XT ARTERIES 523 



Membranous folds, the Eustachian and Thebesian valves, extend 

 into the right auricle between the apertures of the postcaval and left 

 precaval : in the embryo these help to conduct the blood through the 

 aperture in the auricular septum (p. 521) : they afford another example of 

 vestigial structures, representing the remains of the sinu-auricular valve. 



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

 two systemic 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 Mammal, 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 (L 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. <:), which goes to the head and face. 

 Each subclavian forms several branches, the most important 

 of which are a brachial (br) to the fore-limb, a vertebral 



