SENSE ORGANS OF AMIURUS. 383 
chamber to the convex margin of the posterior part, leaving, how- 
ever, the medial end free. These are the points with which fibres 
are left in connection if the air-bladder be removed forcibly trom the 
vertebral column. 
It is easy to demonstrate that if the fibres of the air-bladder 
attached to the sickle-shaped part of the malleus be put on the 
stretch, it (the posterior part of the malleus) is pulled outwards and 
downwards from the vertebral column, the ligament between it and 
the knob serving as a binge. Simultaneously, however, owing to 
the form of the articulation with the third vertebra, the anterior 
end, and consequently the spoon-shaped process of the stapes move 
inwards, the cavity of the atriwm sinus imparis is diminished, and 
the contained fluid urged onwards. As the result of more fluid 
being forced into the cavum sinus imparis, the saccus endolymphaticus 
which floats freely in it must be compressed, and a current of 
endolymph urged forwards which must impinge very directly on the 
macula acustica sacculi of each side. (Fig. 8, Pl. VI.) The position 
of these macule with relation to the saccus and ductus endolym- 
phaticus would appear to render unnecessary the special maculee 
described by Nusbaum in Cyprinus. In any case altered tension in 
the anterior part of the air-bladder wiil be brought within range of 
perception by the auditory nerve. 
Hasse has suggested (loc. cit. p. 596) that in Cyprinus such 
altered tensions will directly affect the spinal cord, the semi-fluid 
tissue surrounding it undergoing compression through the medial 
wall of the atriwm and the claustrum. A glance at Fig. 4 will 
. show that this is hardly likely to be the case in Amiurus, for the 
medial wall of the atriwn is formed of somewhat dense tissue, and 
the claustruwm can be affected very little by the movements of the 
stapes. It is certain, however, in Amiurus that when the fluid in the 
cavum sinus imparis is urged forwards, that in the spinal-canal is 
propelled in the same direction. The reason for this is to be sought 
for in another diverticulum of the cavwm which lies above the spinal 
cord, and communicates with the atria at their points of entrance 
into the cavum. (Fig. 5, Pl. VI.) From this point the sac is con- 
tinued some little distance forwards through the foramen magnum 
into the adipose tissue above the medulla oblongata. It terminates 
there in two lobes, the division into which is indicated in Fig. 6, 
and is filled with the same fluid contents as the cavum and atria. 
