olfactory nerve (I) 



parophysis 



horizontal canal I j! k J 



anterior canal V ^ ,// Q\ J P'"g°l body 



mesencephalon 



saccular otolith 

 endolymphatic sac 



isthotic 



otic vesicle 



stapes in fenestra vestibuli 



Figure 13-21. Endolymphatic sac development in amphibians. A, dorsal view of brain and endo- 

 lymphatic sac of Necturus, with the ear capsule shown on the left side; B, dorsal view of broin, with 

 endolymphatic sacs and inner ear of right side of the Bullfrog; C to D, two stages in the develop- 

 ment of the endolymphatic sac of Xenopus. (B after Witschi, 1956; C to D ofter Paterson, 1949) 



the delicate columella, associated with the more posterior 

 and ventral tympanum seen in the later reptiles or the 

 chain of bones associated with the tympanum in the mam- 

 mal. The assumption that rapid changes or extensive muta- 

 tions are required does not seem to fit the case. The ear has 

 been functional at all times as a hearing device, using one 

 or another mechanism. 



In the development of the middle-ear cavity, a diverticu- 

 lum of this space is observed to expand around the colu- 

 mella or chain of ossicles. It also penetrates adjacent skull 

 bones, particularly in birds. The connection of this cavity to 

 the pharynx may be wide as in the lizard or a long narrow 

 tube as in the mammal. 



The inner ear of the reptile and mammal is like that of 

 the amphibian, but the lagena and its associated basilar 

 papilla are drawn out into the cochlea (Figure 13-22). This 

 is slightly curved in the reptile and bird and spiraled in the 

 mammal. The amphibian papilla is not observed in these 

 groups. The lagenar macula disappears, while the sensory 

 membrane of the basilar papilla becomes the organ of Corti. 

 The lagena is associated with a perilymphatic space which 

 forms the scala vestibulae and scala tympani. These are as- 

 sociated with the oval and round windows respectively. 



The ear of fishes Among the fish a tympanum and a mid- 

 dle-ear apparatus are lacking; transmission of vibrations into 



the inner ear presents no difficulties in a liquid medium. 

 The inner ears of the myxinid and lamprey differ markedly. 

 In the myxinid it consists of a ring-like structure, a part of 

 which is a semicircular canal lying at an angle between the 

 horizontal and vertical canals of other forms. There is an 

 endolymphatic duct which comes off from the inside of this 

 donut-like structure or dorsomedially. This duct extends 

 into the cranial cavity and enters the meninges, where it 

 forms a slim endolymphatic sac. Internally the inner ear 

 has two cristae in anterior and posterior ampullae, or only 

 one, the posterior ampulla. There are several sensory areas 

 (maculae) in what might be called the vestibule. These 

 probably correspond to the utricular, saccular, and lagenar 

 organs. 



In the lamprey there are two semicircular canals corre- 

 sponding to the anterior and posterior vertical canals of 

 higher forms. The crus communis, where these join, opens 

 into the top of the vestibule. The vestibule is divided into 

 anterior and posterior utricular chambers by a fold of the 

 medial wall. The ampullae of the canals open into these. 

 Below these divisions of the utriculus is a small (anterior) 

 sacculus, and a larger (posterior) lagena. The utricular 

 chambers are partly subdivided by ridges along the course 

 of the semicircular canals. Long cilia on the medial walls of 

 these chambers produce currents in the endolymph. 



In the vestibule of the lamprey are utricular, saccular, 



408 • THE NERVOUS SYSTEM 



