DEVELOPMENT OF THE THORACIC DUCT 85 



bearing on the observed embryological conditions. An under- 

 standing of the former is necessary for the correct interpretation 

 of the latter, and vice versa. Hence, while the details of the 

 adult anatomy of the cat's lymphatic system in its relation to 

 the development are reserved for Part V of the second publica- 

 tion of this series, a limited number of adult illustrations are here 

 introduced in order to prepare the ground for the consideration 

 of the ontogenesis of the broncho-mediastinal trunk and of the 

 preazygos segment of the thoracic duct. 



In a certain proportion of adults the ventral or broncho- 

 mediastinal trunk has no direct connection with the preazygos 

 segment of the thoracic duct, but drains solely cephalad into a 

 ventral prolongation from the subclavian approach of the jugular 

 sac, which process also receives the main lymph vessels from the 

 axilla and the forelimb. 



Figs. 92 and 93 (adults, nos. 67 and 13) give good illustrations 

 of this condition. 



In many other adults, however, the left ventral or broncho- 

 mediastinal trunk is connected caudally, with or without the 

 interposition of one or more lymph nodes common to both, with 

 the preazygos segment of the thoracic duct. This junction com- 

 monly occurs about at the level at which the innominate artery 

 divides. The cephalic extremity of the main broncho-mediastinal 

 trunk then usually still drains into the jugular lymph sac by 

 establishing a connection with the ventral prolongation which 

 the subclavian approach of the sac sends caudad over the ventral 

 aspect of the jugulo-subclavian confluence. This process also 

 receives the main lymphatic trunks from the anterior extremity 

 and from the axillary area of the thoracic wall. Occasionally the 

 cephalic end of the broncho-mediastinal trunk also connects with 

 the similar ventral prolongation which the jugular approach of 

 the lymph sac sends down over the ventral aspect of the common 

 jugular angle of confluence. 



The broncho-mediastinal trunk is therefore usually represented 



v by a ventral lymph channel of some size, whose caudal extremity 



joins the preazygos segment of the thoracic duct, while its cephalic 



extremity empties, in common with other axial and appendicular 



