AUDITOR r AND VISUAL ORGANS. 



85 



In the Primates amongst the Mammalia there are present papilla; 

 in the skin (especially on the volar surface) in which the structures 

 known as touch-bodies, containing the termination of tactile nerves, 

 are placed (fig. 82). 



In addition to the general sensibility and the tactile sensations, 

 the higher animals possess, as a special form of sensibility, the 

 capacity of distinguishing different temperatures. 



The sensations of sound are produced through an organ, the 

 auditory organ, which is, in a certain measure, a special modification 

 of a tactile organ. The auditory organ in its simplest form appears 

 as a closed vesicle filled with fluid (endolympfi) and one or more 

 calcareous concretions (otoliths) ; and containing in its walls rod or 

 hair cells in which the nerve fibrillse end (fig. 83). Sometimes the 

 vesicle lies on a ganglion of the central ner- 

 vous system (Worms), sometimes at the end 

 of a shorter or longer nerve, the auditory 

 nerve (Molluscs, Decapoda). In many aqua- 

 tic animals the vesicle may be open and its 

 contents communicate directly with the exter- 

 nal medium, in which case the otoliths may 

 be represented by small particles such as sand- 

 grains which have entered it from the exterior 

 (Decapod Crustaceans). In Molluscs a deli- 

 cate sensory epithelium (macula acustica, fig. 

 83 Cz, Hz.\ marks the percipient portion of 

 the inner wall of the vesicle; while in Crus- FlG - 82. -Tactile papi'.u 



from the volar surface 



tacea the fibres or the auditory nerve end in wit h the touch corpuscle 

 cuticular rods or hairs which project from the and its nerve - v - 

 wall of the vesicle, and, like the olfactory hairs of the antennae, 

 bring about the nervous excitations. In the Vertebrata not only 

 does the auditory vesicle obtain a more complicated form (mem- 

 branous labyrinth), but there are also added to it apparatuses for 

 conducting and magnifying the sound (fig. 84). The tympanum of 

 Acrideidse and LocusticUe, which is generally looked upon as an 

 auditory organ, is built upon quite a different type, since here, 

 instead of a vesicle filled with fluid, air cavities serve for the action 

 of the sound waves on the nerve-endings. 



The visual organs or eyes * are, after the tactile organs, the 

 most widely distributed, and indeed are found in all possible stages 



* Cf. E. Leuckart, " Organologie des Augcs," Graefe and Samisch, Hand* 

 buck der Ophtlialmologie, Bd. II. 



