THE SPINAL CORD AS A REFLEX CENTER 599 



(h) Plantar Refiex. — It consists in a flexion of the toes in consequence of tactile 

 stimulation of the sole of the foot. In certain affections of the pyramidal tracts 

 of the cord, this stimulation elicits an extension of the great toe, instead of a flexion. 

 This constitutes the so-called Babinski phenomenon. 



(i) Bulbocavernosus Reflex. — This muscle may be made to react to stimuli 

 applied to the glans penis. 



(j) Reflexes from the mucosa may be elicited by stimulation of different mucous 

 surfaces. 



(k) Winking Reflex. — The eyelids are closed if a stimulus is applied either 

 to the cornea, conjunctival membrane or skin covering the eyelids. 



(Z) Reflexes from the Facial Muscles. — These responses are obtained by stimu- 

 lating the skin in vicinity of these muscles. 



Among the so-called deep reflexes, i.e., reflexes which are elicited by stimula- 

 tions of the tendons, ligaments and periosteum, may be mentioned the : 



(a) Patellar Reflex or Knee-jerk.^ — A sHght stroke upon the ligamentum patellae 

 produces a contraction of the muse, quadriceps femoris, involving especially the 

 muse, vastus medialis and vastus intermedins. The best results are obtained if 

 the muscle is first put under a slight tension which end can readily be attained by 

 crossing the knees or by sitting upon a chair or table and permitting the leg to 

 hang free across its edge. 



(b) Achillis Jerk. — If the foot is placed in the position of dorsiflexion, a tap 

 upon the tendo calcaneus (Achillis) evokes a contraction of the muse, gastrocne- 

 mius and plantar flexion of the foot. The so-called ankle clonus is obtained if 

 the foot is quickly flexed so that the tendo Achillis and muse, gastrocnemius are 

 suddenly stretched. In certain nervous disorders this reaction acquires a periodic 

 character. 



(c) Wrist Jerk. — This reflex is obtained by tapping the tendons of the muscles 

 of the forearm. Similar effects are yielded by the muse, gracilis, semitendinosus, 

 triceps and biceps. 



{d) Jaw Jerk. — The jaws are closed if the lower jaw is tapped when in the 

 half -open position. 



(e) Periosteal Reflexes. — The muse, supinator longus and biceps contract if the 

 head of the radius is tapped upon. 



(/) Tensor Tympani Reflex. — The muse, tensor tympani contracts as a result of 

 sound impacts of high pitch. The ear drum is in this way rendered more tense. 



The organic or visceral reflexes have been enumerated in part above. They 

 include those pertaining to micturition, defecation and the sexual activities. They 

 are executed chiefly with the help of smooth muscle and glandular tissue, while 

 the superficial and deep reflexes are largely concerned with striated muscle. 



The Nature of the Patellar Reflex. — While the question whether 

 or no the knee-jerk is a true refliex, has been decided in favor of the 

 first view, this decision has not been reached without considerable 

 discussion. To begin with, it was thought that it could not be a 

 true reflex, because the time interposed between the stroke upon the 

 patella and the contraction of the muse, quadriceps, is altogether 

 too short to permit of the passage of the impulse through the spinal 

 cord. This view was based upon the early calculations of the speed 

 of the nerve impulse which, in accordance with Helmholtz, amounts 

 to 33 m. in a second in warm-blooded animals. It was believed, 

 therefore, that the sensory impulse does not enter the spinal center 

 at all, but is transferred to the muscle by way of a peripheral collateral. 

 If this conception were correct, the patellar reflex should really be 



^ Discovered by Erb, Archiv ftir Psychiatric, v. 1875; and Westphal, ibid., 1875. 



