582 THE SPINAL CORD. 



We may therefore on the whole of the evidence conclude that the main- 

 tenance of skeletal tone is one of the functions of the cord ; but we may here 

 repeat that the condition of the cord, on which depends the issue from the 

 cord along the efferent cords of the influences, whatever their nature, which 

 produce tone in the muscle, may be, and indeed is, in its turn dependent on 

 afferent impulses. In the case of the frog quoted above the tone of the pen- 

 dent limbs disappears or is greatly lessened when the posterior roots of the 

 sciatic nerves are divided, though the anterior roots be left intact. In the 

 absence of the usual stream of afferent impulses passing into it, the cord 

 ceases to send forth the influences which maintain the tone. Hence the 

 maintenance of tone presents many analogies with a reflex action, especially 

 when we remember that, as stated above, tone passes insensibly into con- 

 traction ; and it may be a mere matter of words whether we speak of the 

 maintenance of tone as an automatic or as a reflex action of the cord. 

 We may, however, distinguish the part played by the afferent impulses in 

 assisting the cord to a condition in which it is capable of maintaining tone 

 from the part played by an afferent impulse in causing a reflex action ; in 

 the former the action of the afferent impulses seems analogous to that of a 

 supply of arterial blood in maintaining an adequate irritability of the ner- 

 vous substance, in the latter the afferent impulses lead directly to a discharge 

 of energy. And it is convenient to distinguish the two things by different 

 names. 



511. The close connection between tone and reflex action is illustrated 

 by the so-called "tendon phenomena," which, on the one hand, are considered 

 as cases of ordinary reflex action, and, on the other hand, have been regarded 

 as exemplifying a special influence of the spinal cord on the irritability of 

 the muscle. It is well known that when the leg is placed in an easy posi- 

 tion, resting 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 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 rhythmatic contractions, called " clonic " contractions, by suddenly 

 pressing back the sole of the foot, so as to put them on the stretch. These, 

 and other instances of a like kind, at first sight appear to be, and indeed are 

 by many observers maintained to be, cases of reflex action, due to afferent 

 impulses started in the tendon ; hence they have been frequently spoken of 

 as " tendon-reflex." Other observers maintain that they are not reflex, but 

 due to direct stimulation of the muscles, the vibrations set up in the more or 

 less tense tendon being transmitted to the muscles, and so throwing the latter 

 into contractions. The chief arguments against their being reflex are that 

 the interval between the tap and the contraction is very short (0.03 or 0.04 

 second), shorter than the ordinary interval of a reflex action ( 507), and 

 that the movement persists after section of the nerves of the tendon. The 

 first argument is perhaps not a very strong one, and the second may be met 

 by supposing that in such a case at least, if not always, the reflex act really 

 begins in the muscle, being started in it by the vibrations transmitted to it 

 along the tendon. 



But even if we admit that the movements are purely muscular, started 

 and carried out in the muscle without the help of the usual reflex chain of 

 afferent impulses, spinal centre, and efferent impulses, we must at the same 

 time admit that they are closely dependent on the integrity of the spinal 

 cord and of the connections between the cord and the muscle. In the case 



