Page:The American Journal of Psychology Volume 1.djvu/13

 NORMAL KNEE- JERK. 7

muscle with the head of the tibia, is subjected to considerable tension. If, now, the ligamentum patel- lae be struck, it will be suddenly depressed into the cavity of the joint beneath it, and a jerk will be trans- mitted by means of the patella and the quadriceps tendon to the quadriceps muscle. Whether the mus- cle fibres and the motor nerve fibres lying in the muscle are directly stimulated by the mechanical irritation thus brought to them, whether the end organs of the sensory nerves in the end of the tendon near the muscle, and in the muscle itself, are excited by the effect of the blow, and transmit stimuli to the muscle through the afferent spinal nerves, the cen- ters in the spinal cord and the efferent spinal nerves, or whether both of these processes aid to bring about the muscular contraction, is unknown. We only know that the result of the blow is to cause a sudden contraction of the quadriceps, which jerks the foot forward in the characteristic manner.

All the methods by which the knee-jerk may be obtained, are merely different ways of giving the quadriceps muscle a twitch by bringing a sudden strain upon its tendon.

Nature of the Process. — Whatever view is held with regard to the nature of the process, all admit that it is very dependent upon the condition of the reflex arc, and that the only matter of doubt is whether the influence exerted by the spinal cord has the form of a reflex action and occurs after the blow has been struck, or whether it is a continuous reflex influence which prepares the muscle by increasing its tone, and thus renders it more susceptible to the irritation resulting from the