THE CAT 229 



Turn the apex of the heart to the left side and pin it there. 

 Study the dorsal surface of the heart. Find the right and the 

 left auricle. Note the precaval vein entering the right auricle; 

 a short distance from the base of this vein it is joined by the 

 azygos vein. This is a median vein arising in the abdominal cavity 

 and extending along the mid-dorsal line of the thorax to the 

 precaval. It receives the intercostal veins, and is homologous to 

 the right posterior cardinal vein of fishes. 



Entering the posterior margin of the right auricle will be seen 

 the large postcaval vein, which brings the blood of the entire hinder 

 part of the body ; it receives no branches in the thorax. Beneath 

 the base of the postcaval the largest of the coronary veins, which 

 bring blood from the wall of the heart, enters the right auricle. 



Entering the left auricle are the pulmonary veins, which bring 

 arterial blood from the lungs. They are arranged in three groups, 

 each of which contains two or three veins. The right-hand and 

 the left-hand group bring blood from the anterior and middle 

 lobes of the right and the left lung respectively; the posterior 

 group, from the posterior lobes of both lungs. Follow them to the 

 lungs and note their distribution. 



Exercise 18. Draw the dorsal aspect of the heart with the veins just 

 mentioned. 



The Precaval System. The precaval vein and its posterior 

 branch, the azygos vein, have already been observed. A short 

 distance in front of the azygos the precaval receives the internal 

 mammary veins ; they unite and form a single vessel, which joins 

 the precaval opposite the third rib. Each internal mammary may 

 be followed posteriorly to the abdominal cavity, where it anas- 

 tomoses with the epigastric vein. 



The precaval vein is not long ; it is formed by the union of 

 the two large innominate veins. These also are short; they are 

 diagonal in position, and each is formed by the union of the ex- 

 ternal jugular vein, which comes from the head, and the subclavian, 

 from the shoulder and foreleg. The innominate usually receives 

 a single tributary, formed by the union of the vertebral and 

 costocervical veins, although these are subject to considerable 



