272 THE ANATOMY OF YERTEBRATED ANIMALS. 



The pulmonary air-sacs and their prolongations do not 

 communicate with the air-cavities of the skull, which receive 

 their air from the tympana and the nasal chambers. In some 

 birds, the air is conducted from the tympanum to the articular 

 piece of the mandible by a special bony tube, the siphonium. 



In all Sauropsida^ the ureters open directly into the cloaca, 

 which is provided with a urinary bladder in the Lacertilia 

 and the Chelonia^ but not in other Repjtilia^ nor in Aves. 



Organs of copulation present themselves under three 

 forms : 



1. In the Chelonia^ the CrocodUia, and the Ostrich, a sim- 

 ple solid penis, grooved upon its posterior aspect, is attached 

 to the anterior wall of the cloaca, and contains erectile tissue. 

 In the ostrich this penis lies in a sac of the cloaca, into which 

 it can be retracted somewhat as in the 3Ienotremata, 



2. In many birds, such as the Hheidce, Casuaridce, Ap>tery' 

 gidce, TinainomorplicB / Penelope^ and Grax^ among the Alec- 

 toromorphm / and in many aquatic birds, there is also a single 

 penis attached to the front wall of the cloaca, grooved on its 

 dorsal side, and supported by two fibrous bodies coated with 

 more or less erectile tissue. But the distal end of the penis 

 is invaginated, and the involution held in this position, except 

 during erection, by an elastic ligament. 



3. In Lacertilia and Ophidia^ two copulatory organs are 

 developed at the sides of the cloaca. The integument is pro- 

 longed inward, on each side, into a blind sac, which lies upon 

 the inferior caudal muscles. The inner surface is often armed 

 with spiny developments of the epidermis, and presents a 

 groove, which is continued on the parietes of the cloaca to the 

 aperture of the vas deferens. The wall of the blind sac con- 

 tains erectile tissue, and it can be erected or retracted by ap- 

 propriate muscles. 



