182 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
tip of the snout and innervated by the ophthalmicus superficialis 
branch of the seventh nerve; an infraorbital line running in the same 
way beneath the eye and supplied by the buccalis nerve; a hyomandib- 
ular line extending along the lower jaw (and the operculum when 
present), and innervated by the mandibularis externus; 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 supra- 
temporal line extending across the hinder part of the skull, from one 
side to the other. 
The lateral line organs appear in the larve 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 lateralis nerves being lost 
in the adult. In a few cases (Triton, Amblystoma, 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 recognized as sensory and a 
‘sixth sense’ was attributed to them. Recently 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 line organs in nerve supply are the 
ampullz of Savi and Lorenzini which occur on the head of elasmo- 
branchs. Each consists of a long tube, opening by a pore at the surface 
of the skin and ending with a chambered enlargement, the ampulla, 
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 supposed to be connected with a pressure sense. The 
statement is made that when they are removed the fish is unable to 
sink; this may throw some light on their functions. . 
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 
