HEARING. 253 



At the inner canthus of the eye, there is a little spongy- 

 looking bit of mucous membrane, called the caruncula, rest- 

 ing on a fold of smoother membrane, the edge of -which is 

 laid against the eyeball. The fold is the plica semilunaris, 

 and is the vestige or representative of a third eyelid, more 

 distinct in many mammals than it is in man, and developed 

 in birds, especially those of the nocturnal sort, as the mem- 

 brana nictitans, a structure which rapidly sweeps across the 

 eyeball to clear it, instead of the upper lid moving as it does 

 when we wink. This can be readily seen in the owl. 



Opposite the plica semilunaris, the margins of the eyelids, 

 particularly that of the lower lid, change their direction; and 

 on the elevation where this takes place there may be seen in 

 each, when the lid is slightly pulled outwards, a little open- 

 ing which rests against the eyeball. These openings are the 

 puncta lacJirymalia, which lead into two minute ducts called 

 canaliculij each of which passes vertically into the eyelid for 

 a very short distance, then turns abruptly inwards to open 

 into a sac behind the tendo oculi, whence the tears pass 

 downwards by the nasal duct (p. 37) into the nose. Thus 

 the tears, secreted by the lachrymal gland, pass across the 

 eye, washing the conjunctiva; and every time a wink takes 

 place, the puncta lachrymalia, and the parts of the canalicuH 

 in connection with them, are lightly pressed against the eye, 

 so that when the pressure is removed, the moisture is sucked 

 into their interior; and only when there is a redundant 

 secretion, or when by some irregular movement, as in laughter, 

 the puncta lachrymalia are disarranged, do the tears accu- 

 mulate within the lids, and overrun the cheeks. 



188. Hearing. The simplest form of ear may be studied 

 with advantage in the cuttlefishes, in which animals the 

 organs of hearing are imbedded in a cartilaginous collar in 

 the neck, and consist each of a sac supplied with nerves, 

 filled with fluid, and containing some loose particles of hard 

 substance. The vibrations of sound are communicated to 

 the fluid in the sac, and, according to an acoustic law, are 

 strengthened by beating against the solid particles contained 

 in it, and they stimulate the nerve-terminations. But in all 

 vertebrate animals, the ear is much more complex, and, 

 particularly in mammals, not only is the primary sac highly 



