122 



ANATOMY OF THE RABBIT 



spleen is a lymphatic organ, the largest in the body, with func- 

 tions comparable to those of lymph nodes. It contains large 

 vessels which act as reservoirs for blood and it also stores iron. 



The Urinogenital System 

 The urinogenital system comprises two primary systems — re- 

 productive and urinary — differing widely in their central organs, 

 but associated to a certain extent by having common ducts. In 

 the rabbit, as indicated in the accompanying diagram (Fig. 68), 

 this association extends only to the presence in the two sexes of a 

 urinogenital canal, or urinogenital sinus connecting both urinary 

 and genital structures with the outside of the body. This canal is 



Fic. 69. The principal stages in specialization of the female urinogenital 

 ducts in vertebrates. A, frog; B, monotreme; C, marsupial, bl, bladder; 

 cl, cloaca; k, kidney; od, oviduct; ov, ovary; r, rectum; u, ureter; us, 

 urinogenital sinus (vestibulum) ; ut, uterine tube; v, vagina. (Chiefly from 

 figures of Gegenbaur and Wiedersheim.) 



designated in the male as the urethra, but in the female as the 

 vestibulum, since the structure known from the human relation 

 as the female urethra is only a urinary canal leading from the 

 bladder and does not serve as a reproductive duct. 



In primitive vertebrates (Fig. 69), the urinary and genital ducts 

 open into the posterior end of the digestive tube, the latter forming 

 in this relation a common canal, the cloaca. In terrestrial verte- 

 brates, the urinary bladder is developed as a ventral outgrowth of 

 the digestive tube and, except in amphibians, both sets of ducts 

 undergo a migration from their original position on to the wall of 

 its canal, the latter being thus transformed into a urinogenital 

 sinus. This development reaches its extreme in the placental mam- 



