AUDITORY ORGANS 197 



and lastly the lateral line proper (sometimes double) which runs back 

 on either side to the tail and is supplied by the lateralis of the tenth 

 nerve. Frequently the systems of the two sides are connected by a 

 supratemporal line extending across the hinder part of the skull, from 

 one side to the other (fig. 204, soc). 



The lateral-Une organs appear in the larvae of all amphibia, but on 

 the assumption of a terrestrial life they sink beneath the skin and 

 usually degenerate, all traces of them and the lateraHs nerves being 

 lost in the adult. In a few cases {Triton, Ambly stoma, etc.) they are 

 said not to be entirely lost, but to reappear at the surface when the 

 animals return to the water for oviposition. Various functions have 

 been assigned to the lateral-line organs. Since they contain much 

 mucus they were long called slime organs. Then they were recog- 

 nized as sensory and a 'sixth sense' was attributed to them. Re- 

 cently it has been made very probable that they are to recognize 

 vibrations of a slow rate in the water and thus, among other things, 

 to determine currents, etc. 



Closely allied to the lateral-Hne organs in nerve supply are the 

 ampullae of Savi and Lorenzini which occur in the head of elasmo- 

 branchs. Each consists of a long tube, opening by a pore at the sur- 

 face of the skin and ending with a chambered enlargement, the am- 

 pulla, at the deeper end. The tube is filled with a crystal mucus and 

 the ampulla is embraced by fibres of the lateralis nerve. The organs 

 have been supiposed to be connected with a pressure sense. 



THE AUDITORY ORGANS 



Both in character of innervation and in certain peculiarities of 

 development, the sensory parts of the vertebrate ears are closely related 

 to the lateral-line organs. In their most complete expression three 

 parts are recognized in the auditory organs, the outer, middle and 

 inner ears. Of these the last is the essential portion and occurs in all 

 vertebrates, the middle ear first appearing as such in the amphibia 

 and the outer ear, more or less completely developed, is found only in 

 the amniotes. 



The Inner Ear arises as a circular area of thickened ectoderm on 

 either side of the head, between the seventh and ninth nerves (fig. 

 143, oc). This soon becomes cup-shaped and then the cup closes in to 

 form an auditory vesicle (fig. 205), the cavity of which is connected 

 with the exterior by a slender tube, the endolymph duct, the result of 

 incomplete closure. As one portion of the medial wall of the vesicle 



