318 THE ANATOMY OF VEETEBEATED ANIMALS. 



longations above the kidneys to the sacral vertebrse and 

 to the femora, whence these bones, when they are pneu- 

 matic, receive their air. 



The pulmonary air-sacs and their prolongations do not 

 communicate with the air cavities of the skuU, which re- 

 ceive 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 Saurojasida, the ureters open directly into the 

 cloaca, which is provided -with a urinary bladder in the 

 Lacertilia and the Chelonia, but not iu other Reptilia, nor in 

 Aves. 



Organs of copulation present themselves under three 

 forms : — 



1. In the Chelonia, the Crocodilia, and the Ostrich, a 

 simple solid penis, grooved upon its posterior aspect, is at- 

 tached 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 

 Menotremata. 



2. In many birds, such as the EheidcB, Casuaridoe, 

 Aptenjgidai, Tinamomorphce ; Penelope, and Crax, among the 

 Alectoromorphce ; and m 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 

 prolonged inwards, 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 contains erectile tissue, and it can be 

 erected or retracted by appropria,te muscles. 



