INNOMINATE. 



469 



three large arteries arising from the arch of the aorta. This vessel is joined by 

 the left vertebral, left inferior thyroid, left internal mammary, and the left supe- 

 rior intercostal veins, and 

 occasionally some thymic 

 and pericardiac veins. Fi S- 241.-The Vena Cav and AzygOfl Veins, 



There are no valves in the 

 venas innominate. 



, - 



with their Formative Branches. 



Peculiarities. Sometimes 

 the innominate veins open se- 

 parately into the right auri- 

 cle ; in such cases the right 

 vein takes the ordinary course 

 of the superior vena cava, but 

 the left vein, after communi- 

 cating by a small branch with 

 the right one, passes in front 

 of the root of the left lung, 

 and, turning to the back of 

 the heart, receives the cardiac 

 veins, and terminates in the 

 back of the right auricle. 

 This occasional condition of 

 the veins in the adult is a 

 regular one in the foetus at an 

 early period, and the two ves- 

 sels are persistent in birds 

 and some mammalia. The 

 subsequent changes which 

 take place in these vessels are 

 the following : The communi- 

 cating branch between the two 

 trunks enlarges and forms the 

 future left innominate vein; 

 the remaining part of the left 

 trunk is obliterated as far 

 as the heart, where it remains 

 pervious, and forms the coro- 

 nary sinus ; a remnant of the 

 obliterated vessel is seen in 

 adult life as a fibrous band 

 passing along the back of the 

 left auricle and in front of the 

 root of the left lung, called, by 

 Mr. Marshall, the vestigial fold 

 of the pericardium. 



The internal mammary 

 veins, two in number to 

 each artery, follow the 

 course of that vessel, and 

 receive branches corre- 

 sponding with those de- 

 rived from it. The two 

 veins unite into a single 

 trunk, which terminates 

 in the innominate vein. 



The inferior thyroid 

 veins, two, frequently three 

 or four in number, arise 

 in the venous plexus, on 

 the thyroid body, commu- 

 nicating with the middle 

 and superior thyroid veins. 

 The left one descends in 



Superior Thyroid 



