar peristaltic 

 movements in other hollow viscera. These muscle-fibres are for the most part 

 unstriped. So, too, micturition, the cremaster reflex, emission and vaginal 

 peristalsis, and the reactions of parturition are to be classed here. Moreover, 

 the entire vascular system is controlled in this manner, the contraction and 

 distention of the small arteries being in a large measure in response to stimuli 

 originating at a distance ; while as a third group, we have the glands, the 

 activity of which is almost entirely reflex. For the discussion of the various 

 reflexes mediated by the cranial nerves, the reader is referred to the special 

 sections dealing with the cardiac, vasomotor, and respiratory centres in the 

 bulb and the pupillary centres in the mid-brain. 



Periodic Reflexes. — Not all reflexes are to be obtained from the same 

 animal with equal intensity at all times. In general, frogs in the spring-time 

 and in early summer, after reviving from their winter sleep, are highly 

 irregular in their reflex responses — so irregular that students are advised 

 not to attempt the study of these reaction- at this season. On the other 

 hand, it is during the spring that mating occurs, and during this period the 

 male clasps the female and exhibits the peculiar reflex which has already 

 been described. Comparable with this variation in the frog must be the 

 changes which occur in the spinal cords of migratory birds, which, both in 

 the spring and in the fall, are capable of such extended flights, or in the 



