SARGENT: THE OPTIC REFLEX APPARATUS OF VERTEBRATES. 235 



But if the optical reflex function of tliis apparatus is admitted, the 

 above statements then become corollary truths. 



3. Delay in the Conduction Path. 



The conduction path of every nervous impulse is formed of a chain of 

 neurons. In the simplest reflex processes this may consist of only two 

 neurons, one sensory and the other motor, in which case there would be 

 but one place of juncture, that between the cell processes of the sensory 

 cell and the dendrites or cell body of the motor cell. In the more com- 

 jjlex nervous reactions, on the other hand, the impulse must be trans- 

 mitted over a chain of several neurons, with a synapse at the place of 

 contact between each two links of the chain. \Ye have as yet insufficient 

 data from which to ascertain how the delay which accompanies the trans- 



'""Mci 



Figure M. Diagram of tlie optic reflex apparatus. For meaning of abbrevia- 

 tions, see Explanation of Plates, p. 257. 



mission through such a chain of neurons " is to be apportioned between 

 the fine nerve-endings, the unknown field of conjunction, and the 

 branches of the efferent cell through which it is discharged into the 

 issuing efferent fibres. It is logical to assume that in each one of these 

 three situations the diflerence of structure involves a corresponding 

 difference in the rate of propagation, and that the sum of central delay 

 is the sum of the retardation in all of these situations" (Gotch, :00, 

 p. 483). 



"There is reason to believe," says Schafer (:00, p. G08), "that the 

 additional delay ('lost time') which is characteristic of the passage of 

 nervous impulses through the nerve-centres, is due to a block at each 

 synapse ; that, in fact, the nerve-impulses are momentarily arrested at 

 these places of contact of the nerve-cells with one another. And it is 

 not improbable that the relative number of these blocks will furnish a 



