482 



NER VE. 



sciatic nerve in the living animal, with the circulation and central 

 connections undisturbed, is equally excitable throughout its whole 

 length to mechanical stimuli. 1 On the other hand, chemical stimuli 

 (glycerin, NaCl, etc.), are far more effective with the peripheral than 

 with the central portions of such a nerve. The results thus appear to 

 be conflicting as regards comparative excitability of different regions of 

 the same nerve, but undoubtedly some portions are less tolerant of the 

 injurious effects of exposure than others ; it is not improbable that 

 unequal changes of this kind are the chief factors in determining the 

 different excitability of the various nerves already referred to. 



The conductivity of different portions of any nerve does not vary 

 in so striking a manner as the excitability. There are, however, very 

 great differences in the rate of transmission of structurally different 

 nerves. Thus in the non-medullated nerves of the involuntary muscles, 

 the rate of transmission was estimated by Chauveau as at most 8 metres 

 per second ; nearly four times as slow as that of medullated nerves to 

 voluntary muscles. In the non-medullated nerves of Octopus a rate of 

 4 metres per second, in those of the Anodon a far slower rate of 1 cm. 

 per second was found ; whilst in the nerves to the mantle of Eledone, 

 Uexkiill found a rate of only one-half to 1 mm. per second. 2 In the 

 medullated fibres of the electrical nerve of Torpedo, the rate is from 

 8 to 20 metres per second, 3 whilst m the single medullated nerve fibre 

 of the electrical organ of the Malapterurus, it is as rapid as that of the 

 frog's sciatic, 28 metres per second. 4 



Attempts have been made to estimate the rate in afferent nerves, 

 by utilising the reflex movement as an index of the amount of the trans- 

 mitted excitatory state at the central nervous system. The results are 

 undoubtedly ambiguous, through the uncertainty in the time relations 

 of the reflex response, and when, as in experiments on man, the 

 response is a voluntary one, involving the " personal time " of the 

 individual, they become still more dubious. The only reliable data are 

 rheotomic observations of the electrical response, and the rate of trans- 

 mission in the afferent nerves of the frog appears with these methods 

 to resemble closely that in efferent nerves. An inquiry of very great 

 interest is that of the alteration in rate which occurs in the fine terminal 

 arborisations of the nerves. Whether central or peripheral, these 

 endings are such that the excitatory state has finally to leave the 

 continuous axis cylinder and pass into other excitable tissues. It is 

 known that an appreciable delay occurs under these circumstances ; 

 the central delay for afferent endings in the spinal cord is at least 

 •006 sec, and may be considerably more ; the peripheral delay in the 

 case of a voluntary muscle has been already alluded to in the section 

 dealing with muscle, it amounts to from '003 sec. to "005 sec. 5 The 

 electrical organ of Malapterurus furnishes an admirable object for 

 the further investigation for this delay, as in this organ the nerve 

 fibres split up, making a rich network in each electrical plate. The 

 response of the organ producing an electrical shock is one which is 



1 Tigerstedt, " Studien u. mechanisch. Nervenreizung, " 1880. 



2 Ztschr. f. Biol., Bd. xxx. 



3 Jolyet, Ann. d. sc. nat., Bordeaux, Stir. 2,* tome ii. ; Mem. Soc. d. sc. phys. etnat. de 

 Bordeaux, 1883, tome v. 



4 Gotch, Phil. Trans., London, 1887. B, p. 510; Goteh and Burcli, ibid., 1896, B, 

 p. 347. 



6 Bernstein, Arch. f. Physiol, Leipzig, 1882, S. 329. 



