268 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



of higher vertebrates. The walls sometimes contain striated muscle, 

 and in some siluroids and cyprinoids they are more or less calcified, 

 partly by the inclusion of processes from the vertebrae. 



The blood supply is arterial, coming from either the aorta or the 

 coeliac axis, in some instances different portions receiving blood from 

 both. In the walls the arteries break up into networks of minute 

 vessels (*rete mirabile*), these frequently making 'red spots' 

 variously distributed on the inner surface. From the retia the blood 



Fig. 289. — Ventral view of opened air bladder and Weberian apparatus of Macrones, 

 combined from Bridge and Haddon. a, atrial cavity; ac, anterior chamber of air bladder 

 the arrows showing the connexion with the posterior chamber; s, sacculus; sc, scaphium; 

 sk, subvertebral keel; ta, trc, anterior and crescentic processes of tripos; m, utriculus. 



passes to the body veins (postcardinal, hepatic or vertebral). In 

 the ganoids and phystomous species, especially those with a wide 

 pneumatic duct, the gases contained in the swim bladder may be ob- 

 tained directly from the air or water, but in the physoclists this is 

 impossible and the red spots may be the place of its secretion, the 

 probability being increased by the greater abundance of the spots in 

 species with closed ducts. 



The gases in the air bladder consist of oxygen, nitrogen and carbon dioxide, 

 the oxygen usually being in greater relative amount than in the air. It is most 

 abundant in fishes from deep water, rising in some cases to over 90 oer cent, of 



