96 COMPARATIVE ANATOMY. 



first time; eyelids without cilia. Third eyelid (nictitating 

 membrane) present both in reptiles and birds. This struct- 

 ure is a transparent elastic membrane, which while at rest 

 lies folded up at inner side of eyeball. It is moved across 

 the front of the eye by appropriate muscles situated at the 

 back of the organ. These muscles are two in number, and 

 are named from their shapes the quadrate and pyramidal 

 muscles. "The first of these arises from the sclerotica at 

 the upper and back part of the globe of the ej'e, and its 

 fibres slightly converge as they descend toward the optic 

 nerve, above which they terminate in a semilunar tendinous 

 sheath having no expressed or fixed insertion. The pyra- 

 midal muscle arises from the sclerotica, from the lower and 

 nasal side of the eyeball; its fibres converge as they pass 

 to the upper side of the optic nerve, and there terminate in 

 a small round tendon, which glides through the pulley at 

 the free margin of the quadratus, and winding round the 

 optic nerve, passes along a cellular sheath at the lower part 

 of the sclerotica, and is inserted into the lower part of the 

 margin of the third eyelid, along which it is continued for 

 some distance, and is gradually lost." (Owen.) 



In birds an additional gland (gland of Harder) is placed 

 upon inner surface of orbit, and furnishes a secretion resem- 

 bling that from the Meibomian follicles of mammals. 

 Mammals possess both lachrymal glands and Meibomian 

 follicles. Quadrupeds have two orbital muscles in addition 

 to those seen in man, a retractor and compressor. 



EAR. 



The ear is the organ of hearing. The auditory capsule 

 (labyrinth) receives sonorous vibrations upon its anterior sur- 

 face, and conveys them across the fluid contents to act upon 

 the nervous expansion (acoustic nerve). The impression is 

 intensified through the agency of one or more free motile 

 sclerous particles (otolithes, otoconites, lapilli, ear-stones, ear- 

 dust) almost constantly present.* 



* These are wanting in Amphioxus and marsipobranchiate fishes. (M'uller.) 



