CONDUCTION IN SPINAL KOOTS 



97 



with Volta, which led to the latter's discovery 

 of the Voltaic pile and the Voltaic current. 

 The experiment is used in this exercise 

 merely as a convenient device for distinguish- 

 ing, owing to the feeble current it employs, 

 between the easily excited nerve-muscle 

 twitch and the less easily excited 'reflex 

 twitch '. 



The experiment's results may be vitiated, 

 if the induced currents employed are pushed 

 to excessive strength, by escape of the 

 stimulating currents to other structures than 

 the isolated root examined. For this reason 

 mechanical stimulation, by ligating with 

 a rather coarse thread, supplies impor- 

 tant control observations. If preferred, the 

 mechanical stimulation may be employed 

 prior to the faradic. The whole experiment 



can, if the cord be well protected from cooling, 

 &c., be strikingly demonstrated by ' ligation 

 stimuli ' apart from electrical. 



W. M. Bayliss has shown, 1900, that there 

 is an important exception to the Magendie 

 'law' of spinal root conduction, in that 

 {Jnl. of Physiol, vol. xx^vi, p. 173) the 

 dorsal roots contain efferent vasodilatator 

 fibres, and that these fibres, like the afferent 

 fibres of the dorsal roots, belong to nerve- 

 cells of the dorsal root-ganglia. His experi- 

 ments have demonstrated the existence of 

 these efferent vasodilatator fibres in the 

 lumbar dorsal roots you use in this exercise, 

 as well as in various other dorsal roots of the 

 spinal series. Cf. also Bayliss, Principles of 

 Gen. Physiol, p. 692, 2nd ed., 1918. 



EXERCISE XYI 



TONIC KNEE-JEEK; LAW OF CONDUCTION OF THE SPINAL NERVE-ROOTS 



TESTED BY PINNA REFLEX 



I. Bulbo-spinal preparation. See that the preparation is in good circulatory 

 and reflex condition. The tracheal tube has been inserted far back, close to 

 sternum, in order that respiratory ventilation may not be impeded by the 

 posture of the preparation required in the exercise. 



II. There are required in addition to your usual instruments (1) the 

 cutting bone-forceps (exerc. XV) and (2) a fine hook-ended seeker (PI. VIII, 

 fig. 4) ; see that these are provided ready for you. 



Arrange the inductorium circuits for unipolar faradization (see exerc. 

 V) ; attach the copper plate as diffuse electrode to the planta of one hind- 

 foot of preparation ; see that the stigmatic hand-electrode is ready for use. 



III. With the preparation lying on its side, examine the knee-jerk as Obs. 75. 

 in J III of preceding exerc. XV. Note differences between this reflex as ^*^® *°^^° 



now exhibited and as seen in that exercise. The contraction is now as 

 ample or more so, but the return fall is not free. An increment of postural 

 extension at knee persists for a time after each jerk-contraction of the knee- 

 extensor. This is well seen if two or three jerks are ehcited in quick 



knee-jerk. 



