CHORDATA. 351 



eyes. The essential sensory portion of the ear (internal ear) 

 arises as an inpocketing of ectoderm, and consists of a closed, 

 fluid-filled membranous sac which is surrounded by mesoder- 

 mal structures often solid bone. Ordinarily this sac is con- 

 stricted, being thus partially separated into two irregular cham- 

 bers, the dorsal (utriculus) and the ventral (sacculus). From 

 the former arise three semicircular canals which are supplied 

 with sensory hair-cells in the epithelial lining and are looked 

 upon as being an organ to assist in detecting direction of 

 motion and maintaining balance or equilibrium. From the 

 sacculus arises an outgrowth, the cochlea, which in higher 

 forms is well developed. It becomes spiral, and is well sup- 

 plied with sensory cells. It is regarded as the chief auditory 

 organ in those forms possessing it. This membranous sac or 

 labyrinth is completely surrounded by cartilage or bone in 

 fishes, and lies toward the middle line from the spiracle. There 

 is no external ear. In forms above the fishes a membrane 

 (tympanic membrane) stops up the spiracle and incloses what 

 is known as the middle ear, which still communicates with the 

 mouth by the Eustachian canal. A bridge of minute bones is 

 also formed from the tympanic membrane across the middle 

 ear whereby the external vibrations can be communicated to 

 the internal ear. In addition to this, particularly among the 

 mammals, is found an external tube leading to the tympanic 

 membrane. Expanded folds of skin supported by cartilage 

 form a funnel to catch the waves. The tube (external audi- 

 tory meatus} and the funnel or pinna constitute the external 

 ear. 



367. The Eye. The eyes of vertebrates are a single pair 

 of organs lying imbedded in an orbit of cartilage or bone, 

 within which they have considerable freedom of motion. 

 Six muscles, four straight (rectus) and two oblique serve to 

 move the eyeball. These muscles receive the third, fourth, 

 a nd sixth of the cranial nerves. In the higher forms muscular 

 folds of the skin serve to protect the eye in front. The upper 

 and lower lids act vertically, but the third (nictitating mem- 



