312 ZOOTOMY. 



{c.il.a, § 258) which pass backwards and outwards to the 

 hind legs. The aorta is united to the pulmonary artery 

 by a short ligament, the remains of the embryonic ductus 

 arteriosus. 



238. The thoracic duct, or main trunk of the lymphatic system : a 

 slender, thin- walled tube, lying to the dorsal side of the aorta, and 

 pouring its contents into the left subclavian vein at its junction with 

 the left external jugular : it is best seen by pulling the thoracic aorta 

 somewhat to the right. 



239. The thoracic portion of the sympathetic nerves, 

 slender white cords, lying one on each side of the aorta, 

 and having at intervals ganglia, which lie on the heads of 

 the ribs ; each is continued backwards into the abdomen 

 (§ 185) and forwards into the head (§ 245). 



240. The azygos or (right) cardinal vein (az.ii), best 

 seen by turning the heart and lungs over to the left side : it 

 lies immediately to the right of the thoracic aorta, receiving 

 the intercostal veins (i.cs) from the spaces between the 

 seven posterior ribs : anteriorly it pours its contents into 

 the right precaval. The blood from the five anterior inter- 

 costal spaces is returned, on each side, into an aqterior 

 intercostal vein which enters the corresponding pre- 

 caval. 



241. The innominate artery (/«), springing from the 

 arch of the aorta towards its right side ; it gives off at its 

 origin the left common carotid artery {l.c.c) and then, 

 passing forward, divides into the right common carotid 

 (r.c.c) and right subclavian {s.cl.a). 



242. The left subclavian artery, taking its origin 

 from the left side of the arch of the aorta. 



243. The course of the common carotid arteries : 

 each passes up the side of the neck, external to the trachea 

 and internal to the external jugular, gives off branches to 



