598 



COMPARATIVE ANATOMY 



is increased and, with it, the range of audible sounds. A second mem- 

 brane-covered window, the fenestra cochleae, is added to the inner ear. 

 The malleus and incus are added to the stapes, to form a chain of bones so 

 arranged that the amplitude of the vibrations of the ear drum is reduced 

 and their intensity increased as they pass from the drum to the fenestra 



vestibuli. The efficiency of the apparatus is 

 increased by the addition of two muscles, 

 the stapedial, the smallest of skeletal muscles, 

 and the tensor tympani which tightens the 

 drum. (Fig. 501) 



The conclusion of morphologists that the 

 vertebrate membranous ear is a modified 

 lateral line organ, or a group of such organs, 

 seems justified by the fact that the mem- 

 branous ear develops, like a lateral line organ, 

 from a thickened placode of ectoderm on the 

 side of the head, and that its later ontogenetic 

 changes resemble those of a lateral line organ. 

 In both cases, the skin sinks below the surface, 

 and patches of sensory cells are differentiated. 

 Moreover, the eighth nerve develops as a 

 branch of the facial, a nerve which supplies 

 lateral line organs. In the elasmobranchs, the 

 external apertures of the invagination canal of 

 the statocyst lies near the openings of the 

 occipital row of lateral line organs. A similar 

 separation of lateral line organs also occurs 

 in the case of the ampullae of Lorenzini 

 and the vesicles of Savi in elasmobranchs. 

 Both from their development and nerve 

 relations these organs are obviously differen- 

 tiated lateral line organs. The ear, it is be- 

 lieved, has had a similar history. 



The Human Ear 



The ear consists of three parts, external, middle, and internal. The 

 last is the true sensory organ, which has the double function of equilibra- 

 tion and hearing. 



The External Ear. The external ear has two parts, an auricle or pinna, 

 supported by cartilages, and an auditory meatus, which extends to the ear 

 drum. Sound waves are collected by the pinna, and conveyed by the 

 meatus to the ear-drum which lies about an inch below the surface of the 

 head. The deeper portion of the auricle, which forms a vestibule to 



Fig. 495. — Labyrinth of 

 human embryo, 30 mm. long. 

 a, ampulla; ac, anterior canal; 

 c, cochlea; cr, crus; de, endo- 

 lymph duct; nc, cochlear 

 nerve; s, sacculus; se, endo- 

 lymph sac; u, utriculus; v. 

 vestibular nerve and its gan- 

 glion. (From Kingsley's 

 "Comparative Anatomy of 

 Vertebrates," after Streeter.) 



