208 



Basic Structure of Vertebrates 



Fig. 179. Development of ear of Amblystoma (Norris). (A) Beginning invagina- 

 tion of otic vesicle. (B) Vesicle closed. (C) Model of early vesicle. (D) Older 

 vesicle. (E) Parts all outlined. Sensory patches are lined, (a) Anterior canal; 

 (aa) anterior ampulla; (ai) auditory invagination; (rfe) endolymph duct; (e) external 

 canal; (ea) external ampulla; (g) otic ganglion; (/) lagena; (m) medulla oblongata; 

 (n) notochord; (p) posterior canal; (r) vestibular recess; (s) sacculus; (se) endo- 

 lymph sac; (u) utriculus; (») otic vesicle; (7) seventh nerve. (Courtesy, Kingsley: 

 "Comparative Anatomy of Vertebrates," Philadelphia, The Blakiston Company.) 



changes consisting of local expansions, constrictions, and hollow out- 

 growths, which ultimately convert it into the more or less complicated 

 "membranous labyrinths" of various vertebrates (Fig. 180). 



Sir ucl ure of the Otic Sac 



The main or central region of the otic sac is divided, by a con- 

 striction, into a dorsal utriculus and a ventral sacculus (Fig. 180). 

 From the utriculus spring the semicircular canals, usually three 

 of them. Each is a slender tube arching through approximately a half- 

 circle and freely open into the utriculus at each end. Two of the three 

 tubes lie in vertical planes (assuming a horizontal axis in the animal) 

 which are at approximate right angles to one another, and the third 

 tube is horizontal — therefore perpendicular to the plane of each of the 

 others. The ventral end of each vertical tube expands into a so-called 

 "ampulla," and the anterior end of the horizontal tube has a similar 

 enlargement. The cyclostome eels have no horizontal canal. The 

 lamprey eel (Petromyzon) has the two vertical canals. Another cyclo- 

 stome, the hagfish (Myxine) has what appears as a single vertical 

 canal, but it has an ampulla at each end, suggesting that it may corre- 

 spond to the two canals of the lamprey (Fig. 180A, B). 



The sacculus produces a hollow outgrowth which, in fishes and 

 amphibians, is a short protuberance known as the "lagena." In 

 reptiles, especially crocodilians, the lagena is more elongated. It is 

 well developed also in birds, but attains its maximum in mammals, in 

 which it appears as the greatly elongated and spirally coiled cochlea 

 (Fig. 180G). 



The thin wall of the otic labyrinth consists throughout of two 

 layers of tissue. The inner is the original ectoderm of the embryonic 

 placode and is a simple epithelium only one cell in thickness. The 



