SARGENT : THE OPTIC REFLEX APPARATUS OF VERTEBRATES. 231 



animals is in preventing infection and consequent meningitis. To 

 make these experiments conclusive, the time reaction to optical stimuli 

 should be taken both before and after the operation, and also after 

 complete recovery. 



These experiments, though incomplete, show clearly, I believe, that 

 when Reissner's fibre is severed the power to respond quickly to optical 

 stimuli is lost. 



C Physiological Value op the Apparatus. 



We are now naturally led to the consideration of the value of this 

 apparatus in the life activities of the animal. We have seen something 

 of how, as an archaic element of the nervous system, the more complex 

 structures have been built up about it. It probably existed in a form 

 like the single median giant fibre connected with the pigment spot of 

 Amphioxus before the central nei've-paths of the cord were well devel- 

 oped. But not satisfied merely with throwing some light on its origin, 

 ■we must now attempt to explain why this unusual structure should 

 pei'sist throughout the vertebrate series. We are logically led to sup- 

 pose that it is of some advantage to the animal, and the most probable 

 advantage is that it offers a ' short circuit ' for the transmission of 

 motor reflexes, by which a reaction may be brought about more quickly 

 than when the impulse is transmitted through the nerve-tracts of the 

 spinal cord. 



This leads us to the consideration of the time relations involved in 

 these central processes. The data at hand for a comparison of the time 

 of transmission of nervous impulses along different paths in the nervous 

 system are as yet meagre. Sufficient reliable data exist, however, to 

 show that in the reflex response to optical stimuli through the apparatus 

 connected with the fibre of Reissner, tliere are at least two sources of 

 time-saving: (1) in the rate of transmission through the conducting 

 substance ; (2) in the simplicity of the central changes in the optic 

 lobes, where, by a reduction in the number of neurons involved, there 

 is avoidance of delay in the passage through the finer arborizations from 

 neuron to neuron, and through tlie cell bodies of nerve-cells. 



1 . Rate of Transmission of the Nervous Impulse. 



The rate of transmission of the nervous impulse was first measured 

 by Helmholtz ('50), who found it in the sciatic nerve of the frog to be 

 about 27 ni. per second under normal conditions. He afterward de- 



