THE INTERNAL EAR IN VERTEBRATES 583 



Then there are also those peculiar and intricate connexions 

 between the swim-bladder and the ear that are to be found 

 in Carps, Siluroids, Herrings and a few other bony fish. 

 These certainly, by their structure, suggest an organ for trans- 

 ference of vibrations. Though it is of course held by many, 

 including some, like the late Prof. T. W. Bridge, who have 

 made a very special study of these connexions, that they are 

 hydrostatic and serve to inform the fish of the condition of 

 tension in its swim-bladder and therefore of its depth in the 

 water, it should be borne in mind that they occur mainly in 

 bottom freshwater fishes who can have comparative little oppor- 

 tunity for alterations, in the depth at which they swim so great 

 as to be a vital matter. Unsatisfactory as the present condition 

 of the question of hearing in fishes undoubtedly is, the general 

 trend of the above evidence suggests that fishes are sensitive, 

 through the ear, to shock and jarring vibrations of any sort and 

 are also to some extent capable of hearing true sounds if the 

 sounds are sufficiently loud and are originated in the water. 

 The actual receptive organs for vibration are probably the 

 otolith organs. 



We must now consider certain modifications that arise in 

 connexion with the equilibrating ear that ultimately lead to our 

 own organ of hearing — the cochlea and organ of Corti. 



The first appearance of these modifications coincides with 

 the adoption of a terrestrial mode of life, which is quite what 

 one would expect, seeing that in air sound plays an infinitely 

 more important part in life than can be the case in the relatively 

 profound silences of the sea. It is not, therefore, matter for any 

 surprise that the organism responds to its new conditions and 

 attempts to form an organ more sensitive and accessible to 

 sound vibrations than is the deep-seated labyrinth of the fish. 



This object has been attained by modifications in three 

 directions : 



(1) By the formation of a direct path by which vibrations 

 may reach the capsule within which the ear lies (the tympanic 

 apparatus). 



(2) By the provision of efficient means for directing the 

 vibrations after they have entered the ear capsule to certain 

 definite nerve endings. 



(3) By an elaboration of the nerve endings themselves. 

 Dealing with the second of these lines of modification, 



