42 



DISSECTION OP THE DOG 



small. It follows the phrenic nerve as far as the pericardium. (2) Branches 

 to the thymus (aa. thymicae) are naturally most easily demonstrated in 

 the young. (3) Several small arteries are distributed in the precardial 

 mediastinum (aa. mediastinales anteriores). 



V. mammaria interna. — The internal mammary vein is a satellite of the 

 artery of the same name and receives tributaries equivalent to the branches 

 of the artery. The mode of termination of the vein is subject to some variation. 

 It may open into the innominate vein of its own side, or into the cranial vena 

 cava. 



Fig. 15. — Lateral surface of the right lung. 



Thymus. — If the animal be more than two or three years old, there is 

 little chance of seeing more than the merest trace of the thymus. When 

 present and of good size, the organ is a greyish lobulated body lying in the 

 precardial mediastinum, and flattened laterally in conformity with the place 

 of its location. At its maximum development the thymus extends a little 

 beyond the first rib on the one hand, and overlaps the heart slightly on the 

 other. 



Pulmones. — Each lung is a soft, spongy organ occupying a considerable 

 part of one side of the thoracic cavity and, as has been seen, in intimate asso- 

 ciation with the pleura of that side. With the exception of attachments by 

 its root (radix pulmonis) and the pulmonary ligament, each lung lies free. The 

 lungs of the dog are frequently of a greyish colour due to pigmentation. 



