REGULATION OF THE CIRCULATION 449 



the nervous system in two ways — by an acceleration of ihe rate 

 of the heartbeat, and by the contraction of vessels in other 

 parts of the body {in this case, of the alimentary canal and of 

 the spleen), so that the total capacity of the vascular system is 

 not increased. 



ARTERIES 



The aortic arch (Fig. 355) bends over to the left and, as the 

 dorsal aorta, passes backwards under the backbone through 

 the chest and abdomen till it becomes the small caudal artery. 

 A ligamentous band, known as the ductus arteriosus, connects 

 the aortic arch with the pulmonary artery, just before the 

 bifurcation of the latter. At one stage in development this band 

 is represented by an open tube (p. 601). In its course the aorta 

 gives off numerous arteries, of which the following are the most 

 important : (i) a pair of common carotids, each passing up the 

 neck and forking opposite the angle of the jaw into external 

 and internal branches, (2) a pair of subclavians arising from the 

 aortic arch, and going to the shoulder and fore hmbs, (3) the 

 coeliac, which arises from the dorsal aorta shortly behind the 

 diaphragm and divides into the hepatic and the lienogastric, 

 (4) the anterior mesenteric, shortly behind the coeliac, (5) the 

 renal arteries, (6) the genital arteries, (7) the small posterior 

 mesenteric, (8) the common iliac arteries ; these last arise just 

 before the hip girdle and practically end the dorsal aorta, which 

 after them is diminished to the caudal artery. Each common 

 iliac gives off an iholumbar to the body-wall, a vesical to the 

 bladder, and divides into internal and external iliacs. There is 

 considerable variation in the layout of the origins of carotids 

 and subclavians, the commonest variant being the possession 

 of an innominate, from which the right carotid and right sub- 

 clavian branch. Two small coronary arteries from the base of the 

 aorta supply the heart muscle. 



VEINS 



Each superior vena cava (Fig. 356) is formed by the union of 

 a subclavian vein from the shoulder and fore-limb, an external 

 jugular from the surface of the head, and an internal jugular from 

 the brain. The external jugulars are connected by an anastomosis 

 across the ventral surface of the neck. The right superior vena 



