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 different points, and thus ministers to its nourishment. The chief vessels of the brain are: The Anterior Cerebral. Middle Cerebral. Anterior Communicating. Posterior Cerebral. Posterior Communicating. Anterior Cerebellar. Posterior Cerebellar.

3. Occipital Artery. - This artery, given off by the carotid, is the third division. It passes up to the atlas or first bone of the neck, and after giving off 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.