91 



EXERCISE XV 



KNEE-JERK; MAGENDIE'S LAW OF THE SPINAL ROOTS 



I. See that the decapitate preparation provided is being ventilated 

 properly and in good condition. 



II. Get ready the inductorium and circuits and the ordinary bipolar hand- 

 electrodes. See that in addition to your usual operating instruments you have 

 (provided for you) the strong cutting bone-pliers for laminectomy and the 

 * lifting * vertebral forceps (text-fig. 35). Also a zinc- and copper-wire fork for 

 the Galvani observations. 



III. With the preparation lying on its side, lift one hind-limb by the Obs. 72. 

 thigh and, holding it vertical and semiflexed at hip, elicit the knee-jerk by a T*^® atonic 

 slight tap, e. g. with the scalpel-handle on the patellar tendon. Note the free ^®®"J®^ 

 flail-like fall of the leg at knee which follows on the brief contraction. This 

 indicates the full relaxation of the vastocrureus muscle after its brief reflex 

 contraction. The completeness of this fall is characteristic of the knee-jerk in 

 conditions where, as in the decapitate preparation, the vastocrureus and the 



other extensor muscles are not exhibiting tonic (postural) action. It should 

 be noted for contrast with the tonic form of the knee-jerk to be examined in 

 the next following exercise (XVI, obs. 75). 



IV. Magendie's ' law ' states that of the spinal roots the dorsal are Obs. 73. 

 afferent and not efferent, and the ventral efferent and not afferent. The Conductive 

 following experiment tests this upon the 6th and 7th lumbar roots. fpinaf roots^ *^^ 



Operation. Place the preparation prone, with limbs symmetrically 

 extended backward and pelvis raised and supported on a warm bottle, so that 

 the circulation and the ventilation of the lungs are not impeded by pressure 

 against table. 



The 6th and 7th lumbar nerves are chosen because their roots are large 

 and long, not difficult to expose, and are distributed to tibialis anticus muscle 

 among others, and that muscle's tendon is easily used for myograph if desired. 



Skin-incision (PI. VII, fig. 1). Make out, by feeling through the skin, the 

 tops of the iliac crests. Along the mid-dorsal line make out the tip of the 

 spinous process of the 7th lumbar vertebra ; it lies (PI. VII, fig. 1) at the level 



N 2 



