ORGANS OF THE SENSES. 287 



crescentic fold of the skin extending upwards from the su- 

 perior margin of the external meatus. This rudimentary 

 concha is perceptible in most birds, which can erect or de- 

 press the feathers which bound its upper margin, but it is 

 most developed in the nocturnal predaceous birds, which 

 most require this sense to direct them to their prey in the 

 night. The extent of this crescentic membranous concha is 

 encreased in some of the owls by the long feathers which 

 radiate from around its free margin, and it is raised or 

 depressed at will like a valve or an eye-lid. The solid pa- 

 rietes of the tympanum are pierced by several foramina 

 which lead to the large cells between the two tables of the 

 skull, as those in the tympanum of man and quadrupeds lead 

 to the mastoid cells, and the tympanic cavity is here traversed 

 by the vidian nerve as in mammalia. The cartilaginous 

 malleal portion of the single long tympanic ossiculum divides 

 into three parts where it is attached to the inner surface of 

 the membrana tympani, but in place of the three muscles of 

 the malleus observed in man and quadrupeds, there is here 

 but one long muscle which extends forwards from near the 

 occipital condyle to the exterior end of the malleus. 



We thus arrive at the most perfect condition of this com- 

 plex acoustic instrument presented by the inammifervus 

 animals, where all its internal essential parts and all its 

 external accessory apparatus have attained their full deve- 

 lopment. The petrous portion of the temporal bone, of 

 great density, embraces closely all the most important in- 

 ternal parts, and the exterior concha for collecting the 

 sonorous vibrations and directing them to the membrana 

 tympani, is generally of great size. The thin membranous 

 labyrinth, highly vascular and of exquisite delicacy, is filled 

 with a fluid ento-lymph enveloping two cretaceous bodies 

 composed of minute calcareous crystals, and analogous to 

 those found in the labyrinth of fishes and even of many in- 

 vertebrated animals. These two cretaceous bodies, nearly of 

 equal size, are contained, one in the large sacculus of the 

 vestibule, and the other in the long elliptical sacculus 

 formed by the meeting of the semi-circular canals near their 

 ampullae, and they are supported by the delicate expanded 

 extremities of distinct branches of the acoustic nerves as in 

 the inferior classes. The membranous labyrinth is surrounded 



