THE INTERNAL EAR IN VERTEBRATES 579 



does not even lie in the same plane throughout its length but 

 takes a sinuous course between one end and the other. The 

 facts, so far as we have them, seem to suggest that the course, 

 length and width of a canal are not of vital physiological im- 

 portance, provided that the sensory ridges and the stretch of 

 the canals leading to them are accurately aligned at right angles 

 to one another and in the three planes of space. The rest of 

 the canal, if approximately in the same plane, serves its purpose 

 by facilitating the flow of the endolymph across the sensory 

 ridge when the head rotates. 



In the lowest fishes — the semi-parasitic Hags — the saccular 

 chamber into which the two ends of the combined vertical canals 

 open is single and has a single sensory area covered by a single 

 mass of calcareous material. 



But in all other fishes most if not all of the chambers and 

 sense organs normal to the vestibular labyrinth are recognisable. 

 Interesting stages in the separation of the different parts may 

 be observed in many Sharks and other fishes, especially in the 

 Lamprey, 1 the general tendency being towards a more complete 

 isolation of the different sense organs. In the Wolf-fish and 

 some other teleosteans, this tendency may, in fact, be carried 

 to such an extreme that the sacculus and lagena are completely 

 cut off and lie more than half an inch away from the rest of 

 the labyrinth. 



Above the lowest fishes, all parts of the vestibular labyrinth 

 can be traced either in adult or embryonic life throughout the 

 whole Vertebrate Class, although in some cases one part, in 

 some another, may suffer degeneration. In all, however, there 

 are the three semi-circular canals lying in approximately the 

 same relative positions ; and in all, except in mammals other 

 than the monotremes, there are three sensory areas covered 

 by calcareous material. 



In comparing the whole labyrinth of a fish with that of man, 

 for instance, it is plain that although in the fish all parts of the 

 human labyrinth except the cochlea are represented, they are 

 represented in excess, being vastly larger and more complete. 

 The vestibular or equilibrating labyrinth in man and all higher 

 vertebrates is, in fact, to a certain extent degenerated. This 

 fact requires some explanation if it indicate a diminution of 

 efficiency, for it entails no apparent loss of balancing power. 



1 Kraqse, A?iat, 4nz. 29, 1906, p. 257. 



