DISSECTION OF THE DOG 41 



Thus constituted, the portal vein makes directly for the porta of the liver, 

 and has been previously seen in relation to the hepatic artery and the bile- 

 duct. Close to the porta it is joined by the gastro-duodenal vein, and generally 

 also by a fair-sized vessel from the pancreas. 



Within the hver the portal vein comports itself after the manner of an 

 artery ; that is to say, it repeatedly divides and finally ends in capillary spaces, 

 or sinusoids, between the microscopic cells of the liver. 



Dissection. — Strip the mediastinal pleura from the surface of the pericardium 

 and expose the phrenic nerves. At the same time the fat lying in the 

 precardial mediastinum should be removed, in order that the entire length 



gastro-dtiodenalis 



V. pancreatico-duodenalis 



V. gastro-epiploica 

 dextra 



\ 'v. mesmferica crnnialis 



\ V. mesenierica caudalis 



I 



V. liencdis 



V. gastro-epiploica sinistra 

 Fig. 14. — Portal vein : semi-diagrammatic. 



of the phrenic nerves may be demonstrated. If the subject be a young 

 one, the thymus should be found in this position. Other structures to be 

 sought are the pericardiaco-phrenic, thymic, and mediastinal branches of 

 the internal mammary artery. 



A. MAMMABiA iNTEBNA. — The greater part of the internal mammary artery 

 has aheady been examined. Its origin, the first part of its course, and some 

 of its earher branches remain for consideration. The internal mammary is a 

 branch of the subclavian artery and arises at the first rib. From this point 

 it runs obliquely in a caudal and ventral direction in the precardial mediastinum, 

 and thus reaches the sternum, where its relationship to the transverse thoracic 

 muscle begins. The collateral branches now to be dissected are as follows : 



(1) The pericardiaco-phrenic artery (a. pericardiaco-phrenica) is often very 



