?12 PROF. T. W. BRIDGE ON TEE 



position c£ the apertui^e of the ductus pneumaticus slightly to 

 the left of the median line, is (1) tliat the left lateral compart- 

 ment is smaller than the right; (2) that the orifice of communi- 

 cation between the subspherical sac nnd the left lateral chamber 

 is reduced to the condition of a relatively small aperture, which 

 is in striking contrast to the widely open^ direct cmnmunication 

 between the sac and the lateral compartment of the right side ; 

 and. (3) that while the ductus pneumaticus opens directly into 

 the left compartment, its connection with the right chamber is 

 pomewhat indirect and takes place through the cavity of the 

 subspherical sac, round the free anterior margin of the ventral 

 portion of the longitndinal septum (fig. 3). 



Like the abdominal portion of the bladder, the subspherical 

 sac (PI. 36. fig. 3,sb.s.) is firmly attached along its dorso-lateral 

 inai'gins to the transverse coatiferous processes of certain of the 

 trunk vertebrae, viz., the second to the fifth, inclusive. Its 

 dorsal wall is extremely thin, and, in fact, is represented by 

 the tunica interna alone ; the latter stratum, in the absence of 

 the mesonephros in this region, being closely adherent to the 

 lateral and ventral surfaces of the anterior vertebral centra. 

 Internally, the sac is partially subdivided by an extremely thin 

 median longitudinal septum (Ls.'), which projects downwards 

 i'rom its dorsal wall, and is a direct continuation of the dorsal 

 half of the longitudinal septum previously mentioned, as unsym- 

 metrically dividing the cavity of the abdominal portion of the 

 air-bladder. Tlie septum is best developed behind, and gradually 

 diminishes in height towards the anterior limit of the sac. Its 

 free ventral margin is remarkably thick and forms a strong, arch- 

 like band, the fibres of which diverge behind to strengthen and 

 stiffen the inner and outer lips of the orifice by which the sac 

 communicates with the left lateral compartment of the preceding 

 portion of the bladder. Anteriorly also the band divides, but 

 into three fibrous bundles, the more laterally-situated of which 

 diverge downwards into the lateral walls of the sac and strengthen 

 the outer lips of the aperture through which the sac com- 

 municates with its anterior tu buhl r prolongation, while the mesial 

 fibres continue their forward course. 



