CHAP, i.] THE SPINAL CORD. 705 



abdomen or the testicle is retracted upon stroking the inside 

 of the thigh. A special form of reflex action, or at least an 

 action maintained by many to be a reflex action, is called forth 

 by sharply striking certain tendons. It is well known for 

 instance that when the leg is placed in an easy position, rest- 

 ing for instance on the other leg, a sharp blow on the patellar 

 tendon will cause a sudden jerk forward of the leg, brought 

 about by a contraction of part of the quadriceps femoris ; it is 

 necessary or at least desirable for a good development of the 

 jerk that the tendon (and muscle) should be somewhat on the 

 stretch. Similarly the muscles of the calf may be thrown into 

 action by tapping the tendo Achillis put somewhat on the 

 stretch by flexion of the foot ; and in some cases the same 

 muscles may be made to execute a series of regular rhythmic 

 contractions, called ' clonic ' contractions, by suddenly pressing 

 back the sole of the foot so as to put them on the stretch. The 

 "knee-jerk," as the sudden extension of the leg when the 

 patella is struck is familiarly called, has acquired great interest 

 in medical practice, and has been carefully studied not only in 

 man but in various animals. It has been contended by some 

 that the act is not truly a reflex one, but the evidence on the 

 whole goes to shew that it is. From various experiments we 

 learn that in it as in an ordinary reflex movement there are 

 three factors, namely, an afferent limb, a spinal centre and an 

 efferent limb ; if any one of these three factors fails, the whole 

 act fails. The efferent limb is furnished by fibres in the 

 anterior crural nerve which reaching that nerve by the anterior 

 roots of (in man) the 3d and 4th lumbar nerves, and passing 

 to the vastus internus muscle and adjoining portions of tin- 

 crureus muscle, these being the parts of the great quadriceps 

 concerned. The spinal centre lies in the lumbar portion of the 

 cord, in what in man corresponds to the 3d and 4th lumbar 

 segments. The afferent limb is furnished by fibres starting in 

 the patellar tendon, running in the anterior crural nerve, and 

 reaching the cord by the posterior root of (in man) the 4th 

 lumbar nerve. It is further worthy of notice as an illustration 

 of what we were urging a little while back that the vigour of 

 the knee-jerk is influenced by the antagonistic hamstring mus- 

 cles, and that not merely in a mechanical way but by nervous 

 action. The knee-jerk is reinforced by cutting the nerves sup- 

 plying the hamstring muscles, or by simple division of certain 

 posterior roots, fibres of which take origin in those muscles or 

 their tendons ; it is depressed by central stimulation of those 

 nerves, or by simply stretching the hamstring muscles. It- 

 would appear that these flexor muscles antagonistic to the 

 extensor muscles, send up to the spinal cord, according to their 

 condition, afferent impulses which influence the spinal centre 

 for the knee-jerk. The activity of the same centre may also be 



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