sakgent: the onic reflex apparatus of vertebrates. 233 



Aside from the observations of Chauveau, there have not been, to my 

 knowledge, any other investigations on the relative rate of transmission 

 in medullated and non-niedullated nerve in the same animal. Other 

 i:)hysiological differences between these two kinds of nerve have, how- 

 ever, been brought out by Kiihne und Steiuer ('79). Having demon- 

 strated the resting current in a suitably prepared olfactory nerve of a 

 fish, they proceeded to compare the value of this current with that ob- 

 tained from medullated nerve of the same diameter in the same fish, as 

 well as from the sciatic nerve of the frog. The result showed a much 

 higher electromotive force for the non-medullated olfactory nerve. This 

 result has been confirmed by Biedermann ('86) and others, working on 

 the non-medullated nerves of Anodonta, and by Sowton (:00), on 

 fishes. Sowton (:00) found still other physiological differences between 

 medullated and non-medullated nerve. " AVhereas in medullated nerve 

 successive effects diminish very slightly, or not at all, or actually in- 

 crease (' staircase effect '), non-medullated nerve always exhibits a 

 comparatively rapid decrease of successive effects." ..." Isolated gray 

 nerve appears to be far less resistant than white nerve." 



I have elsewhere shown (Sargent, :04) that the medullary sheath of 

 the nerve-fibre has for its principal function the insulation^ of the separate 

 nerve-fibres. 



There are, then, considerable physiological differences between medul- 

 lated and non-medullated nerve-fibres, and especially in the rate of 

 transmission of the nervous impulse. In those fibres which are well 

 insulated, the transmitting impulse is conserved and the rate of trans- 

 mission is more rapid. The conducting substance in the fibre of Eeissuer 

 is not only surrounded by a medullated sheath, but it is also isolated 

 from the other conduction paths of the nervous system, and further 

 insulated by the surrounding cerebro-spinal fluid. If, then, the degree 

 of insulation so directly aiTects the rate of transmission of the nervous 

 impulse, it is at least probable that in the fibre of Reissner transmission 

 is more rapid than in other conduction paths of the same nervous sys- 

 tem. It seems, indeed, not improbable that in this separated and 

 highly specialized conduction path the rate of transmission may be 

 several times as fast as through the ordinaiy axis-cylinder. 



2. Time Relations of Central Processes. 



As is well known, the period which necessarily elapses between the 

 presentation of a stimulus and the time at which a signal of recognition 

 is given by the observer is known as the ' physiological time,' This 



