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 from 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 hinge. 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 atrium sinus imparls is diminished, and 

 the contained fluid urged onwards. As the result of more fluid 

 being forced into the cavum sinus imparls, the saceus endolymphatlcus 

 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 maculce with relation to the saceus and ductus endolym- 

 phatlcus would appear to render unnecessary the special maculae 

 described by Nusbaum in Cyprinus. In any case altered tension in 

 the anterior part of the air-bladder will 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 atrium and the claustrum. A glance at Fig. 4 will 

 show that this is hardly likely to be the case in Amturus, for the 

 medial wall of the atrium is formed of somewhat dense tissue, and 

 the claustrum can be affected very little by the movements of the 

 stapes. It is certain, however, in Amlurus that when the fluid in the 

 <cavicm sinus Imparls 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 cavum which lies above the spinal 

 cord, and communicates with the atria at their points of entrance 

 into the cavum. (Fig. 5, PI. VI.) From this point the sac is con- 

 tinued some little distance forwards through the foi-amen 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. 



