826 THE CENTRAL NERVOUS SYSTEM 



ter of gravity of the body — postural reflexes, as they are called. The 

 best type of this reflex is the knee-jerk, another good example being the 

 extensor thrust. The scratch reflex contains a certain element of the 

 nociceptive in it, and of the simpler reflexes it comes second in its claim 

 on the final common path. In brief, then, in reflexes which in an intact 

 animal would cause the sensation of pain and probably some reflex ac- 

 tivity of the vocal organs, we gel in the spinal animal a reflex flexion 

 movement of the part stimulated with the evident object of removing 

 that part from the stimulating agency. This reflex flexion secures pos- 

 session of the final common path whatever other reflex may at the time 

 be occupying it. Thus, if the animal is scratching itself and something 

 occurs to hurt its foot, then immediately the scratching movement will 

 give place to one of flexion, and so on. 



Some integration between distant reflex arcs in the nervous system 

 is to a certain extent an application of the principle of reciprocal in- 

 hibition of the muscles moving a joint. In this broader integration the 

 inhibition affects more removed fields of reflex activity so as to harmonize 

 the activities of one part of the animal -with those of every other part. 



The manner in which the stimulation may spread along the various 

 available pathways also depends on the strength of the afferent im- 

 pulses. If a very feeble stimulus is applied to the skin of the leg in a 

 spinal animal, the reflex will be represented only by a slight contraction 

 of the inner ends of the hamstring muscles. As the stimulus is increased 

 in strength the reaction will spread, until at last it involves all the 

 flexors in contraction and the antagonistic extensors in inhibition. If it is 

 still further increased, the flexion movement will be accompanied by an 

 extension of the muscles of the opposite hind limb — the crossed exten- 

 sion reflex. Further increase of the stimulus will cause the reflex move- 

 ment to spread to the anterior extremities, involving, first of all, the 

 fore limb of the same side (extension at the elbow and contraction at 

 the shoulder), and then that of the opposite side (flexion at the elbow 

 and extension at the wrist). A very powerful stimulus applied to the 

 hind limb will even spread to other more distant muscular groups, such 

 as those of the neck, causing a turning of the head to the side stimu- 

 lated, opening the mouth, etc. 



This spread or irradiation of the reflex in the spinal cord can not be 

 entirely explained on anatomic grounds, and must depend, therefore, 

 upon varying resistance to the flow of the afferent impulse to different 

 motor neurons, some of which it excites while others it inhibits. 



The necessity for adjustable resistance to the transmission of different 

 afferent stimuli on to the final common path becomes evident when Ave 

 remember that, not only are there about five times as many fibers en- 



