2345. In many Reptiles (such as the Tortoises and Frogs) the urinary cyst gives off two cæca or blind sacs, as is the case in the larynx of many apes.
2346. In the Bird the two cul-de-sacs of the urinary cyst are much more developed and have assumed the form of two cæcal intestines, so that they have been actually viewed as such, and their number two been assigned as characteristic of the bird, the other animals meanwhile having only one. The cæca intestinalia of Birds are the lateral and upper extremities of the urinary cyst. The true cæcum of the bird is the vitelline canal or duct, just as in the fishes and all higher organized animals; this being distinctly retained in the aquatic birds.
2347. In the Bird the rectum properly opens into the urinary cyst between the two blind sacs or cæca, and that indeed with a regular vulva, which is a sphincter muscle.
2348. The cloaca of the Bird is an urinary cyst, into which the anus opens.
2349. The orifice of the cloaca is properly that of the urethra; ova and fæces are moistened with urine. In a bird both these egesta are combined.
2350. Like the urethra, so is the trachea membranous in Fishes, and in most Reptiles also.
2351. Urinary cyst and urethra stand in sympathetic relation with the trachea and larynx, and have also similar diseases, such as catarrh, inflammation, &c.
2352. The proper proof, however, of the urinary cyst belonging to the respiratory system resides in its genesis. It proceeds from the allantois, which in Birds is a decided organ of respiration, a branchia.
2353. Out of this urinary cyst issue in the embryo what have been called primordial kidneys, which shrink or dwindle down at a later period, but have entirely the structure of branchiæ.
2354. Here then is a sexuo-respiratory process, which corresponds to the anal respiration, upon a higher stage, of many worms and aquatic larvæ, e. g. Holothuriæ and Libellulæ. This anal respiration is in its signification a sexual respiration.