SARGENT. — REISSNER'S FIBRE. 451 



tomus) the apparatus is highly developed, Eeissner's fibre having a di- 

 ameter of 10 ^. In Lophius, which is a much larger fish, but is 

 usually sedentary and quiescent in its habits, and more dependent upon 

 tactile than visual sensations for obtaining its food, the apparatus is 

 degenerate and Reissner's fibre inconspicuous, {b) The corpora quad- 

 rigemina in higher vertebrates are degenerate organs, having given up 

 most of their functions to other parts of the brain. In mammals they 

 are, it is generally believed, concerned only with reflex functions. As- 

 suming this to be true, the apparatus under discussion, since it has its 

 centre in the corpora quadrigemina, must have, in the higher vertebrates 

 at least, a purely reflex function. 



(4) Embryology, (a) The apparatus does not reach full development 

 until just before the animal attains free life. In the active fry of trout 

 and salmon, which give reflex responses at this early age, it is fully devel- 

 oped afe the time of hatching. In those forms in which the young are 

 retained within the uterus of the mother until a late stage of develop- 

 ment, as in mammals and some selachians, the apparatus is not developed 

 until a relatively much later stage, {b) In sluggish larvjB the apparatus 

 is not established until after the time of hatching; in Amia not until the 

 fourth or fifth day, in Petromyzon not until the second month. This 

 corresponds closely with the time at which these larvai begin to respond 

 promptly to definite optical stimuli from surrounding objects, (c) In 

 those mammals which are born blind, as the mouse and kitten, the 

 apparatus at birth is in a very incomplete state, so that it cannot be 

 functional. In the mouse at birth the axons are just penetrating into 

 the ventricle, and Reissner's fibre is as yet unformed. 



(5) Degeneration. A study of the blind vertebrates of the cave fauna 

 from the collections of Professor C. H. Eigenmann show that the optic 

 reflex apparatus is reduced in direct proportion to the degeneracy of the 

 eye. In those species which are totally blind no trace of this apparatus 

 is to be found. 



Experiments are now in progress to determine the effect of artificial 

 extirpation of the eye on this apparatus. 



Such a " short circuit " for the transmission of optical motor reflexes 

 must be of great importance in saving time. If the impulse were trans- 

 mitted from the termination of the optic nerve to the posterior muscula- 

 ture through the cord, it would involve transmission through a chain of 

 from two to three neurons. Now, it is well known to psychologists that 

 the time of transmission of a nerve impulse is delayed by passing through 



