5io TEXT-BOOK OF PHYSIOLOGY. 



2. The tendo achillis reflex or ankle- jerk. This is characterized by a corf- 



traction of the gastrocnemius muscle and a flexion of the foot. 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 

 extension of the foot is produced. 



3. Ankle clonus. This consists of a series of rhythmic reflex contractions 



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 dorsi-flex the foot at the ankle suddenly and energetically, thus 

 putting the tendo Achillis and the gastrocnemius muscle upon 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. 



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

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



The organ reflexes, e.g., the activities of the genito-urinary organs, the 

 stomach, intestines, gall-bladder, etc., which are induced by peripheral 

 stimulation have been considered in connection with the physiologic action 

 of these organs. The genito-urinary center is located in the lumbar region 

 of the spinal cord. In diseased conditions of this region the genito-urinary 

 reflexes are sometimes increased, at other times decreased or even abolished. 



Reflex Irritability. The general irritability or quickness of response of 

 the mechanism involved in reflex action can be approximately 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 Ttirck 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, solution of sulphuric acid. The time is determined by means 

 of a metronome 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. The currents must 

 follow each other with a rapidity sufficient to give rise to a summation of 

 effects in the nerve-centers which will then be followed by a muscle response. 

 It is highly probably that the chemic stimulation gives rise to a similar sum- 

 mation of effects. 



The period of time thus obtained is distributed over the entire mechan- 

 ism. The true reflex time, however i.e., the time occupied in the passage 

 of the nerve impulses through the spinal mechanism is shorter and is obtained 

 by subtracting from the whole period the time occupied by the passage of the 

 impulses through the afferent and efferent nerves as well as the latent period 

 of muscle contraction. This corrected period, the true reflex time, has been 

 found to be twelve times longer than the time occupied by the passage of the 

 nerve impulse through the nerves, including the latent period of the muscle. 



The reflex irritability is increased by : 



i. Separation of the Brain from the Cord. This is at once followed by an in- 

 crease in reflex irritability, and is taken as evidence that the brain 

 normally exerts an inhibitor influence over the reflex centers of the cord. 



