312 STRUCTURE OF THE VERTEBRATES 



and quadrate is lost in the mammals, these bones are incor- 

 porated into the middle ear as the malleus and incus. The stapes 

 is in contact with the inner ear, passing through a foramen in 

 the bone and resting against a thin membrane of the utriculus. 

 The malleus is in contact with the tympanic membrane with 

 the incus lying between. The three assist in transmitting vibra- 

 tions of the tympanum to the inner ear. 



External Ear. The external ear is often limited to the conch, 

 or shell-like funnel, which projects outward in most mammals. 

 It may also include the external auditory meatus, the canal 

 from the outside to the tympanic membrane. 



The anura and lower 'reptiles have a tympanum flush with 

 the surface of the body. In other reptiles the membrane sinks 

 below the surface; and in the crocodiles and birds the meatus 

 is partially covered by flaps of tissue. The conch, a mechanism 

 for concentrating the sound waves, appears first in mammals. 

 It is supported by cartilages, and in the primitive condition 

 rises to a dorsal point. Darwin's point, on the upper rim of the 

 human ear, is a remnant of this primitive structure. The conch 

 varies widely in size and shape. It is particularly large in bats. 

 It tends to be small in animals adapted for water life, and has 

 been secondarily lost in the Atlantic seals, sirenia and whales. 



