218 MANUAL OF ANIMAL BIOLOGY 



the auditory nerves (eighth cranial) and thereby give rise to 

 the auditory sensations. (W. f. 147.) 



The utriculus, in which the sense of position is located, consists 

 of a basal part attached to the sacculus, and bears three semi- 

 circular canals which lie primarily at right angles to each other. 

 The utriculus also contains endolymph surrounding the sensory 

 nerve cells. Movements in the endolymph stimulate the sensory 

 cells and thus bring about a sense of position. Experimental work 

 has shown clearly that when the labyrinth with the semicircular 

 canals is removed from both sides of the head, the Frog is not able 

 to regulate its position at all. If the labyrinth is destroyed on only 

 one side an asymmetrical attitude of the Frog results ; the head 

 being tipped one way or the other depending upon which side has 

 been operated on. 



In the higher Vertebrates, including Man, a greatly coiled struc- 

 ture, known as the cochlea, develops from the sacculus. It is 

 regarded as a derivative of the sacculus of the ear in the lower 

 Vertebrates. The cochlea becomes the essential part of the ear 

 for the function of hearing. It contains an extremely complex 

 structure, known as the organ of corti, in which the nerve end- 

 ings are located. The vibrations of the auditory ossicles are com- 

 municated to the endolymph present in the cochlea and in the rest 

 of the membranous labyrinth at a certain region of the vestibule 

 known as the oval foramen. (W. f. 148.) 



D. The Sense of Sight. The eyes, which have as their specific 

 function the reception of light, or photic, stimuli, lie in special 

 cavities, or orrits, on the dorsal and lateral wall of the head. The 

 eye is spherical in shape, and is composed of several layers of tissue. 

 It is covered on the outside by a strong, connective tissue sheath, 

 known as the sclerotic coat, which forms a continuous covering 

 of the eye, except for one small area in the posterior region where it 

 is pierced by the optic nerve running from the eye to the brain. 

 Posteriorly, the sclerotic coat of the eye is opaque, but in the front 

 there is a transparent region, known as the cornea, through which 

 the light rays can pass into the interior of the eye. (W. f. 150.) 



Attached to the sclerotic coat are several eye muscles which move 

 the eye in various directions. In the first place, there is a large 

 retractor rulri muscle attached to the posterior portion of the 

 eyeball. When this muscle contracts the eye is drawn back into 

 the orbit. The eye is protruded from the orbit by the contraction 



