96 



EXERCISE XV 



fork of copper and zinc wire soldered together at the handle end. The 

 stalk end of the Y is held in the hand, and of the two free ends, one copper, 

 one zinc, and 3 mm. apart, one is brought under the root-bundle it is desired 

 to test and the root-bundle lifted on it so as to be insulated in air for about 

 6-7 mm. The handle of the Y-wire is then rotated slightly, so as to make its 

 other free end touch the root-bundle ; if the root is motor a twitch results. 



Or the experiment can be done by raising the root-bundle with the small 

 brass handle-hook used for separating the roots, and then touching the root 

 with the point of the fine scissors, touching with the scissors at the same time 

 some part of the brass wire of the hook. 



ANNOTATION 



Ohs. 72. The knee-jerk stands as proto- 

 type of a numerous group of reflexes of the 

 proprioceptive class. A difficulty for re- 

 garding it as truly reflex on account of the 

 brevity of its latent time has been removed 

 by new knowledge of the quick speed of 

 conduction obtaining in mammalian nerves, 

 and by the improved measurements of the 

 reflex time of the jerk, by Jolly {Brit. Med. 

 Jnl. Oct. 1910 ; Quart. Jnl. of Expl. Physiol. 

 vol. iv, p. 67, 1911) and Snyder {Atn. Jnl. 

 of Physiol, vol. xxvi, p. 474, 1910). 'Pro- 

 prioceptive ' is a term applied to reflexes in 

 which the exciting stimulus is some part of 

 the body acting upon its own afferent nerves. 

 In the knee-jerk the actual stimulus is a 

 sudden stretching of the muscle, the stretch 

 serving to excite the afferent nerve-fibres in 

 the muscle itself, and these in turn exciting 

 a responsive discharge of motor impulses 

 from the reflex centre. The knee-jerk 

 muscle is vastocrureus ; the rectus femoris 

 portion of quadriceps extensor is probably 

 not involved. 



Obs. 73. Follows closely the original 

 experiment made by F. Magendie in 1822 

 {Jnl. de Physiologie, Paris, tome ii, pp. 

 276-9 and 366-71), which discovered the 

 functional difference between the ventral 



spinal roots and the dorsal spinal roots, the 

 former being efferent, the latter afferent. 

 Magendie's observation was conducted on 

 the lumbo-sacral roots of the puppy ; for 

 exciting the roots he used the Voltaic 

 current as well as mechanical stimuli. 



The well-known slight variation in level 

 of the spinal roots in the posterior region of 

 the vertebral column affects this exercise 

 practically not at all. The directions given 

 apply to what may be termed the standard 

 cat, but in some cats the spinous process of 

 seventh lumbar vertebra lies slightly behind 

 the transverse level of the iliac crests, while 

 in some it lies farther forward from it than 

 usual. In the latter case it may happen that 

 the student mistakes seventh lumbar root for 

 sixth, but there is no practical difference in 

 the procedure desirable for the one and the 

 other, and the seventh root supplies motor 

 fibres to tibialis anticus as does the sixth. 

 The seventh root has an available length of 

 about 30 mm., whereas the available length 

 of the sixth is about 25 mm. 



Obs. 74. Galvani's 'experiment with metals' 

 was made on the sciatic nerve and muscles of 

 the leg of the frog, 1791 {De Viribus Electrici- 

 tatis in Motu Muscularis, Bologna). From it 

 sprang the long and important controversy 



