=The Arterial System.=—The blood leaving the heart passes from the left ventricle through the aortic semi-lunar valve, into the ascending aorta. Here the two coronary arteries come off which go to supply the muscular tissues of the heart. The ascending aorta passes into the arch of the aorta. Here are given off the innominate artery to the right and the common carotid and the subclavian to the left. The innominate is only about an inch or two in length, and divides into the right common carotid and the right subclavian arteries. On each side the subclavian passes down beneath the clavical bone and enters the axillary space where it is known as the axillary artery. After leaving the axillary space, the artery passes down the arm and is known as the brachial artery. About one inch below the bend of the elbow the artery divides into two branches, known as the radial and ulnar. The radial goes to the thumb side of the hand, and the ulnar to the little finger side of the hand. The ulnar artery and a branch of the radial form the palmar arch, which gives off the branches to the fingers. Coming off the brachial are the deep brachial arteries and the anastomotica magna arteries which anastomose and give collateral circulation to the forearm, by means of the recurrent radial and ulnar arteries. [Illustration: FIG. 28—The arch of the aorta and its branches. (Gray)] The common carotid arteries pass up each side of the neck to a point opposite the Adam's apple, where they, divide into the external carotid, which supplies the muscular tissue of the face, and the internal carotid artery, which goes up through the skull and helps to form the circle of Willis. The vertebral arteries come off the subclavian arteries on either side and pass upward, winding through the foramen of the vertebrae, until finally arriving inside the cranial cavity, unite to form one artery called the basilar, which helps to form the circle of Willis. [Illustration: FIG. 29—The internal carotid and vertebral arteries. (Gray)] The circle of Willis is situated at the base of the brain and gives off to the front the two anterior cerebral arteries, to the sides the two middle cerebral arteries, and to the back the two posterior cerebral arteries. The two anterior cerebral arteries are connected by the anterior communicating branch, and the middle cerebral artery and the posterior cerebral arteries on each side are connected by the posterior communicating branches. The cerebral arteries terminate in the piamater as a dense capillary network, and from there supply the substance of the brain with nutrition. [Illustration: FIG. 30—The circle of Willis. (Spalteholz)]