THE SENSE ORGANS 



419 



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 

 aperture of the invagination canal of the 

 statocyst lies near the openings of the occipital 

 row of lateral-line organs. A similar separa- 

 tion of lateral-line organs occurs also 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 differentiated lateral-line 

 organs. The ear, it is believed, 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 equilibration 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 , , . 



vestibular nerve and its gan- 



deeper portion of the auricle, which forms a giion. (From Kingsley's 



vestibule to the meatus, is the concha. "Comparative Anatomy of 

 . Vertebrates," after Streeter.) 



The opening of the meatus is guarded by two 



processes, a ventral tragus next the cheek and a dorsal antitragus opposite. 

 The incurved outer rim of the auricle is the helix. The antihelix is a 

 smaller ridge which bounds the concha dorsally. The walls of the meatus 

 are supported laterally by fibro-cartilage and medially by bone and lined 

 by skin„ The meatus is beset with hairs, and contains many tubular 

 glands which secrete the ear-wax. 



The Middle Ear. A tympanic membrane or ear-drum separates the 

 external meatus from the tympanic cavity or middle ear, within which lie 

 the ear bones. Leading from the tympanic cavity to the pharynx is the 

 auditory or Eustachian tube, which serves to equalize the atmospheric 

 pressure on the two sides of the tympanic membrane, so that this may 



Fig. 368. — 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. 



