546 TEXT-BOOK OF PHYSIOLOGY 



health. They are increased or exaggerated when the gray matter of the 

 cord is abnormally excited, as in tetanus, strychnin-poisoning, and disease 

 of the lateral columns. 



The tendon or deep reflexes are characterized by a movement of cer- 

 tain parts of the body due to the contraction of certain muscles and are 

 elicited by a sharp tap on their tendons. The fundamental condition for 

 the production of the tendon reflex is a certain degree of tonus of the muscle, 

 which is a true reflex, maintained by afferent nerve impulses developed in 

 the muscle itself in consequence of its extension and hence compression of 

 the end-organs, the muscle spindles, of the afferent nerves. When the mus- 

 cle is passively extended, as it must be when the reflex is to be elicited, there 

 is an exaltation of the tonus and an increase in the irritability. To this 

 condition of the muscle due to passive tension, the term myotatic irritability 

 has been given. If the muscle extension be now suddenly increased, as it 

 is when the tendon is sharply tapped, the increased compression of the muscle 

 spindles will develop additional afferent impulses which after transmission 

 to the spinal cord will give rise to contraction of the corresponding muscle. 

 The tendon reflexes are of much value in the diagnosis of certain lesions of 

 the spinal cord. 



The following are the principal forms of the tendon reflexes: 



1. ThePatellar tendon reflex or knee-jerk. This phenomenon is characterized 



by a quick extension of the leg from the knee downward, due to the 

 contraction of the extensor muscles of the thigh, when the ligamentum 

 patellae is struck between the patella and tibia. This reflex is best ob- 

 served when the legs are freely hanging over the edge of a table. The 

 patella reflex is generally present in health, being absent in only 2 per 

 cent.; it is greatly exaggerated in lateral sclerosis, in descending degen- 

 eration of the cord; it is absent in locomotor ataxia and in atrophic 

 lesions of the cells of the ventral horns. 



2. The tendo-Achillis reflex or ankle-jerk. This phenomenon is characterized 



by a flexion of the foot due to a contraction of the gastrocnemius muscle 

 when the tendo-Achillis is struck. To elicit the contraction, the leg 

 should be extended and the dorsum of the foot be pressed toward the 

 leg so as to give to the gastrocnemius a slight degree of extension. If 

 the tendon be now sharply struck a quick flexion of the foot is produced. 



3. Ankle clonus. This phenomenon consists of a series of rhythmic con- 



tractions of the gastrocnemius muscle, varying in frequency from six to 

 ten per second. To elicit this reflex, pressure is made upon the sole of 

 the foot so as to extend the foot at the ankle suddenly and energetically, 

 thus putting the tendo-Achillis and the gastrocnemius muscle on the 

 stretch. The rhythmic movements thus produced continue so long as 

 the tension within limits is maintained. Ankle clonus is never present 

 in health, but is very marked in lateral sclerosis of the cord. 



4. The Toe Reflex. This phenomenon is characterized by a flexion of the 



foot, then of the leg and perhaps of the thigh when the great toe is 

 strongly and suddenly flexed. It is present in those diseases of the 

 spinal cord in which there is a pronounced patellar reflex. 



5. The Wrist and Elbow Reflex. These phenomena are characterized by an 



extension movement of the hand and arm when the tendons of the ex- 

 tensor muscles are sharply tapped. These reflexes are especially 

 marked in primary lateral sclerosis of the cord in the upper portion. 



