SENSE OF SIGHT. 423 



tions. No doubt this faculty which the organ of hearing 

 possesses for judging of such different qualities (fulness, 

 rapidity, and form, or combinations of the sound waves), 

 renders necessary that remarkable complication of structure 

 in the internal ear which will long continue to baffle physio- 

 logists. According to Flourens, the semicircular canals 

 have great influence on the equilibration of the animal. This 

 physiologist discovered that injury to these canals produces 

 rotatory movements. Yulpian has made similar experiments 

 on a pigeon, and shown that rotatory, rolling, or falling move- 

 ments are produced, according as the horizontal canal, the 

 anterior vertical canal, or the posterior vertical canal is de- 

 stroyed. These effects are, however, caused rather by a 

 vertigo, and are no proof that the semicircular canals govern 

 the equilibrium and the co-ordination of movements. It may 

 be asked, finally, whether the phenomena observed in these 

 experiments are really owing to lesions of the semicircular 

 canals, and not to that of the adjacent parts. Bottcher made 

 the experiment of carefully detaching the semicircular canals 

 in a frog, and succeeded in destroying them without injuring 

 any other part of the labyrinth or of the encephalon, and he 

 never found in the batrachians this operation followed by any 

 difficulty in walking or standing. This effect is produced 

 only when the lesion is deep-seated. We may conclude from 

 this that the semicircular canals really form an organ of 

 hearing, and not an organ which governs the equilibrium of 

 walking and standing. (Bottcher, " Medic. Zeitschr.," 1872.) 



V. THE SENSE OF SIGHT. 



THE sense of sight enables us to judge of the luminous 

 properties of the objects by which we are surrounded, and, 

 consequently, of their color, their form, and their position. 

 The organ of sight (the eye ) is essentially composed, 1, of a 

 membrane (the retina), connected with the nerve termina- 

 tions, and upon which the impressions of the luminous rays 

 are made ; 2, of a dioptrical organ intended to collect and 

 condense the rays of light upon the membrane just men- 

 tioned, where they represent in miniature external objects, as 

 on a screen in a dark room ; 3, of membranes, attached to 

 these two organs, for the purpose of securing and modifying 

 their functions. These different parts (Fig. 112) are con- 

 nected like the other organs of the senses, in a physiological 

 point of view, with the study of the surfaces of the organism, 



