ON THE ANATOMY OF FISHES. 
277 
One of the earliest theories of the hydrostatic function of the air-bladder with which 
we are acquainted is that propounded by Boeelli in 1685. According to his theory 
the function of the air-bladder, in its mean condition, is to render the specific gravity 
of the Fish equal to that of the water in which it lives, and also aid the Fish in its 
movements of ascent and descent in the water, such movements being due either to 
an increase in the specific gravity of the animal as the result of muscular compression 
of the air-bladder, or to the diminished specific gravity which would result from 
muscular relaxation and the consequent expansion of the contained gases.* From 
the time of Boeelli until the publication of the results of Moeeau’s experimental 
researches in 1876, the function of the air-bladder proved a fruitful source of discussion 
among zoologists and comparative physiologists, among whom the names of Dela- 
EOCHE, CuviEE, Stannius, Goueiet, Owen, Biot, Monnoyee, and Johannes 
Mullee may be mentioned. Most of these writers, however, adopted the theory of 
Boeelli with scarcely any modification, and with a like absence of any attempt at 
experimental proof. 
Johannes Mullee (28), while accepting Boeelli’s views for Fishes in general, is 
responsible for another theory as to the hydrostatic function of the air-bladder in 
certain Fishes, which has been widely quoted with tacit approval, but like Boeelli’s 
theory, has never received the sanction of experimental proof. Mullee’s theory is : 
that those Fishes (Cyprinidse and Characinidm) in which the air-bladder is partially 
constricted into an anterior and a posterior sac, possess the power by muscular con- 
traction of driving the air from one chamber to the other, and thus producing a 
displacement of the centre of gravity with reference to the longitudinal axis of the 
animal. By this means such Fishes were supposed by Mullee to be able by see- 
saw movements to vary in a vertical direction the long axis of the body, the fins then 
causing the Fish to advance in the new direction which the axis of its body had 
taken.t 
More recently the theories of Boeelli and Johannes Mullee have been success- 
fully opposed on experimental grounds, first, by Moeeau (27), in 1876, and, secondly, 
by Chaebonnel-Salle (6), in 1887. Summarizing the conclusions which these 
researches appear to justify, it may be afiirmed for Fishes in general : — 
(1.) That the function of the air-bladder is to render the Fish, bulk for bulk, of the 
same weight as the medium in which it lives.| In this mean condition, or plane of 
* It is worthy of notice that the theory usually associated with the name of Boeelli is practically 
identical with one propounded by the authors of two papers in the ‘ Phil. Trans.’ for 1675, and therefore 
published several years earlier than the former. See “ Conjecture concerning the Bladders of Air found 
in certain Fishes,” and illustrated by an experiment suggested by the Hon Robert Boyle; and also 
“ On the Swimming Bladders in Fishes,” by Mr. Ray (loc. cit.). 
+ Monnoyee (26) on purely theoretical grounds also adopted Muller’s theory, but admits his inability 
to support his conclusions by any experimental evidence. 
X The experiments of Delaroche (11a) prove that a Fish without an air-bladder has a den.sity always 
