226 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



the medulla caudad through the cord, in which they lie one on either 

 side of the median plane, ventral to the central canal. They constitute 

 the conduction path for short-circuit transmission of motor reflexes from 

 the auditory centres to the musculature. In the Amblyopsidae the 

 sense of hearing has become, with the degeneration of the eye, more 

 acute and the auditory reflexes have an increased importance, which has 

 resulted in the greater size of Mauthner's fibres. 



I have examined two other blind vertebrates. In Typhlotriton, a 

 blind salamander, no evidence of lieissner's fibre was found. Rhineura, 

 the blind lizard of Florida, has a very small Eeissner's fibre. In speci- 

 mens 28 to 30 cm. long, the diameter of the fibre was 0.8 micra, which 

 is small for the size of the animal. 



4: Relation between the Optic Lobes ami the Optic Reflex Apparatus. 



The corpora quadrigemina in higher vertebrates are degenerate organs, 

 having given up most of their functions to other parts of the brain. 

 "In lower forms the optic lobes — that is, the region of the corpora 

 quadrigemina — are the main visual organs. In higher forms the 

 corpora quadrigemina appear to be active mainly in x-eflex functions, 

 while the lateral geniculate body represents the way station in the 

 visual path to the occipital cortex " (Barker, '99, p. 804). " The 

 superior colliculus of the corpora quadrigemina, so largely developed 

 in lower animals, is but rudimentary in man " (ibid. p. 808). Since, 

 then, the optic reflex apparatus has its centre in the superior colliculus, 

 it must have, in the higher vertebrates at least, a jjurely reflex function. 



5. Relative Importance of the Visual Sense. 



In the lower vertebrates the relatively high development of the eye, 

 as compared with the ear and other sense organs, naturally suggests 

 that the visual sense is of the greatest importance in warning the animal 

 of the presence of danger. This is probably especially true in am- 

 phibians and reptiles, in which there are no Mauthner's fibres as in 

 fishes, and consequently no short circuit exists between the auditory 

 centres and the musculature. Moreover, the sense of hearing is so 

 little developed in the lower vertebrates that it cannot be relied 

 upon to present promptly evidence of approaching danger. Parker 

 (03, p. 18G) has said, ''The sense of hearing is probably the most 

 recently acquired of the senses," and further, " though the irogs, toads, 

 turtles, and their like have a sense of hearing, the efficiency of this sense 



