DISTRIBUTION OF THE SYSTEMIC AETEEIES 451 



Carotid Arteries. — These vessels ascend the neck, one on the right 

 and the other on the left side of the trachea, in company with two 

 important nerves, the vago-sympathetic and the recurrent. On reaching 

 the larynx, they each divide into three vessels — the external carotid, the 

 internal carotid, and the occipital. 



1. The External Carotid supplies on each side the external parts of 

 the head. It runs beneath the parotid gland, behind the angle of the jaw, 

 and distributes its branches to the muscles of mastication, the submaxillary 

 and sublingual salivary glands, the tongue, the palate, the pharynx, the 

 lips, the ear, the teeth of the upper and lower jaw, and parts of the eye 

 and membranes of the brain. 



2. The Internal Carotid ascends on the outer side of the guttural 

 pouch and enters an opening [Foramen lacerum) at the base of the skull; 

 while on the floor of the cranium it unites by a cross branch with its fellow 

 on the opposite side, and forms with it an arterial circle (Circle of Willis). 

 It sends branches upward into the substance of the brain at diff'erent 

 points, and thus ministers to its nourishment. The chief vessels of the 

 brain are : The Anterior Cerebral. Middle Cerebral. Anterior Communica- 

 ting. Posterior Cerebral. Posterior Communicating. Anterior Cerebellar. 

 Posterior Cerebellar. 



3. Occipital Artery. — This artery, given oft' by the carotid, is the 

 third division. It passes up to the atlas or first bone of the neck, and 

 after giving ofi" the retrograde, mastoid and occipito-muscular branches to 

 the small muscles of the poll, enters the spinal canal by an opening in the 

 first vertebra, and divides into the cerebro-spinal and occipito-muscular 

 branches. The former on entering the spinal canal divides into two 

 branches, one passing backwards along the spinal cord, and the other, going 

 into the cranium, joins with its fellow on the opposite side to form the 

 basilar. 



Posterior Aorta. — This is a large vessel of considerable length 

 situated immediately beneath the spine, along which it runs from the 

 seventh or eighth dorsal vertebra as far back as the sacrum. It is the 

 largest division of the common aorta, and in the first part of its course 

 describes an arch backward, termed the aortic arch. The anterior portion 

 of the vessel is situated within the chest, and is hence distinguished as the 

 thoracic aorta, while the posterior segment occupies the abdomen, and is 

 known as the abdominal aorta. It is, however, one continuous vessel, and 

 these difi"erent terms are only used to denote its anatomical relations. As 

 it proceeds backwards it passes from the chest into the abdomen through 

 the hiahis aorticus, an opening between the two pillars of the diaphragm. 



The posterior aorta in its course beneath the spine gives off' a number 



