390 COMPARATIVE ANATOMY 



autonomic system, and are of two sorts, vasoconstrictor nerves which 

 stimulate the contraction of the blood vessels and thus check the flow of 

 blood, and vasodilator nerves which act as inhibitors and thus permit the 

 dilation of the blood vessels. 



SYSTEMIC CIRCULATION 



The Arteries. All of the blood of the head, trunk, and limbs leaves 

 the heart by way of the aorta, the largest artery of the body, with a 

 diameter of over an inch where it leaves the heart. Beginning at the 

 left ventricle, the aorta ascends to the level of the second rib, curves to 

 the left over the bronchial tube, and then descends along the backbone 

 to the fourth lumbar vertebra, where it divides into right and left common 

 iliac arteries. The divisions of the aorta near the heart are known 

 successively as the ascending aorta, aortic arch, and descending or dorsal 

 aorta. From the ascending aorta in the region of the semilunar valves, 

 two small arteries, the right and left coronary arteries, are given ofif to 

 the walls of the heart. From the upper side of the aortic arch three 

 large arteries arise. The first is the innominate artery, which extends 

 towards the head for a short distance and then divides into right sub- 

 clavian and right common carotid arteries. Near the origin of the 

 innominate the left common carotid artery leaves the aorta and passes 

 to the head. Immediately to the left of the origin of the left common 

 carotid the left subclavian artery extends from the aorta into the left 

 arm. From the lower side of the aortic arch near the ligamentum arteri- 

 osum the upper and lower bronchial arteries pass to the right and left 

 bronchial tubes. (Fig. 326) 



The two common carotid arteries extend towards the head to about 

 the level of the larynx and there each divides into internal and external 

 carotids. The internal carotid supplies chiefly the brain, while the 

 external carotid carries blood to the remaining parts of the head and face. 

 Among the many branches of the external carotid artery are superior 

 thyroid, ascending pharyngeal, lingual, external and internal maxillary, 

 sterno-cleido-mastoid, occipital, posterior auricular, and superficial 

 temporal arteries. The names of these vessels suggest their distribution. 



The subclavian artery extends beneath the clavicle giving off as 

 branches vertebral, internal mammary, superficial cervical, thyroid axis, 

 and costo-cervical arteries. The vertebral arteries, after passing through 

 the costo-transverse foramina of the cervical vertebrae, unite beneath the 

 brain to form the basilar artery which, with the internal carotids, supplies 

 the brain. The internal mammary artery conveys blood to the sternum, 

 diaphragm, and intercostal muscles. The thyreo-cervical or thyroid axis 

 artery supplies thyroid gland, esophagus, trachea, and muscles of the 

 shoulder blade and neck. 



