i ARTERIES 523 



A membranous fold, the Eustachian valve, extends into the 

 right auiicle between the apertures of the postcaval 

 and left precaval : in the embryo this helps to conduct 

 the blood through the aperture in the auricular septum 

 (p. 521). It, like the " ductus arteriosus " (p. 550, and 

 compare Fig. 120), affords another example of a vestigial 

 structure, representing the remains of the sinu-auricular 

 valves (pp. 89 and 449). 



You will remember that in the frog (p. 80) there are 

 two systematic trunks, representing the second arterial 

 arch of the tadpole and fish (p. 451 and Fig. 120), and 

 uniting above to form the dorsal aorta. In the Mam- 

 mal, one of these the right disappears in the course 

 of development and all the blood from the left ventricle 

 passes into the .single left aortic arch (Figs. 135 and 140) 

 from the base of which both carotid arteries arise, the 

 aortic arch then curving over the left bronchus to pass 

 into the dorsal aorta (d. ad). 



Close to the origin of the aortic arch, just beyond its 

 semilunar valves, two small coronary arteries are given off 

 to the walls of the heart ; and more anteriorly, at the 

 curve of the arch, arise the vessels which supply the 

 head and fore-limb. There is a certain amount of 

 variation as to the origin of these, which is asymmetrical, 

 and is usually as follows. Springing from the arch of 

 the aorta towards the right side is an innominate artery 

 (Fig. 140, in), which gives off close to its origin the left 

 common carotid (I. c. c), and then, passing forwards, 

 divides into the right common carotid (r. c. c) and the 

 right subclavian (s. cl. a), the left subclavian (br) taking its 

 origin independently from the left side of the arch. 

 Each common carotid passes forwards along the neck, 

 close to the trachea, and at about the level of the larynx 

 divides into an internal carotid (i. c), which supplies the 

 brain, and an external carotid (e. c), which goes to the 

 head and face. Each subclavian forms several branches, 



K K 2 



