THE SPINAL CORD. 459 



a given tendon. Though spoken of as reflexes, they are not such in 

 reality, but rather contractions of the muscles whose irritability has 

 been increased in consequence of extension. To this condition of 

 the muscle Gowers has applied the term myotatic irritability. It has 

 already been stated that the slight contraction of the muscle which 

 gives rise to muscle tonus is a true reflex, the afferent impulses for 

 the same arising in the muscle itself in consequence of extension and 

 compression of the muscle spindles. If the muscle is extended sud- 

 denly, as by a short tap on the tendon, reflex contraction at once 

 ensues. 



The following are the principal forms of the tendon reflexes : 



1. Patellar reflex or knee-jerk, consisting of a contraction of the ex- 



tensor 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. 

 . + ifhe patella reflex is generally present in health, being absent 

 in only 2 per cent.; it is greatly exaggerated in lateral sclerosis, 

 in descending degeneration of the cord; it is absent in locomotor 

 ataxia and in atrophic lesions of the anterior gray cornua.^^ 



2. Ankle-jerk or Reflex. If the extensor muscles of the leg be placed 



upon the stretch and the tendo Achillis be sharply struck, a 

 quick extension of the foot will take place. 



3. Ankle Clonus. This consists of a series of rhythmic reflex 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 so as to suddenly and energetically flex the foot at the 

 ankle, thus putting the tendo Achillis upon the stretch. The 

 rhythmic movements thus produced continue so long as the 

 tension is maintained, fo 1 !^ 6 clonus is, never present in health,, 

 hnt is v^ pMjfaMJ-fa ktemi adfiromq rrf thg rH 

 The toe reflex, peroneal reflex, and wrist reflex are also present in 

 sclerosis of the lateral columns and in the late rigidity of hemiplegia. 

 Reflex Irritability. The general irritability or quickness of 

 response of the mechanism involved in reflex action can be approxi- 

 mately determined by observation of the length of time that elapses 

 between the application of a minimal stimulus and the appearance of 

 the muscle response. The method of Tiirck is sufficiently accurate 

 for general purposes. This consists in suspending a frog, after 

 removal of the brain, and immersing the foot in a 0.2 per cent, solu- 

 tion of sulphuric acid. The time is determined by means of a metro- 

 nome beating one hundred times a minute. Stimulation of the skin 

 can also be effected by the induced electric current, as suggested by 

 Gaskell. A single shock is, however, ineffective. When the shocks 

 follow each other with sufficient rapidity, they give rise to a summa- 

 tion of effects in the nerve-centers which will soon be followed by a 



