46 Anatomy of the Rabbit. 



of canals, beginning as lymphatic capillaries in peripheral organs, and 

 ending as lymphatic trunks which empty into the great veins. The 

 lymphatic trunks of the anterior portion of the body are designated 

 from their association with the corresponding veins as jugular and 

 subclavian. They enter the venous system on either side at the point 

 of junction of the internal and external jugular veins or of the common 

 jugular and subclavian (Fig. 52). 



The lymphatic vessels of the posterior portion of the body, 

 including the intestine, unite to form a common canal, the thoracic 

 duct. The latter lies for the most part betwneen the aorta and the verte- 

 bral column, and traverses the thorax in this position to enter the 

 venous system at the same point as the jugular and subclavian trunks 

 of the left side. 



The lymphatic capillaries are terminal, absorptive vessels, differing 

 from blood capillaries both in the character of their walls and in their 

 relations to other portions of the system, since they are not interposed 

 as in the vascular system between vessels of a larger order. The lym- 

 phatic vessels connecting the capillaries with the lymphatic trunks form 

 extensive plexuses, in connection with which the lymph nodes are dis- 

 tributed. 



THE URINOGENITAL SYSTEM. 



The urinogenital system comprises two primary systems — reproductive 

 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. 25), 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 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 in man is not associated with the reproductive ducts. 



In general, however, the relations of the urinary and reproductive 

 organs involves two chief features. First, in primitive vertebrates, 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 vertebrates, 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 higher mammalia, where 

 the urinogenital sinus is completely separated from the digestive tube, and 

 where the urinary ducts are also transferred from a posterior or hypo- 

 cystic position on the wall of the urinogenital sinus to an anterior or 

 epicystic position on the dorsal wall of the bladder. 



Secondly, there is a more fundamental association between the 

 reproductive and excretory organs, depending on the circumstance that 

 the former are primarily in the vertebrates organs connected with the 



