COMPARATIVE ANATOMY OF THE CIRCULATORY SYSTEM 251 



tract into the capillaries of the liver. The purpose of this arrangement is that the liver cells 

 may remove from the blood the digested food materials. There is no renal portal system in 

 mammals, it having been completely usurped by the postcaval vein. 



3. The systemic veins. — There are three systemic veins in the rabbit— two 

 precavals and one postcaval — and two in the cat — one precaval and one post- 

 caval. The condition in the cat is due simply to the union of the two precavals 

 anterior to the heart (Fig. 59, p. 268.). Although the branches are similar in 

 the two animals, they will be described separately . 



a) The branches of the precaval vein: This vein is also called the anterior 

 vena cava and descending vena cava. 



Rabbit: Turn the apex of the heart forward and examine the great veins 

 which enter the right auricle. The left precaval vein comes from the left, and 

 passing around the left auricle enters the left side of the right auricle. It receives 

 small coronary veins from the heart wall. The right precaval passes directly into 

 the right anterior part of the right auricle. Note additional coronary 1 veins 

 entering the right auricle directly. 



Carefully trace the right precaval forward, clearing away connective tissue 

 and muscle from about its course, and follow it away from the heart. At the point 

 of entrance into the right auricle it receives from behind the azygos vein. Press 

 the lungs to the left and follow the azygos posteriorly along the dorsal thoracic 

 wall near the median line. Note the intercostal veins which enter it at segmental 

 intervals; they course along the posterior margin of each rib. Entering the 

 precaval immediately anterior to the entrance of the azygos is the supreme 

 intercostal vein, the first of the series of intercostal veins. Shortly anterior to 

 this the internal mammary vein enters the precaval. This vein ascends on the 

 internal surface of the chest very near the midventral line. Trace it posteriorly, 

 noting branches from the intercostal muscles. It continues posteriorly on the 

 abdominal wall as the superior epigastric vein. The next tributary of the pre- 

 caval is the vertebral vein. It enters the medial side of the precaval at about the 

 same level as the internal mammary joins the lateral side. It may be traced 

 deep dorsally to the cervical vertebrae from which it emerges receiving a costocer- 

 vical tributary from the neck. Beyond this point the precaval receives the large 

 subclavian vein from the fore limb. Follow this laterally. It passes between 

 the first and second ribs into the axilla and is then known as the axillary vein. 

 Expose the axilla by cutting down through the pectoral muscles near the mid- 

 ventral line and at their insertion on the humerus. The pectoral muscles should 

 then be separated from the underlying serratus ventralis but should not be 

 removed. The large, stout white cords seen crossing the axilla are the nerves 

 of the brachial plexus and are not to be injured. Lymph glands— small rounded 

 masses — will also be noted in the axilla. 



1 The term coronary (meaning literally a crown or wreath) is in mammals unfortunately applied to 

 two vessels, those of the heart wall and a vessel of the stomach wall having radiating branches. ^ To 

 avoid confusion the latter is referred to as the coronary vessel of the stomach (coronaria ventriculi). 



