136 VERTEBRATE ZOOLOGY 



almost immediately into the right and left subclavian and the right 

 and left carotid arteries. Each subclavian, after giving off branches 

 to the thyroid gland and the neck, becomes the axillary artery ; 

 this artery gives off branches to the pectoral muscles, and finally, 

 as the brachial artery, enters the leg. The carotids pass along the 

 neck to the head. 



Next to the innominate artery is the left aorta, which springs 

 from the right side of the ventricle, passes to the left side of the 

 body, turns dorsally and posteriorly, and finally joins the right 

 aorta just beneath the spinal column, forming thus the dorsal 

 aorta. The right aorta, in a ventral view, lies behind the innom- 

 inate (which branches from it), and will not be seen at present ; it 

 springs from the left side of the ventricle, and passes to the right 

 side of the body to aid in forming the dorsal aorta. 



At the left of the aorta is the pulmonary artery. It divides im- 

 mediately into a right and a left pulmonary, which go to the lungs. 



These vessels will be studied in greater detail when the whole 

 vascular system is dissected. 



The thymus gland— a small, loose, yellowish mass of tissue- 

 will be seen on each side lying dorsal to and in contact with the 

 arterial trunks, a short distance from the heart. 



Exercise 7. Draw the ventral aspect of the heart and the arterial 

 trunks and their branches so far as they have been observed. 



Bend the heart forward and pin it there. Observe the right and 

 left auricles ; also the large sinus venosus, which is a large, dark- 

 colored vessel extending across the pericardial space and opening 

 into the right auricle. Four large veins enter the sinus, bringing 

 blood to the heart,— the right and left precaval veins, which bring 

 blood from the forward part of the body; the left hepatic vein, 

 from the left lobe of the liver ; and the postcaval vein, which brings 

 blood from the hinder part of the body and the right lobe of the liver. 



Observe the large pulmonary vein, which brings arterial blood to 

 the left auricle from the lungs. It is formed by the union of a 

 right and a left pulmonary, each of which may be seen lying along- 

 side the bronchus on each side; the bronchi are the two large 

 branches into which the trachea divides. 



