THE VASCULAR SYSTEM. 135 



and the muscles of the innominate region. The caudal 

 artery is the continuation of the aorta beyond the origin of 

 the internal iliacs. It extends into the tail (Fig. 72). 



ARTERIES OF THE HEAD AND NECK. 



From the arch of the aorta arise the innominate and 

 left subclavian, which supply the sternum, neck, head, and 

 anterior extremities with blood. The innominate gives off 

 the left carotid and then divides into the right carotid 

 and right subclavian (Fig. 73). Sometimes the two caro- 

 tids arise as a single trunk from the innominate, which is 

 then continued as the right subclavian. In this case the 

 common trunk of the carotid usually bifurcates within one 

 or two centimeters of its origin, forming the right common 

 carotid and the left common carotid, lying on the respective 

 sides of the trachea beneath the sternomastoid and the 

 sternohyoid muscles. The vagus or tenth cranial nerve and 

 the sympathetic trunk lie in the same sheath with the caro- 

 tid (Fig. 66). 



Each carotid artery in the neck region gives off the fol- 

 lowing branches : a thyroid to the thyroid cartilage and 

 gland, and a muscular to the muscles of the neck. At the 

 base of the skull, about the middle of the bulla of the tem- 

 poral bone, a slight enlargement of the vessel is seen, from 

 which arises the very small internal carotid, leading through 

 the foramen lacerum medius to the base of the brain, where 

 it joins the circle of Willis. The continuation of the com- 

 mon carotid is now known as the external carotid. This, 

 after giving off a lingual branch to the tongue, an external 

 maxillary branch to the lower jaw, a post-auricular branch, 

 and a temporal branch, turns to pass along the medial 

 aspect of the mandible, where it is named the internal 

 maxillary, whose main branches are the inferior alveolar, 

 the middle meningeal supplying the dura mater, several 



