238 HENRY C. TRACY 



The anterior membranous vesicle enclosed in the osseous capsule, 

 the enlargement of the lateral-line canal in the lateral recesses, 

 the lateral recess of the skull, the perilabyrinthine canals, and 

 the otolithic membrane are to be considered as accessory struc- 

 tures by which the stimulus is brought to bear on the receptor. 

 This mechanism as a whole may be conceived to function in 

 one of the following ways: 



1. Merely as a sense organ which acquaints the fish with 

 changes in water level. 



2. As a part of a reflex mechanism actuated by changes in 

 water level, originating nerve impulses which pass out by effer- 

 ent fibers of the spinal nerves to the skeletal muscles, thereby 

 producing compensatory movements which tend to maintain the 

 fish at a nearly constant water level. 



3. As a part of a reflex mechanism which through efferent 

 visceral nerves transmits nerve impulses to the secretory cells 

 and smooth muscle fibers in the swimbladder wall and which 

 thus adjusts the swimbladder to changes in hydrostatic pressure. 



4. By some combination of two or possibly of all three of 

 these functions. * 



To consider this ear-swimbladder mechanism "merely as a 

 sense organ" (whatever that term may mean) is hardly adequate 

 in view of our present conceptions of the nervous system in gen- 

 eral and the nerve connections of the membranous labyrinth 

 in particular. In all vertebrates the maculae and cristae seem 

 to function primarily (and possibly exclusively) through reflex 

 mechanisms. It is probably safe to affirm this positively in 

 case of the lower vertebrates. It is therefore almost certain 

 that this apparatus acts reflexly according to the second or 

 third possibility as stated above or by combination of the two. 

 We know, through the work of Moreau, Baglioni, and others, 

 that in the case of the Physoclisti the gas pressure inside the 

 swimbladder is adjusted to changes in the outside water pres- 

 sure as a result of an interaction between the absorptive and 

 secretory mechanisms in the wall of the organ. In the Clu- 

 peoids there is an epithelium, apparently secretory, in the 

 pneumatic duct and probably in the tubular portion of the 



