38 HIGH SCHOOL ZOOLOGY. 



openings into the utriculiis are dilated into ampullae eacli of 

 wliicli receives a twig of the auditory nerve. 



47. Reference was made to the communication between the 

 air-bladder and the ear of the catfish ; this is brought about in 

 the following way. Near the narrow tube between the 

 upper and lower parts of the labyrinth, there is a cross duct 

 connecting both, and projecting backward into a little pear- 

 shaped sac, which lies in a groove on the npper surface of the 

 basi-occipital bone. The whole labyrinth, and especially tliis 

 l)art of it float compai'atively freely in the fluid contents of the 

 ci-anial cavity (perilymph) ; if currents should be caused 

 in this perilymph it is obvious that currents would also be es- 

 tablislied in the endolymph, and thus excite or disturb the 

 terminal cells of the auditory nerve-fibres which project into it. 

 Such an arrangement for causing currents in the perilymph 

 exists ; it consists in the fact that each half of the neural arch 

 of the first vertebra, can be pressed closer into the neural 

 canal or pulled out by means of a lever (m, Fig. 7), the hinder 

 end of which is attached to the front end of the air-bladder ] 

 consequently any changes of pressure in the air-bladder are 

 ti-ansmitted through this lever to the pei'ilymph, and so to the 

 auditory nerve. Whetl>er sound-waves can aflfect the density 

 of the air in the air-bladder (there is a spot behind the shoulder- 

 blade where it comes immediately underneath the skin) or 

 whether some other function is performed by this singular ap- 

 paratus, cannot be decided with the knowledge at our disposal. 



48. Considerable resemblance will be detected in the microscopical 

 structure of the ends of the nerves of special sense. The labyrinth is 

 lined with epithelial cells which here and there present a difierent 

 character (neuro-epitlielium) where the auditory nerve terminates within 

 them. Two kinds of cells are to be seen, the supporting cells and the 

 hair-cells, the latter alone being connected with the nerves. In the 

 ampulL'e the hairs of the hair-cells are very long and delicate, in tiie- 

 other sensitive spots, short and stouter, for they carry on their 

 tijis the otoliths referred to above. The ampulUe are supposed to be 



