56 BRAIN AND BODY OF FISH 



in the bone just above the auditory fenestra of the saccule and enter 

 on either side a small pear-shaped cavity ; from this again a small 

 duct passes upwards and outwards into the posterior spherical air- 

 vesicle, while from its anterior end another duct leads forward 

 into the anterior spherical air- vesicle. Both of these membranous 

 extensions of the swim-bladder are enclosed in bony capsules ; in 

 the bony capsule of the posterior vesicle are embedded the semi- 

 circular canals which thus surround this vesicle just as we have seen 

 the semicircular canals of Mormyrus surround its air- vesicle. If the 

 anterior wall of the anterior air-vesicle is carefully chipped away, 

 the shiny coat of the anterior extension of the swim-bladder will be 

 seen to occupy the outer half of the bony capsule, which is divided 

 by a transverse membrane ; the inner half of the bony capsule 

 communicates by a slit-like opening with the perilymphatic space 

 and near the medial margin of the orifice there lies an end-organ 

 with a layer of hair-cells sm-mounted by an otolith ; from the base 

 of this organ nerve fibres pass into a ganghon connected with the 

 auditory nerve. 



If we consider the position of the posterior air-vesicle and its 

 connections with the exterior it will be noted that it lies in the 

 pterotic bone near the surface of the cranium and that immediately 

 in front there is a temporal foramen occupied by a bay-like expan- 

 sion of a lateral-line canal, and that the inner wall of the lateral-line 

 canal belonging to the lateral wing of the frontal bone is absent. 

 The place of this wall is taken by a tense membrane, which is 

 attached posteriorly to the front of the bony capsule of the posterior 

 air-vesicle, and anteriorly to the bone which connects the anterior 

 capsule to the outer waU of the cranium. How are we to construct 

 a reasonable theory as to the physiological use of this complicated 

 mechanism. Looking at the position of the posterior air-vesicle, 

 its site in the temporal region, and its proximity to the external 

 surface, suggest the theory that its function is to receive vibrations 

 from the surrounding water, which may also be received through 

 vibrations received from the tense membrane which forms the inner 

 wall of the adjacent lateral-line sinus, as this membrane is attached 

 to its capsule. The analogy of its envelopment by the semicircular 

 canals, as is seen in the air-vesicle of Mormyrus, also supports this 

 view. 



Vibrations received by the air in the posterior vesicle would, 

 according to this suggestion, be carried to the air in the outer seg- 

 ment of the anterior spherical air-vesicle, and so to the transverse 

 membrane which divides the bony capsule ; this membrane would 

 convey the vibrations to the perilymph in the inner division, and 



