AIR-BLADDER. 



145 



direction, the nearer they are to the 

 posterior extremity of the air-bladder, 

 where they form an assemblage 

 giving the appearance of a cauda 

 equina. All these appendages soon 

 bifurcate in a dorsal and ventral 

 stem; these stems bifurcate again 

 and again, and either terminate after 

 the first or second bifurcation or are 

 so far prolonged as to reach the 

 median line of the ventral and dorsal 

 sides, anastomosing with the branches 

 of the other side. The branches 

 being enveloped in lamina? of the 

 peritonaeum, form a dorsal and ventral 

 sac of beautiful appearance, caused 

 by the regular arrangement of the 

 air-vessels. The dorsal sac is situated 

 between the air-bladder and the roof 

 of the abdominal cavity without 

 being attached to the latter. The 

 ventral sac receives within its cavity 

 the intestine, liver, and ovaries. — 

 A peculiar mechanism has been ob- 

 served in the air-bladder of the 

 Ophidiidce, the anterior portion of 

 which can be prolonged by the con- 

 traction of two muscles attached to 

 its anterior extremity, with or with- 

 out the addition of a small bone. 



Air-bladders with a pneumatic 

 duct are found in Ganoids and Phy- 

 sostomes, the duct entering the dor- 

 sal side of the intestinal tract, with the exception of Polyp- 



Fig. 63.— Air-bladder of 

 a Scisenoid. 



I. Visceral surface opened at 

 h, to show openings of tlie 

 lateral brandies. 



II. Isolated lateral branch ; 

 a, its opening into the cavity 

 of the air-bladder. 



