672 



FUNCTIONS OF THE SPINAL CORD 



[Oil. XLVII. 



are those of the eye (i) the conjunctival reflex, the movement of the 

 eyelids when the front of the eyeball is touched ; and (ii) the con- 

 traction of the pupil on exposure of the eye to light, and its dilatation 

 on stimulation of the skin of the neck. 



Tendon Reflexes. When the muscles are in a state of slight 

 tension, a tap on their tendons will cause them to contract. The two 



so-called tendon reflexes which 

 are generally examined are the 

 patella tendon reflex or knee- 

 jerk, and the foot phenomenon 

 or ankle-clonus. 



The knee-jerk. The quad- 

 riceps muscle is slightly 

 stretched by putting one knee 

 over the other; a slight blow 

 on the patella tendon causes a 

 movement of the foot for- 

 wards, as indicated in the 

 dotted line of fig. 488. The 

 phenomenon is present in 

 health. 



Ankle-clonus. This is eli- 

 cited as depicted in the next 

 figure: the hand is pressed 

 against the sole of the foot, the calf muscles are thus put on the 

 stretch and they contract, and if the pressure is kept up a quick 

 succession or clonic series of 

 contractions is obtained. This, 

 however, is not readily obtained 

 in health. 



These phenomena are not 

 true reflexes ; the time that in- 

 tervenes between the tap and 

 the response is so short that 

 they must be due to direct 

 stimulation of the muscles by 

 the sudden stretching of their 

 tendons. 



Nevertheless, the idea that 

 they are reflex is supported by 

 the following facts : 



1. There are nerves in tendon. 



2. The phenomena depend for their occurrence on the integrity 

 of the reflex arc. Disease or injury to the afferent nerve, efferent 

 nerve, or spinal grey matter, abolishes them. Thus they cannot 



FIG. 488. The Knee-jerk. (Gowers.) 



FIG. 489. Ankle-clonus. (Gowers.) 



