VASCULAR SYSTEM.] Su-cat Glands, or sudoriparous glands, are found generally distributed throughout the skin, but are most abundant in the palms and soles, where they number 2500 to 3000 in each square inch. In the skin of the back, again, there are only between 400 and 500 in the square inch. Each gland consists of a ball-like body lying in the subcutaneous .tissue, from which a tubular duct proceeds through the ekin to open on its free surface. The ball is composed of a convoluted tube continuous with the tubular duct, and terminating in a blind end. The wall of the gland tube consists of a delicate nucleated membrane lined by columnar secreting cells. It is surrounded by connective tissue containing capillary blood-vessels. As the gland-duct pierces the cutis it passes between the papillae; in its course through the cuticle it pursues a spiral direction, and has its walls formed, not of a distinct membrane, but of the cuticle cells themselves. The epithelial lining of the duct is continuous with the cells of the rete Malpighii of the cuticle. In the axilla and groin the sweat glands are much larger than in the skin generally. The sweat glands arise as flask-shaped pouches of the rete Malpighii projecting into the cutis, which in course of time become elongated into tubes, and the cells contained in which become the secreting cells of the gland. VASCULAR SYSTEM. The human body and the bodies of all the more highly organised animals are traversed by numerous tubes or pipes, technically called Vessels, some of which in man are nearly an inch in diameter, others so small as to require a micro scope for their examination, others again of every interme diate size. In connection with the vessels is a central organ, the Heart. The heart and the vessels collectively constitute the Vascular System. Of these vessels some contain blood, and form the Blood- vascular system ; others contain lymph, and form the Lymph-vascular system. The lymph-vascular system is not independent of the blood-vascular system, but communicates with it at several points. The vascular system is a hydraulic apparatus, possessing a pump, pipes, and valves. The heart is the pump, which works, not by the movements of a piston, but by the contraction of its muscular walls ; the vessels are the pipes, which convey the contained fluid, and they are provided in certain locali ties with valves for modifying its flow. BLOOD-VASCULAR SYSTEM. The movement of the blood in the blood-vascular system is called the circulation of the blood. In the lower Vertebrata the heart is a single organ, and the blood flows from it through the vessels back again into the part of the heart from which it had proceeded, forming a simple circulation. In man and the higher ver tebrates the heart is a double organ, i.e., it consists of a right and left portion, intimately united to, but not directly communicating with, each other. The blood which flows from its right side passes through vessels which traverse the lungs, and is conveyed to the left side of the heart ; whilst the blood which flows from the left side passes through vessels which traverse the body generally, and is conveyed to the right side of the heart. This is called a double circulation ; that which appertains to the lungs is the pulmonic circulation; that which appertains to the body generally is the systemic circulation. The vessels which carry the blood away from the heart are called arteries ; those which convey it back to the heart are veins. The arteries and veins do not cornnntnicate directly with each other, but through the intermediation of a network of extremely minute vessels, the capillaries. Hence, both in the pulmonary and systemic circulation, the blood in its passage from the arteries into the veins must go through capillaries. The blood which flows from the left side of the heart into the systemic arteries is pure or arterial blood ; ANATOMY 899 as it traverses the systemic capillaries it parts with certain of its constituents to nourish the organs and tissues, and as it receives from them waste products it becomes impure blood ; in which condition it flows back to the right side of the heart by the systemic veins as venous blood; hence the right side of the heart is often called the venous side. The blood which flows from the right side of the heart along the pulmonary artery is this impure blood ; as it traverses the pulmonary capillaries it is purified by the action of the air in the lungs, and is changed into arterial or pure blood, in which condition it flows back by the pulmonary veins to the left side of the heart, which conse quently is called the arterial side. The object of the pulmonary circulation, therefore, is to reconvert into pure blood the blood which has been rendered impure during its passage through the systemic capillaries. The Heart. The heart is a hollow muscle contained He; in the cavity of the chest, and enclosed within a bag called the Pericardium. The pericardium, with its enclosed heart, occupies the space called mediastinum, between the twc lungs ; it lies therefore behind the sternum, and in front of the spinal column, but projects more to the left than to the right side of the mesial plane. The bag of the pericardium is formed externally of a strong fibrous membrane, which is attached below to the central tendon of the diaphragm, but blends above with the sheaths of the great vessels which pass to and from the heart. When the bag is cut open its inner surface is seen to possess a smooth glistening serous aspect, for it is lined by a layer of squamous endothelium, which layer is continuous with the serous membrane that invests the heart, and forms the visceral layer of the pericardium. The continuity of the serous lining of the bag with the serous investment of the heart takes place where the great blood-vessels pierce the fibrous bag. The heart lies obliquely from above downwards, from right to left, and from behind forwards. For descriptive Fia 83. The Thoracic Viscera. In this diagram the lungs are turned to the side, and the pericardium re moved to display the heart, a, upper, a , lower lobe of left lung; 6, upper* V, middle, 6", lower lobe of*. right lung; c, trachea; d, arch of aorta; , superior vena cava; /, pulmonary artery; g, left, and h, right auricle; *, right, and I, left ventricle ; m, inferior vena cava; n, descending aorta; 1, innominate artery ; 2, right, and 4, left common carotid artery ; 3, right, and 6, left sub- clavian artery; 6, 6, right and left innominate vein; 7 and 9, left and right internal jugular veins; 8 and 10, left and right subclavian veins; 11, 12, 13, left pulmonary artery, bronchus, and vein ; 14, 15, 1G, right pulmonary bronchus artery, and vein ; 17 and 18, left and right coronary arteries. purposes it may be regarded as possessing a base, an apex, an anterior and a posterior surface, a right an^. left border.
The base lies backwards, upwards, and to the right, opposite
