AUDITORY ORGAN 



297 



laginous capsules of Cyclostomes, those of Gnathostomes in general 

 became gradually more and more drawn into the skull in the 

 course of development, and on the partial reduction of their 

 cartilage, certain of the cranial bones having primarily no relation 

 to them may take part in enclosing the labyrinth (many Teleosts 1 ). 

 The auditory organ of the Dipnoi most nearly resembles that of 

 Elasmobranchii, and more particularly that of Chima?ra. In 

 Protopterus the swollen endolymphatic duct of either side gives 

 rise to a number of tube-like diverticula filled with otolithic 

 substance. These almost entirely cover the fourth ventricle of the 

 brain, and extend backwards as far as the sensory root of the first 

 spinal nerve. 



Amphibians. The membranous labyrinth of Amphibians re- 

 sembles that of Fishes and Dipnoans in many respects, but 

 important differences are seen more particularly as regards the 

 lagena, which, especially in the Anura, becomes further constricted 



FIG. 217. RIOHT MEMBRANOUS LABYRINTH OF Rani esculent a. from the inner 



side. (After G. Retzius. ) 



an, aperture of utriculus ; cus, utriculo-saccular canal ; I, lagena cochleae ; mit, mx, 

 mti, macula acustica recessus utriculi, sacculi, and neglecta ; pb, pai's basilaris 

 cochleae ; pl,pj)b, papilla acustica lagense and basilaris : run, rap, rs, rti, rl, ///, 

 branches of auditory nerve to the anterior and posterior ampullae, sacculus, 

 macula neglecta, lagena, and pars basilaris. (Other letters as in Fig. 216, 

 A 1 C.) 



off from the sacculus and has close relations to the perilymphatic 

 system, reaching a higher stage of development. The sacculus 

 itself in Anura is considerably reduced, while in Urodeles it is 

 relatively larger than in Fishes. 



1 In certain Teleosts (Siluroidei, Gymnotidae, Characinidae, Gynmarchidse, 

 Cobitidse, Cyprinoidae) the auditory organ conies into relation with the air-bladder 

 by means of a chain of bones (" Weberian ossicles ") surrounded by a continuation 

 of the dura mater and derived from certain parts of the four anterior vertebra- 

 and corresponding pairs of ribs : it is possible that by this means the relative 

 fulness of the air-bladder can be appreciated by the Fish. Connections between 

 processes of the air-bladder and the internal ear are also met with in several other 

 Teleosts. 



