SARGENT : THE OPTIC REFLEX APPARATUS OF VERTEBRATES. 227 



is low compared with that which it attains in the birds, and particularly 

 in the mammals, the highest vertebrates. It is thus evident that the 

 sense of hearing is coupled with high organization." The observations 

 of Yerkes (:02, p. 630) on the importance of the visual sense in the 

 frog are of great interest in this connection. " One can approach to 

 within a few feet of a green frog or bull frog and make all sorts of noises 

 without causing it to give any sign of uneasiness. Just as soon, how- 

 ever, as a quick movement is made by the observer, the animal jumps. 

 I have repeatedly crept up very close to frogs, keeping myself screened 

 from them by bushes or trees, and made various sounds, but have never 

 succeeded in scaring an animal into a motor response so long as I was 

 invisible. Apparently they depend almost entirely upon vision for the 

 avoidance of dangers. Sounds like the splash of a plunging frog, or 

 the croak or pain-scream of another member of the species, serve as 

 warnings, but the animals do not jump into the water until they see 

 some sign or an unusual or dangerous object." The senses of hearing and 

 smell give such inadequate warning of the presence of danger to these 

 lower animals, that it is of the most vital importance that they should 

 be able to react quickly to optical stimulus. This accounts for the 

 greater development and greater importance of the optic reflex appara- 

 tus in the lower vertebrates, and its decreased value in mammals, where 

 the sense of hearing and smell are more acute. 



■■o 



B. Experimental Physiology. 



The desirability of physiological experiment to determine the func- 

 tion of this apparatus early presented itself to me. The only practical 

 method of accomplishing this was to sever Reissner's fibre and study 

 the reaction of the animal after the operation. In the great majority 

 of animals this is practically impossible because of the difficulty of the 

 operation and the great disturbances resulting from it, seriously impair- 

 ing the validity of subsequent observations. Success is probable only 

 in the lower vertebrates, where the fibre can be reached in the fourth 

 ventricle through the thin tela chorioideus which covers it. To insure 

 success, an animal should be chosen in which the brain flexures ai-e 

 slight, so that the ventricle is easily accessible from the surface. My 

 first attempt was upon the alligator, which seemed to off'er a favor- 

 able subject. My experiments, however, failed to yield any results be- 

 cause of the sluggishness of these animals and their slow response to 

 optical stimuli. This may have been partly due to an abnormal condi- 



