AIR-BLADDER. 



203 



another and from the cranial cavity by bony plates, but they open into 

 the cranial cavity in front. The foveae sacculi (16) end blindly behind, 

 but the cavum sinus imparis opens behind into two laterally-placed 

 chambers the floor of which is formed by the basioccipital, the sides and 

 roof by thick fibrous walls. These chambers are the atria sinus imparis 

 (13). To the outer wall cf these is attached a process of the scaphium (S), 

 the anterior of the three ossicles. Of these, the posterior or tripus (11) 

 is by far the largest, and is inserted behind into the fibres of the anterior 

 chamber of the air-bladder ; laterally it has a process articulating with 

 the complex centrum, and anteriorly it is connected by a strong ligament 

 the interossicular ligament (12) to the scaphium. In this ligament 

 and between the 

 tripus and scaph- 

 ium, is the inter- 

 calarium (8). The 

 tripus and inter- 

 calarium are partly 

 enclosed in a thin- 

 walled fibrous sac, 

 containing a deli- 

 cate fibrous net- 

 work and called 

 the saccus para- 

 vertebralis. The 

 air-bladder is 

 divided into a n 

 anterior and pos- 

 terior chamber, of 

 which the anterior 

 is usually especially 

 distensible, and in 

 the Siluridae comes 

 into close contact 

 with the skin on 

 each side. The pos- 

 terior division is 

 generally divided 

 into two by a 

 longitudinal septum 

 which frequently 

 gives off incomplete transverse septa, 

 into the anterior chamber. 



FIG. 119. A view from above of the cranial floor and anterior 

 vertebrae of Macrones ntmurus, semidiagrammatic (after 

 Bridge and Haddon). The brain has been removed, and the 

 bone cut awav, so as to expose more completely the mem- 

 branous labyrinth. 1 sphenotic, 2 prootic, 3 pterotic, 

 4 ductus endolymphaticus, 5 saccus endolymphaticus, 

 6 epiotic, 7 exoccipital, 8 scaphium, 9 intercalarium, 10 

 complex centrum, 11 tripus, 12 interossicular ligament, 

 13 atrium sinus imparis, 14 cavum sinus imparis, 15 sacculus, 

 16 fovea sacculi, 17 ductus sacculo-utricularis, 18 utricle. 



The ductus pneumaticus opens 



Striped muscles are frequently present in the wall of the air- 

 bladder (species of Trigla, Batrachus, Pogonias, Zeus, and others). 

 Sometimes there are extrinsic muscles passing from the ventral 

 surface of the vertebral column on to the air-bladder (species of 

 Gadus, Diodon, Tetrodon, etc.). 



The blood supply of the air-bladder is arterial from the system 

 of the dorsal aorta, either from the efferent branchial vessels, 

 from the coeliac artery or from the dorsal aorta. In Gym- 



